Carbon dioxide
Overview
Carbon dioxide is a naturally occurring chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 composed of two oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s covalently bonded
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

 to a single carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atom. It is a gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

 at standard temperature and pressure and exists in Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 in this state, as a trace gas
Trace gas
A trace gas is a gas which makes up less than 1% by volume of the Earth's atmosphere, and it includes all gases except nitrogen and oxygen . The most abundant trace gas at 0.934% is argon, which is being continually produced by radioactive decay of in the earth's rocks...

 at a concentration of 0.039% by volume.

As part of the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

 known as photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s, algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, and cyanobacteria absorb carbon dioxide, light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

, and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 to produce carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

 energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 for themselves and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 as a waste product.
Encyclopedia
Carbon dioxide is a naturally occurring chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 composed of two oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s covalently bonded
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

 to a single carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atom. It is a gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...

 at standard temperature and pressure and exists in Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 in this state, as a trace gas
Trace gas
A trace gas is a gas which makes up less than 1% by volume of the Earth's atmosphere, and it includes all gases except nitrogen and oxygen . The most abundant trace gas at 0.934% is argon, which is being continually produced by radioactive decay of in the earth's rocks...

 at a concentration of 0.039% by volume.

As part of the carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

 known as photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s, algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, and cyanobacteria absorb carbon dioxide, light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...

, and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 to produce carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

 energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 for themselves and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 as a waste product. But in darkness photosynthesis cannot occur, and during the resultant respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...

 small amounts of carbon dioxide are produced. Carbon dioxide also is a by-product of combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

; is emitted from volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es, hot springs
Hot Springs
Hot Springs may refer to:* Hot Springs, Arkansas** Hot Springs National Park, Arkansas*Hot Springs, California**Hot Springs, Lassen County, California**Hot Springs, Modoc County, California**Hot Springs, Placer County, California...

, and geysers; and is freed from carbonate rock
Carbonate rock
Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite and dolostone, which is composed of the mineral dolomite .Calcite can be either dissolved by groundwater or precipitated by...

s by dissolution
Dissolution
Dissolution or dissolve may refer to:* Dissolution , in law, means to end a legal entity or agreement such as a marriage, adoption, or corporation...

.

, carbon dioxide in the Earth's atmosphere
Carbon dioxide in the Earth's atmosphere
The concentration of carbon dioxide in Earth's atmosphere is approximately 392 ppm by volume and rose by 2.0 ppm/yr during 2000–2009. 40 years earlier, the rise was only 0.9 ppm/yr, showing not only increasing concentrations, but also a rapid acceleration of concentrations...

 is at a concentration of 392 ppm by volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

. Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons, driven primarily by seasonal plant growth in the Northern Hemisphere
Northern Hemisphere
The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

. Concentrations of carbon dioxide fall during the northern spring and summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant, die and decay. Taking all this into account, the concentration of CO2 grew by about 2 ppm in 2009. Carbon dioxide is a greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

 as it transmits visible light
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

 but absorbs strongly in the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 and near-infrared, before slowly re-emitting the infrared at the same wavelength as what was absorbed.

Before the advent of human-caused release of carbon dioxide to the atmosphere, concentrations tended to increase with increasing global temperatures, acting as a positive feedback
Positive feedback
Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...

 for changes induced by other processes such as orbital cycles
Milankovitch cycles
Milankovitch theory describes the collective effects of changes in the Earth's movements upon its climate, named after Serbian civil engineer and mathematician Milutin Milanković, who worked on it during First World War internment...

. There is a seasonal cycle in CO2 concentration associated primarily with the Northern Hemisphere growing season.

Carbon dioxide has no liquid state at pressures below 5.1 standard atmospheres (516,757.5 Pa). At 1 atmosphere (near mean sea level pressure), the gas deposits
Deposition (physics)
Deposition is a process in which gas transforms into solid . The reverse of deposition is sublimation.One example of deposition is the process by which, in sub-freezing air, water vapor changes directly to ice without first becoming a liquid...

 directly to a solid at temperatures below -78 C and the solid sublimes directly to a gas above −78 °C. In its solid state, carbon dioxide is commonly called dry ice
Dry ice
Dry ice, sometimes referred to as "Cardice" or as "card ice" , is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue...

.

CO2 is an acidic oxide
Acidic oxide
An acidic oxide is an oxide that either*reacts with water to form an acid; or*reacts with a base to form a salt.Examples include:...

: an aqueous solution turns litmus
Litmus test (chemistry)
Litmus is a water-soluble mixture of different dyes extracted from lichens, especially Roccella tinctoria. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. Blue litmus paper turns red under acidic conditions and red...

 from blue to pink. It is the anhydride of carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

, an acid which is unstable in aqueous solution, from which it cannot be concentrated. In organisms carbonic acid production is catalysed by the enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

, carbonic anhydrase
Carbonic anhydrase
The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...

.
+


CO2 is toxic in higher concentrations: 1% (10,000 ppm) will make some people feel drowsy. Concentrations of 7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.

Chemical and physical properties

Carbon dioxide is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor. It can cause asphyxiation and irritation. When inhaled at concentrations much higher than usual atmospheric levels, it can produce a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the mucous membranes and saliva
Saliva
Saliva , referred to in various contexts as spit, spittle, drivel, drool, or slobber, is the watery substance produced in the mouths of humans and most other animals. Saliva is a component of oral fluid. In mammals, saliva is produced in and secreted from the three pairs of major salivary glands,...

, forming a weak solution of carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

. This sensation can also occur during an attempt to stifle a burp after drinking a carbonated beverage
Carbonation
Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...

. Amounts above 5,000 ppm are considered very unhealthy, and those above about 50,000 ppm (equal to 5% by volume) are considered dangerous to animal life.

At standard temperature and pressure
Standard conditions for temperature and pressure
Standard condition for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data...

, the density of carbon dioxide is around 1.98 kg/m3, about 1.5 times that of air
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

. The carbon dioxide molecule (O=C=O) contains two double bonds
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

 and has a linear shape. It has no electrical dipole
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...

, and as it is fully oxidized
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

, it is moderately reactive
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 and is non-flammable, but will support the combustion of metals such as magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

.

At atmospheric pressure and a temperature of −78.51 °C, carbon dioxide changes directly from a solid phase to a gaseous phase through sublimation, or from gaseous to solid through deposition
Deposition (chemistry)
In chemistry, deposition is the settling of particles or sediment from a solution, suspension and mixture or vapor onto a pre-existing surface...

. Solid carbon dioxide is commonly called "dry ice
Dry ice
Dry ice, sometimes referred to as "Cardice" or as "card ice" , is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue...

", a generic trademark. It was first observed in 1825 by the French chemist Charles Thilorier
Charles Thilorier
Charles Thilorier was a scientist who gave the earliest description of dry ice, the solid form of carbon dioxide. In 1834 he opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid .-References:...

. Dry ice is commonly used as a cooling agent, and it is relatively inexpensive. A convenient property for this purpose is that solid carbon dioxide sublimes directly into the gas phase, leaving no liquid. It can often be found in grocery stores and laboratories and is also used in the shipping industry. The largest non-cooling use for dry ice is blast cleaning
Dry ice blasting
Dry ice-blasting is a form of abrasive blasting, where dry ice, the solid form of carbon dioxide, is accelerated in a pressurized air stream and directed at a surface in order to clean it....

.

Liquid carbon dioxide forms only at pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

s above 5.1 atm; the triple point
Triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium...

 of carbon dioxide is about 518 kPa
KPA
KPA may refer to:* Kenya Ports Authority* Kiln phosphoric acid, a dry process to produce phosphoric acid at high temperature in a kiln* Kilopascal , a unit of pressure* Known-plaintext attack, a method of cryptanalysis* Korean People's Army...

 at −56.6 °C (see phase diagram, above). The critical point
Critical point (thermodynamics)
In physical chemistry, thermodynamics, chemistry and condensed matter physics, a critical point, also called a critical state, specifies the conditions at which a phase boundary ceases to exist...

 is 7.38 MPa at 31.1 °C.

Another form of solid carbon dioxide, an amorphous glass-like solid, is known, although not at atmospheric pressure. This form of glass, called carbonia
Amorphous carbonia
Amorphous carbonia, also called a-carbonia or a-CO2, is an exotic amorphous solid form of carbon dioxide that is analogous to amorphous silica glass. It was first made in the laboratory in 2006 by subjecting dry ice to high pressures , in a diamond anvil...

, was produced by supercooling
Supercooling
Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid....

 heated CO2 at extreme pressure (40–48 GPa or about 400,000 atmospheres) in a diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 (silica glass) and germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....

. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure is released.

History

Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the seventeenth century, the Flemish
Flemish people
The Flemings or Flemish are the Dutch-speaking inhabitants of Belgium, where they are mostly found in the northern region of Flanders. They are one of two principal cultural-linguistic groups in Belgium, the other being the French-speaking Walloons...

 chemist Jan Baptist van Helmont
Jan Baptist van Helmont
Jan Baptist van Helmont was an early modern period Flemish chemist, physiologist, and physician. He worked during the years just after Paracelsus and iatrochemistry, and is sometimes considered to be "the founder of pneumatic chemistry"...

 observed that when he burned charcoal
Charcoal
Charcoal is the dark grey residue consisting of carbon, and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen...

 in a closed vessel, the mass of the resulting ash
Ash (analytical chemistry)
In analytical chemistry, ashing is the process of mineralization for preconcentration of trace substances prior to chemical analysis. Ash is the name given to all non-aqueous residue that remains after a sample is burned, and consist mostly of metal oxides....

 was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestre).

The properties of carbon dioxide were studied more thoroughly in the 1750s by the Scottish physician Joseph Black
Joseph Black
Joseph Black FRSE FRCPE FPSG was a Scottish physician and chemist, known for his discoveries of latent heat, specific heat, and carbon dioxide. He was professor of Medicine at University of Glasgow . James Watt, who was appointed as philosophical instrument maker at the same university...

. He found that limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 (calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

) could be heated or treated with acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

s to yield a gas he called "fixed air." He observed that the fixed air was denser than air and supported neither flame nor animal life. Black also found that when bubbled through an aqueous solution of lime (calcium hydroxide
Calcium hydroxide
Calcium hydroxide, traditionally called slaked lime, is an inorganic compound with the chemical formula Ca2. It is a colourless crystal or white powder and is obtained when calcium oxide is mixed, or "slaked" with water. It has many names including hydrated lime, builders lime, slack lime, cal, or...

), it would precipitate
Precipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...

 calcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemist Joseph Priestley
Joseph Priestley
Joseph Priestley, FRS was an 18th-century English theologian, Dissenting clergyman, natural philosopher, chemist, educator, and political theorist who published over 150 works...

 published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...

 (or oil of vitriol as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas. This was the invention of Soda water.

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy
Humphry Davy
Sir Humphry Davy, 1st Baronet FRS MRIA was a British chemist and inventor. He is probably best remembered today for his discoveries of several alkali and alkaline earth metals, as well as contributions to the discoveries of the elemental nature of chlorine and iodine...

 and Michael Faraday
Michael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....

. The earliest description of solid carbon dioxide was given by Charles Thilorier
Charles Thilorier
Charles Thilorier was a scientist who gave the earliest description of dry ice, the solid form of carbon dioxide. In 1834 he opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid .-References:...

, who in 1834 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO2.

Isolation and production

Carbon dioxide can be obtained by or from air distillation
Distillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

, however, this method is inefficient. A variety of chemical routes to carbon dioxide are known, such as the reaction between most acids and most metal carbonates. For example, the reaction between hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

 and calcium carbonate (limestone or chalk) is depicted below:
The carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

 (H2CO3) then decomposes to water and CO2. Such reactions are accompanied by foaming or bubbling, or both. In industry such reactions are widespread because they can be used to neutralize waste acid streams.

The production of quicklime (CaO), a chemical that has widespread use, from limestone by heating at about 850 °C also produces CO2:

The combustion
Combustion
Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

 of all carbon-containing fuels, such as methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 (natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

), petroleum distillates (gasoline
Gasoline
Gasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...

, diesel, kerosene
Kerosene
Kerosene, sometimes spelled kerosine in scientific and industrial usage, also known as paraffin or paraffin oil in the United Kingdom, Hong Kong, Ireland and South Africa, is a combustible hydrocarbon liquid. The name is derived from Greek keros...

, propane
Propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...

), but also of coal and wood, will yield carbon dioxide and, in most cases, water. As an example the chemical reaction between methane and oxygen is given below.
Iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 is reduced from its oxides with coke
Coke (fuel)
Coke is the solid carbonaceous material derived from destructive distillation of low-ash, low-sulfur bituminous coal. Cokes from coal are grey, hard, and porous. While coke can be formed naturally, the commonly used form is man-made.- History :...

 in a blast furnace
Blast furnace
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron.In a blast furnace, fuel and ore and flux are continuously supplied through the top of the furnace, while air is blown into the bottom of the chamber, so that the chemical reactions...

, producing pig iron
Pig iron
Pig iron is the intermediate product of smelting iron ore with a high-carbon fuel such as coke, usually with limestone as a flux. Charcoal and anthracite have also been used as fuel...

 and carbon dioxide:
Yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

 metabolizes sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

 to produce carbon dioxide and ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

, also known as alcohol, in the production of wines, beers and other spirits, but also in the production of bioethanol:


All aerobic
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...

 organisms produce when they oxidize carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s, fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...

s, and proteins in the mitochondria of cells. The large number of reactions involved are exceedingly complex and not described easily. Refer to (cellular respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...

, anaerobic respiration
Anaerobic respiration
Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen...

 and photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

). Photoautotrophs (i.e. plants, cyanobacteria) use another modus operandi: Plants absorb from the air, and, together with water, react it to form carbohydrates:
nCO2 + nO → n + n


Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO2 and (carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

). The relative concentrations of , and the deprotonated forms (bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

) and (carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

) depend on the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

. In neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater, while in very alkaline water (pH > 10.4) the predominant (>50%) form is carbonate. The bicarbonate and carbonate forms are very soluble, such that air-equilibrated ocean water (mildly alkaline with typical pH = 8.2 – 8.5) contains about 120 mg of bicarbonate per liter.

Industrial production

Industrial carbon dioxide can be produced by several methods, many of which are practiced at various scales. In its dominant route, carbon dioxide is produced as a side product of the industrial production of ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

. These processes begin with the reaction of water and natural gas (mainly methane).

Although carbon dioxide is not often recovered, carbon dioxide results from combustion of fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

s and wood
Wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...

 as well fermentation
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...

 of sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

 in the brewing
Brewing
Brewing is the production of beer through steeping a starch source in water and then fermenting with yeast. Brewing has taken place since around the 6th millennium BCE, and archeological evidence suggests that this technique was used in ancient Egypt...

 of beer
Beer
Beer is the world's most widely consumed andprobably oldest alcoholic beverage; it is the third most popular drink overall, after water and tea. It is produced by the brewing and fermentation of sugars, mainly derived from malted cereal grains, most commonly malted barley and malted wheat...

, whisky
Whisky
Whisky or whiskey is a type of distilled alcoholic beverage made from fermented grain mash. Different grains are used for different varieties, including barley, malted barley, rye, malted rye, wheat, and corn...

 and other alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

ic beverages. It also results from thermal decomposition of limestone, , in the manufacture of lime (Calcium oxide
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....

). Directly from natural carbon dioxide springs
Spring (hydrosphere)
A spring—also known as a rising or resurgence—is a component of the hydrosphere. Specifically, it is any natural situation where water flows to the surface of the earth from underground...

, where it is produced by the action of acidified water on limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 or dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....

.

Uses

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry. It is used in many consumer products that require pressurized gas because it is inexpensive and nonflammable, and because it undergoes a phase transition from gas to liquid at room temperature at an attainable pressure of approximately 60 bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

 (870 psi, 59 atm), allowing far more carbon dioxide to fit in a given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Aluminum capsules of CO2 are also sold as supplies of compressed gas for airguns
Air gun
An air gun is a rifle , pistol , or shotgun that fires projectiles by means of compressed air or other gas, in contrast to a firearm, which burns a propellant. Most air guns use metallic projectiles as ammunition. Air guns that only use plastic projectiles are classified as airsoft...

, paintball
Paintball
Paintball is a sport in which players compete, in teams or individually, to eliminate opponents by tagging them with capsules containing water soluble dye and gelatin shell outside propelled from a device called a paintball marker . Paintballs have a non-toxic, biodegradable, water soluble...

 markers, inflating bicycle tires, and for making carbonated water
Carbonated water
Carbonated water is water into which carbon dioxide gas under pressure has been dissolved, a process that causes the water to become effervescent....

. Rapid vaporization of liquid carbon dioxide is used for blasting in coal mines. High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used in supercritical drying
Supercritical drying
Supercritical drying is a process to remove liquid in a precisely controlled way. It is useful in the production of microelectromechanical systems , the drying of spices, the production of aerogel, and in the preparation of biological specimens for scanning electron microscopy.As the substance in...

 of some food products and technological materials, in the preparation of specimens for scanning electron microscopy and in the decaffeination
Decaffeination
Decaffeination is the act of removing caffeine from coffee beans, cocoa, tea leaves and other caffeine-containing materials. Despite removal of caffeine, many decaffeinated drinks still have around 1-2% of the...

 of coffee
Coffee
Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...

 beans.

Foods

Carbon dioxide is a food additive
Food additive
Food additives are substances added to food to preserve flavor or enhance its taste and appearance.Some additives have been used for centuries; for example, preserving food by pickling , salting, as with bacon, preserving sweets or using sulfur dioxide as in some wines...

 used as a propellant and acidity regulator in the food industry. It is approved for usage in the EU (listed as E number
E number
E numbers are number codes for food additives that have been assessed for use within the European Union . They are commonly found on food labels throughout the European Union. Safety assessment and approval are the responsibility of the European Food Safety Authority...

 E290), USA and Australia and New Zealand (listed by its INS number 290).

A candy called Pop Rocks
Pop Rocks
Pop Rocks is a carbonated candy with ingredients including sugar, lactose , corn syrup, and flavoring. It differs from typical hard candy in that it creates a fizzy reaction when it dissolves in one's mouth.-Background and history:...

 is pressurized with carbon dioxide gas at about 40 bar (580 psi). When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.

Leavening agent
Leavening agent
A leavening agent is any one of a number of substances used in doughs and batters that cause a foaming action which lightens and softens the finished product...

s cause dough to rise by producing carbon dioxide. Baker's yeast
Baker's yeast
Baker's yeast is the common name for the strains of yeast commonly used as a leavening agent in baking bread and bakery products, where it converts the fermentable sugars present in the dough into carbon dioxide and ethanol...

 produces carbon dioxide by fermentation of sugars within the dough, while chemical leaveners such as baking powder
Baking powder
Baking powder is a dry chemical leavening agent used to increase the volume and lighten the texture of baked goods such as muffins, cakes, scones and American-style biscuits. Baking powder works by releasing carbon dioxide gas into a batter or dough through an acid-base reaction, causing bubbles in...

 and baking soda release carbon dioxide when heated or if exposed to acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

s.

Beverages

Carbon dioxide is used to produce carbonated
Carbonation
Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...

 soft drinks and soda water. Traditionally, the carbonation in beer and sparkling wine came about through natural fermentation, but many manufacturers carbonate these drinks with carbon dioxide recovered from the fermentation process. In the case of bottled and kegged beer, recycled carbon dioxide carbonation is the most common method used. With the exception of British Real Ale, draught beer is usually transferred from kegs in a cold room or cellar to dispensing taps on the bar using pressurized carbon dioxide, sometimes mixed with nitrogen.

Wine making

Carbon dioxide in the form of dry ice
Dry ice
Dry ice, sometimes referred to as "Cardice" or as "card ice" , is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue...

 is often used in the wine making process to cool down bunches of grape
Grape
A grape is a non-climacteric fruit, specifically a berry, that grows on the perennial and deciduous woody vines of the genus Vitis. Grapes can be eaten raw or they can be used for making jam, juice, jelly, vinegar, wine, grape seed extracts, raisins, molasses and grape seed oil. Grapes are also...

s quickly after picking to help prevent spontaneous fermentation
Fermentation (wine)
The process of fermentation in wine turns grape juice into an alcoholic beverage. During fermentation, yeast interact with sugars in the juice to create ethanol, commonly known as ethyl alcohol, and carbon dioxide...

 by wild yeast
Yeast
Yeasts are eukaryotic micro-organisms classified in the kingdom Fungi, with 1,500 species currently described estimated to be only 1% of all fungal species. Most reproduce asexually by mitosis, and many do so by an asymmetric division process called budding...

s. The main advantage of using dry ice over regular water ice is that it cools the grapes without adding any additional water that may decrease the sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

 concentration in the grape must, and therefore also decrease the alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

 concentration in the finished wine.

Dry ice is also used during the cold soak phase of the wine making process to keep grapes cool. The carbon dioxide gas that results from the sublimation of the dry ice tends to settle to the bottom of tanks because it is heavier than air. The settled carbon dioxide gas creates a hypoxic environment which helps to prevent bacteria from growing on the grapes until it is time to start the fermentation with the desired strain of yeast.

Carbon dioxide is also used to create a hypoxic environment for carbonic maceration
Carbonic maceration
Carbonic maceration is a winemaking technique, often associated with the French wine region of Beaujolais, in which whole grapes are fermented in a carbon dioxide rich environment prior to crushing. Conventional alcoholic fermentation involves crushing the grapes to free the juice and pulp from the...

, the process used to produce Beaujolais
Beaujolais
Beaujolais is a French Appellation d'Origine Contrôlée wine generally made of the Gamay grape which has a thin skin and is low in tannins. Like most AOC wines they are not labeled varietally. Whites from the region, which make up only 1% of its production, are made mostly with Chardonnay grapes...

 wine.

Carbon dioxide is sometimes used to top up wine bottles or other storage vessels such as barrels to prevent oxidation, though it has the problem that it can dissolve into the wine, making a previously still wine slightly fizzy. For this reason, other gases such as nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

 or argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

 are preferred for this process by professional wine makers.

Fire extinguisher

Carbon dioxide extinguishes flames, and some fire extinguishers, especially those designed for electrical fires, contain liquid carbon dioxide under pressure. Carbon dioxide extinguishers work well on small flammable liquid and electrical fires, but not on ordinary combustible fires, as it is so dry. Carbon dioxide has also been widely used as an extinguishing agent in fixed fire protection systems for local application of specific hazards and total flooding of a protected space.International Maritime Organization standards also recognize carbon dioxide systems for fire protection of ship holds and engine rooms. Carbon dioxide based fire protection systems have been linked to several deaths, because it does not support life in the concentrations used to extinguish fire (40% or so), however, it is not considered to be toxic to humans. A review of CO2 systems identified 51 incidents between 1975 and the date of the report, causing 72 deaths and 145 injuries.

Pharmaceutical and other chemical processing

Liquid carbon dioxide is a good solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

 for many lipophilic
Lipophilic
Lipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic — the axiom that like dissolves like generally holds true...

 organic compounds
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 and is used to remove caffeine
Caffeine
Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug. Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants...

 from coffee
Coffee
Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...

. Carbon dioxide has attracted attention in the pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as organochloride
Organochloride
An organochloride, organochlorine, chlorocarbon, chlorinated hydrocarbon, or chlorinated solvent is an organic compound containing at least one covalently bonded chlorine atom. Their wide structural variety and divergent chemical properties lead to a broad range of applications...

s. It is used by some dry cleaners
Dry cleaning
Dry cleaning is any cleaning process for clothing and textiles using a chemical solvent other than water. The solvent used is typically tetrachloroethylene , abbreviated "perc" in the industry and "dry-cleaning fluid" by the public...

 for this reason. (See green chemistry
Green chemistry
Green chemistry, also called sustainable chemistry, is a philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances...

.)

Carbon dioxide is used as an ingredient in the production of urea
Urea
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....

, carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

s and bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

s, and sodium salicylate
Sodium salicylate
Sodium salicylate is a sodium salt of salicylic acid. It can be prepared from sodium phenolate and carbon dioxide under higher temperature and pressure...

. Carbon dioxide is known to react with Grignard reagents to form carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

s. In a metal carbon dioxide complex
Metal carbon dioxide complex
Metal carbon dioxide complexes are coordination complexes that contain carbon dioxide ligands. Aside from the fundamental interest in the coordination chemistry of simple molecules, studies in this field are motivated by the possibility that transition metals might catalyze useful transformations...

es, CO2 serves as a ligand, which can facilitate the conversion of CO2 to other chemicals.

Agricultural and biological applications

Plants require carbon dioxide to conduct photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. Greenhouses may (if of large size, must) enrich their atmospheres with additional CO2 to sustain and increase plant growth. A photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

-related drop (by a factor less than two) in carbon dioxide concentration in a greenhouse compartment would kill green plants, or, at least, completely stop their growth. At very high concentrations (a factor of 100 or more higher than its atmospheric concentration), carbon dioxide can be toxic to animal life, so raising the concentration to 10,000 ppm (1%) or higher for several hours will eliminate pests such as whiteflies
Whitefly
The whiteflies, comprising only the family Aleyrodidae, are small hemipterans. More than 1550 species have been described. Whiteflies typically feed on the underside of plant leaves.-Agricultural threat:...

 and spider mite
Spider mite
Spider mites are members of the Acari family Tetranychidae, which includes about 1,200 species. They generally live on the under sides of leaves of plants, where they may spin protective silk webs, and they can cause damage by puncturing the plant cells to feed...

s in a greenhouse. Carbon dioxide is used in greenhouses as the main carbon source for Spirulina
Spirulina (genus)
Spirulina is a genus of cyanobacteria. As a photosynthetic organism capable of fixing nitrogen, its non-protein nutrient values are relatively low. Spirulina is used in some foods for aquarium fish...

algae.

In medicine, up to 5% carbon dioxide (130 times the atmospheric concentration) is added to oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 for stimulation of breathing after apnea
Apnea
Apnea, apnoea, or apnœa is a term for suspension of external breathing. During apnea there is no movement of the muscles of respiration and the volume of the lungs initially remains unchanged...

 and to stabilize the balance in blood.

It has been proposed that carbon dioxide from power generation be bubbled into ponds to grow algae that could then be converted into biodiesel
Biodiesel
Biodiesel refers to a vegetable oil- or animal fat-based diesel fuel consisting of long-chain alkyl esters. Biodiesel is typically made by chemically reacting lipids with an alcohol....

 fuel.

Oil recovery

Carbon dioxide is used in enhanced oil recovery
Enhanced oil recovery
Enhanced Oil Recovery is a generic term for techniques for increasing the amount of crude oil that can be extracted from an oil field...

 where it is injected into or adjacent to producing oil wells, usually under supercritical
Supercritical fluid
A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. It can effuse through solids like a gas, and dissolve materials like a liquid...

 conditions. It acts as both a pressurizing agent and, when dissolved into the underground crude oil, significantly reduces its viscosity, enabling the oil to flow more rapidly through the earth to the removal well. In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

Refrigerant

Liquid and solid carbon dioxide are important refrigerant
Refrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...

s, especially in the food industry, where they are employed during the transportation and storage of ice cream and other frozen foods. Solid carbon dioxide is called "dry ice" and is used for small shipments where refrigeration equipment is not practical. Solid carbon dioxide is always below −78 °C at regular atmospheric pressure, regardless of the air temperature.

Liquid carbon dioxide (industry nomenclature R744 or R-744) was used as a refrigerant prior to the discovery of R-12
Dichlorodifluoromethane
Dichlorodifluoromethane , is a colorless gas, and usually sold under the brand name Freon-12, is a chlorofluorocarbon halomethane , used as a refrigerant and aerosol spray propellant. Complying with the Montreal Protocol, its manufacture was banned in the United States along with many other...

 and may enjoy a renaissance due to the fact that r134a contributes to climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

. Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to its operation at pressures of up to 130 bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

 (1880 psi
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...

), CO2 systems require highly resistant components that have already been developed for mass production in many sectors. In automobile air conditioning, in more than 90% of all driving conditions for latitudes higher than 50°, R744 operates more efficiently than systems using R-134a. Its environmental advantages (GWP
Global warming potential
Global-warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time...

 of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, hot water heat pumps, among others. Coca-Cola has fielded CO2-based beverage coolers and the U.S. Army
United States Army
The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...

 is interested in CO2 refrigeration and heating technology.

The global automobile industry is expected to decide on the next-generation refrigerant in car air conditioning. CO2 is one discussed option.(see The Cool War
The Cool War
The Cool War refers to the debate about the next-generation refrigerant in car air conditioning worldwide, with an ongoing dispute between the Alliance for CO2 Solutions supporting the uptake of sustainable carbon dioxide as a refrigerant in passenger cars, and chemical giants developing new...

)

Coal bed methane recovery

In enhanced coal bed methane recovery
Enhanced coal bed methane recovery
Enhanced coal bed methane recovery is a method of producing additional coalbed methane from a source rock, similar to enhanced oil recovery applied to oil fields. Carbon dioxide injected into a bituminous coal bed would occupy pore space and also adsorb onto the carbon in the coal at...

, carbon dioxide is pumped into the coal seam to displace methane.

Inert gas

It is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems in portable pressure tools. Carbon dioxide also finds use as an atmosphere for welding
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...

, although in the welding arc, it reacts to oxidize most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are more brittle
Brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...

 than those made in more inert atmospheres, and that such weld joints deteriorate over time because of the formation of carbonic acid. It is used as a welding gas primarily because it is much less expensive than more inert gases such as argon
Argon
Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

 or helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...

. When used for MIG welding, CO2 use is sometimes referred to as MAG welding, for Metal Active Gas, as CO2 can react at these high temperatures. It tends to produce a hotter puddle than truly inert atmospheres, improving the flow characteristics. Although, this may be due to atmospheric reactions occurring at the puddle site. This is usually the opposite of the desired effect when welding, as it tends to embrittle the site, but may not be a problem for general mild steel welding, where ultimate ductility is not a major concern.

Niche uses

Carbon dioxide is so inexpensive and so innocuous, that it finds many small uses that represent what might be called niche uses. For example it is used in the carbon dioxide laser
Carbon dioxide laser
The carbon dioxide laser was one of the earliest gas lasers to be developed , and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available...

, which is one of the earliest type of lasers.

Carbon dioxide can be used as a mean of controlling the pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 of swimming pools, by continuously adding gas to the water, thus keeping the pH level from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids. Similarly, it is also used in the maintaining reef aquaria
Reef aquarium
A reef aquarium or reef tank is an marine aquarium that prominently displays live corals and other marine invertebrates as well as fish that play a role in maintaining the coral reef environment...

, where it is commonly used in calcium reactor
Calcium reactor
In marine and reef aquariums, a calcium reactor is a device used to create a balance of alkalinity in the system. An acidic solution is produced by injecting carbon dioxide into a chamber with salt water and calcium rich media. The carbon dioxide lowers the pH by producing a solution high in...

s to temporarily lower the pH of water being passed over calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 in order to allow the calcium carbonate to dissolve into the water more freely where it is used by some corals to build their skeleton.

In the Earth's atmosphere

Carbon dioxide in earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

 is considered a trace gas
Trace gas
A trace gas is a gas which makes up less than 1% by volume of the Earth's atmosphere, and it includes all gases except nitrogen and oxygen . The most abundant trace gas at 0.934% is argon, which is being continually produced by radioactive decay of in the earth's rocks...

 currently occurring at an average concentration of about 390 parts per million by volume or 591 parts per million by mass. The total mass of atmospheric carbon dioxide is 3.16×1015 kg (about 3,000 gigatonnes). Its concentration varies seasonally (see graph at right) and also considerably on a regional basis, especially near the ground
Planetary boundary layer
The planetary boundary layer , also known as the atmospheric boundary layer , is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface forcing in an hour or less...

. In urban areas concentrations are generally higher and indoors they can reach 10 times background levels. Carbon dioxide is a greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

.

Five hundred million years ago carbon dioxide was 20 times more prevalent than today, decreasing to 4–5 times during the Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

 period and then slowly declining with a particularly swift reduction
Azolla event
The Azolla event occurred in the middle Eocene period, around , when blooms of the freshwater fern Azolla are thought to have happened in the Arctic Ocean...

 occurring 49 million years ago. Human activities such as the combustion of fossil fuels and deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....

 have caused the atmospheric concentration of carbon dioxide to increase by about 35% since the beginning of the age of industrialization
Industrial Revolution
The Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, transportation, and technology had a profound effect on the social, economic and cultural conditions of the times...

.

Up to 40% of the gas emitted by some volcano
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

es during subaerial eruption
Subaerial eruption
A subaerial eruption is a volcanic eruption that has occurred on the surface. They generally produce pyroclastic flows, lava fountains, and lava flows, which are commonly classified in different subearial eruption types, including Plinian, Peléan, and Hawaiian eruptions. Subaerial eruptions...

s is carbon dioxide. It is estimated that volcanoes release about 130–230 million tonnes (145–255 million tons) of CO2 into the atmosphere each year. Carbon dioxide is also produced by hot springs such as those at the Bossoleto site near Rapolano Terme
Rapolano Terme
Rapolano Terme is a comune in the Province of Siena in the Italian region Tuscany, located about 60 km southeast of Florence and about 20 km east of Siena in the area known as the Crete Senesi....

 in Tuscany
Tuscany
Tuscany is a region in Italy. It has an area of about 23,000 square kilometres and a population of about 3.75 million inhabitants. The regional capital is Florence ....

, Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

. Here, in a bowl-shaped depression of about 100 m diameter, local concentrations of CO2 rise to above 75% overnight, sufficient to kill insects and small animals, but it warms rapidly when sunlit and the gas is dispersed by convection during the day. Locally high concentrations of CO2, produced by disturbance of deep lake water saturated with CO2 are thought to have caused 37 fatalities at Lake Monoun
Lake Monoun
Lake Monoun is a lake in West Province, Cameroon, that lies in the Oku Volcanic Field . On August 15, 1984, the lake exploded in a limnic eruption, which resulted in the release of a large amount of carbon dioxide that killed 37 people. At first, the cause of the deaths was a mystery, and...

, Cameroon
Cameroon
Cameroon, officially the Republic of Cameroon , is a country in west Central Africa. It is bordered by Nigeria to the west; Chad to the northeast; the Central African Republic to the east; and Equatorial Guinea, Gabon, and the Republic of the Congo to the south. Cameroon's coastline lies on the...

 in 1984 and 1700 casualties at Lake Nyos
Lake Nyos
Lake Nyos is a crater lake in the Northwest Region of Cameroon, located about northwest of Yaoundé. Nyos is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity...

, Cameroon in 1986. Emissions of CO2 by human activities are currently more than 130 times greater than the quantity emitted by volcanoes, amounting to about 27 billion tonnes per year.

In the oceans

There is about fifty times as much carbon dissolved in the sea water of the oceans in the form of CO2 and carbonic acid, bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 and carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

 ions as exists in the atmosphere. The oceans act as an enormous carbon sink, and have taken up about a third of CO2 emitted by human activity. Gas solubility decreases as the temperature of water increases (except when both pressure exceeds 300 bar and temperature exceeds 393 K, only found near deep geothermal vents) and therefore the rate of uptake from the atmosphere decreases as ocean temperatures rise.

Most of the CO2 taken up by the ocean, which is about 30% of the total released into the atmosphere, forms carbonic acid in equilibrium with bicarbonate and carbonate ions. Some is consumed in photosynthesis by organisms in the water, and a small proportion of that sinks and leaves the carbon cycle. Increased CO2 in the atmosphere has led to decreasing alkalinity
Alkalinity
Alkalinity or AT measures the ability of a solution to neutralize acids to the equivalence point of carbonate or bicarbonate. The alkalinity is equal to the stoichiometric sum of the bases in solution...

 of seawater and there is concern that this may adversely affect organisms living in the water. In particular, with decreasing alkalinity, the availability of carbonates for forming shells decreases, although there's evidence of increased shell production by certain species under increased CO2 content.

NOAA states in their May 2008 "State of the science fact sheet for ocean acidification
Ocean acidification
Ocean acidification is the name given to the ongoing decrease in the pH and increase in acidity of the Earth's oceans, caused by the uptake of anthropogenic carbon dioxide from the atmosphere....

" that:

"The oceans have absorbed about 50% of the carbon dioxide (CO2) released from the burning of fossil fuels, resulting in chemical reactions that lower ocean pH. This has caused an increase in hydrogen ion (acidity) of about 30% since the start of the industrial age through a process known as “ocean acidification.” A growing number of studies have demonstrated adverse impacts on marine organisms, including:
  • The rate at which reef-building corals produce their skeletons decreases, while production of numerous varieties of jellyfish increases.
  • The ability of marine algae and free-swimming zooplankton to maintain protective shells is reduced.
  • The survival of larval marine species, including commercial fish and shellfish, is reduced."


Also, the Intergovernmental Panel on Climate Change (IPCC) writes in their Climate Change 2007: Synthesis Report:

"The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with an average decrease in pH of 0.1 units. Increasing atmospheric CO2 concentrations lead to further acidification [...] While the effects of observed ocean acidification on the marine biosphere are as yet undocumented, the progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species."

Some marine calcifying organisms (including coral reefs) have been singled out by major research agencies, including NOAA, OSPAR commission, NANOOS and the IPCC, because their most current research shows that ocean acidification should be expected to impact them negatively.

The Champagne hydrothermal vent, found at the Northwest Eifuku volcano at Marianas Trench Marine National Monument
Marianas Trench Marine National Monument
The Marianas Trench Marine National Monument is a United States National Monument created by President George W. Bush by an executive order given on January 6, 2009.-Scope:...

, produces almost pure liquid carbon dioxide, one of only two known sites in the world.

Biological role

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s with oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 as part of their metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

, in a process known as cellular respiration
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...

. This includes all plants, animals, many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues to the lungs where it is exhaled. In plants using photosynthesis, carbon dioxide is absorbed from the atmosphere.

Photosynthesis and carbon fixation

Carbon fixation
Carbon fixation
In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...

 is the removal of carbon dioxide from the air and its incorporation into solid compounds. Plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s, algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

, and many species of bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 (cyanobacteria) fix carbon and create their own food by photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. Photosynthesis uses carbon dioxide and water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 to produce sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

s and occasionally other organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s, releasing oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 as a waste product.

Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 involved in the first major step of carbon fixation
Carbon fixation
In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...

, a process by which atmospheric carbon dioxide is converted by plants to energy-rich
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s such as glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

. It is also thought to be the single most abundant protein on Earth.

These phototroph
Phototroph
Phototrophs are the organisms that carry out photosynthesis to acquire energy. They use the energy from sunlight to convert carbon dioxide and water into organic material to be utilized in cellular functions such as biosynthesis and respiration.Most phototrophs are autotrophs, also known as...

s use the products of their photosynthesis as internal food sources and as raw material for the construction of more complex organic molecules, such as polysaccharides, nucleic acid
Nucleic acid
Nucleic acids are biological molecules essential for life, and include DNA and RNA . Together with proteins, nucleic acids make up the most important macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information...

s and protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s. These are used for their own growth, and also as the basis for the food chain
Food chain
A food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...

s and webs whereby other organisms, including animals such as ourselves, are fed. Some important phototrophs, the coccolithophore
Coccolithophore
Coccolithophores are single-celled algae, protists, and phytoplankton belonging to the division of haptophytes. They are distinguished by special calcium carbonate plates of uncertain function called coccoliths , which are important microfossils...

s synthesise hard calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 scales. A globally significant species of coccolithophore is Emiliania huxleyi
Emiliania huxleyi
Emiliania huxleyi, often abbreviated "EHUX", is a species of coccolithophore with a global distribution from the tropics to subarctic waters. It is one of thousands of different photosynthetic plankton that freely drift in the euphotic zone of the ocean, forming the basis of virtually all marine...

whose calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

 scales have formed the basis of many sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s such as limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

, where what was previously atmospheric carbon can remain fixed for geological timescales.

Plants can grow up to 50 percent faster in concentrations of 1,000 ppm CO2 when compared with ambient conditions, though this assumes no change in climate and no limitation on other nutrients. Research has shown that elevated CO2 levels cause increased growth reflected in the harvestable yield of crops, with wheat, rice and soybean all showing increases in yield of 12–14% under elevated CO2 in FACE experiments.

Studies have shown that increased CO2 leads to fewer stomata developing on plants which leads to reduced water usage. Studies using FACE
Free-Air Concentration Enrichment
Free-Air Carbon dioxide Enrichment is a method used by ecologists and plant biologists that raises the concentration of in a specified area and allows the response of plant growth to be measured...

 have shown that increases in CO2 lead to decreased concentration of micronutrients in crop plants. This may have knock-on effects on other parts of ecosystem
Ecosystem
An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s as herbivores will need to eat more food to gain the same amount of protein.

Plants also emit CO2 during respiration, and so the majority of plants and algae, which use C3 photosynthesis, are only net absorbers during the day. Though a growing forest will absorb many tons of CO2 each year, the World Bank writes that a mature forest will produce as much CO2 from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in biosynthesis
Biosynthesis
Biosynthesis is an enzyme-catalyzed process in cells of living organisms by which substrates are converted to more complex products. The biosynthesis process often consists of several enzymatic steps in which the product of one step is used as substrate in the following step...

 in growing plants. However six experts in biochemistry, biogeology, forestry and related areas writing in the science journal Nature that "Our results demonstrate that old-growth forests can continue to accumulate carbon, contrary to the long-standing view that they are carbon neutral." Mature forests are valuable carbon sinks, helping maintain balance in the Earth's atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere.

Toxicity

Carbon dioxide content in fresh air (averaged between sea-level and 10 kPa level, i.e., about 30 km altitude) varies between 0.036% (360 ppm) and 0.039% (390 ppm), depending on the location.

Prolonged exposure to moderate concentrations can cause acidosis and adverse effects on calcium phosphorus metabolism resulting in increased calcium deposits in soft tissue. Carbon dioxide is toxic to the heart and causes diminished contractile force.

Toxicity and its effects increase with the concentration of CO2, here given in volume percent of CO2 in the air:
  • 1% can cause drowsiness with prolonged exposure.
  • At 2% it is mildly narcotic and causes increased blood pressure and pulse rate, and causes reduced hearing.
  • At about 5% it causes stimulation of the respiratory center, dizziness, confusion and difficulty in breathing accompanied by headache and shortness of breath. Panic attacks may also occur at this concentration.
  • At about 8% it causes headache, sweating, dim vision, tremor and loss of consciousness after exposure for between five and ten minutes.


Due to the health risks associated with carbon dioxide exposure, the U.S. Occupational Safety and Health Administration says that average exposure for healthy adults during an eight-hour work day should not exceed 5,000 ppm (0.5%). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues is significantly less. For short-term (under ten minutes) exposure, the U.S. National Institute for Occupational Safety and Health (NIOSH) and American Conference of Government Industrial Hygienists (ACGIH) limit is 30,000 ppm (3%). NIOSH also states that carbon dioxide concentrations exceeding 4% are immediately dangerous to life and health although physiological experiments show that such levels can be tolerated for some time.

Adaptation to increased levels of CO2 occurs in humans. Continuous inhalation of CO2 can be tolerated at three percent inspired concentrations for at least one month and four percent inspired concentrations for over a week. It was suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a submarine
Submarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...

) since the adaptation is physiological and reversible. Decrement in performance or in normal physical activity does not happen at this level. However, it should be noted that submarines have carbon dioxide scrubber
Carbon dioxide scrubber
A carbon dioxide scrubber is a device which absorbs carbon dioxide . It is used to treat exhaust gases from industrial plants or from exhaled air in life support systems such as rebreathers or in spacecraft, submersible craft or airtight chambers...

s which reduce a significant amount of the CO2 present.

These figures are valid for pure carbon dioxide. In indoor spaces occupied by people the carbon dioxide concentration will reach higher levels than in pure outdoor air. Concentrations higher than 1,000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing CO2 concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by CO2 itself. At 2,000 ppm the majority of occupants will feel a significant degree of discomfort, and many will develop nausea and headaches. The CO2 concentration between 300 and 2,500 ppm is used as an indicator of indoor air quality.

Acute carbon dioxide toxicity is sometimes known by the names given to it by miners: blackdamp
Blackdamp
Blackdamp is an asphyxiant, reducing the available oxygen content of air to a level incapable of sustaining human or animal life. It is not a single gas but a mixture of unbreathable gasses left after oxygen is removed from the air and typically consists of nitrogen, argon, carbon dioxide and...

 (also called choke damp or stythe). Blackdamp is primarily nitrogen and carbon dioxide and kills via suffocation (having displaced oxygen). Miners would try to alert themselves to dangerous levels of blackdamp and other gasses in a mine shaft by bringing a caged canary with them as they worked. The canary is more sensitive to environmental gasses than humans and as it became unconscious would stop singing and fall off its perch. The Davy lamp
Davy lamp
The Davy lamp is a safety lamp with a wick and oil vessel burning originally a heavy vegetable oil, devised in 1815 by Sir Humphry Davy. It was created for use in coal mines, allowing deep seams to be mined despite the presence of methane and other flammable gases, called firedamp or minedamp.Sir...

 could also detect high levels of blackdamp (which collect near the floor) by burning less brightly, while methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

, another suffocating gas and explosion risk would make the lamp burn more brightly).

Carbon dioxide differential above outdoor levels at steady state conditions (when the occupancy and ventilation system operation are sufficiently long that CO2 concentration has stabilized) are sometimes used to estimate ventilation rates per person. CO2 is considered to be a surrogate for human bio-effluents and may correlate with other indoor pollutants. Higher CO2 concentrations are associated with occupant health, comfort and performance degradation. ASHRAE Standard 62.1–2007 ventilation rates may result in indoor levels up to 2,100 ppm above ambient outdoor conditions. Thus if the outdoor ambient is 400 ppm, indoor levels may reach 2,500 ppm with ventilation rates that meet this industry consensus standard. Levels in poorly ventilated spaces can be found even higher than this (range of 3,000 or 4,000). [Mendell and Shendell references]

Content

The body produces approximately 2.3 pounds (1 kg) of carbon dioxide per day per person.

In humans, this carbon dioxide is carried through the venous system and is breathed out through the lungs. Therefore, the carbon dioxide content in the body is high in the venous system, and decreases in the respiratory system
Respiratory system
The respiratory system is the anatomical system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles...

, resulting in lower levels along any arterial system. Carbon dioxide content in this sense is often given as the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....

, which is the pressure which carbon dioxide would have had if it alone occupied the volume.

In humans, the carbon dioxide contents are as follows:
Reference range
Reference range
In health-related fields, a reference range or reference interval usually describes the variations of a measurement or value in healthy individuals...

s or averages for partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....

s of carbon dioxide (abbreviated PCO2)
Unit Venous
Vein
In the circulatory system, veins are blood vessels that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart...

 blood gas
Alveolar pulmonary
gas pressures
Pulmonary gas pressures
The factors that determine the values for alveolar PO2 and PCO2 are:*The pressure of outside air*The partial pressures of inspired oxygen and carbon dioxide*The rates of total body oxygen consumption and carbon dioxide production...

 !! Arterial blood carbon dioxide
kPa  5.5-6.8 4.8 4.7-6.0
mmHg  41-51 36 35-45

Transport in the blood

CO2 is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood).
  • Most of it (about 70% to 80%) is converted to bicarbonate
    Bicarbonate
    In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

     ions by the enzyme carbonic anhydrase
    Carbonic anhydrase
    The carbonic anhydrases form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons , a reversible reaction that occurs rather slowly in the absence of a catalyst...

     in the red blood cells, by the reaction CO2 + H2O → H2CO3 → H+ + .
  • 5% – 10% is dissolved in the plasma
    Blood plasma
    Blood plasma is the straw-colored liquid component of blood in which the blood cells in whole blood are normally suspended. It makes up about 55% of the total blood volume. It is the intravascular fluid part of extracellular fluid...

  • 5% – 10% is bound to hemoglobin
    Hemoglobin
    Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

     as carbamino
    Carbamino
    Carbamino refers to a compound composed by the addition of carbon dioxide with a free amino group in an amino acid or a protein, such as hemoglobin forming carbaminohemoglobin....

     compounds


Hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

, the main oxygen-carrying molecule in red blood cell
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...

s, carries both oxygen and carbon dioxide. However, the CO2 bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric
Allosteric regulation
In biochemistry, allosteric regulation is the regulation of an enzyme or other protein by binding an effector molecule at the protein's allosteric site . Effectors that enhance the protein's activity are referred to as allosteric activators, whereas those that decrease the protein's activity are...

 effects on the hemoglobin molecule, the binding of CO2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreased binding to carbon dioxide in the blood due to increased oxygen levels is known as the Haldane Effect
Haldane effect
The Haldane effect is a property of hemoglobin first described by the Scottish physician John Scott Haldane. Deoxygenation of the blood increases its ability to carry carbon dioxide; this property is the Haldane effect. Conversely, oxygenated blood has a reduced capacity for carbon...

, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO2 or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr Effect
Bohr effect
Bohr effect is a property of hemoglobin first described in 1904 by the Danish physiologist Christian Bohr , which states that an increasing concentration of protons and/or carbon dioxide will reduce the oxygen affinity of hemoglobin...

.

Regulation of respiration

Carbon dioxide is one of the mediators of local autoregulation
Autoregulation
Autoregulation is a process within many biological systems, resulting from some internal adaptive mechanism that works to adjust the systems response to stimuli. While most systems of the body show some degree of autoregulation, it is most clearly observed in the kidney, the heart, and the brain...

 of blood supply. If its levels are high, the capillaries expand to allow a greater blood flow to that tissue.

Bicarbonate ions are crucial for regulating blood pH. A person's breathing rate influences the level of CO2 in their blood. Breathing that is too slow or shallow causes respiratory acidosis
Respiratory acidosis
Respiratory acidosis is a medical condition in which decreased ventilation causes increased blood carbon dioxide concentration and decreased pH ....

, while breathing that is too rapid leads to hyperventilation
Hyperventilation
Hyperventilation or overbreathing is the state of breathing faster or deeper than normal, causing excessive expulsion of circulating carbon dioxide. It can result from a psychological state such as a panic attack, from a physiological condition such as metabolic acidosis, can be brought about by...

, which can cause respiratory alkalosis
Alkalosis
Alkalosis refers to a condition reducing hydrogen ion concentration of arterial blood plasma . Generally, alkalosis is said to occur when pH of the blood exceeds 7.45. The opposite condition is acidosis .-Causes:...

.

Although the body requires oxygen for metabolism, low oxygen levels normally do not stimulate breathing. Rather, breathing is stimulated by higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (such as pure nitrogen) can lead to loss of consciousness without ever experiencing air hunger. This is especially perilous for high-altitude fighter pilots. It is also why flight attendants instruct passengers, in case of loss of cabin pressure, to apply the oxygen mask to themselves first before helping others; otherwise, one risks losing consciousness.

The respiratory centers try to maintain an arterial CO2 pressure of 40 mm Hg. With intentional hyperventilation, the CO2 content of arterial blood may be lowered to 10–20 mm Hg (the oxygen content of the blood is little affected), and the respiratory drive is diminished. This is why one can hold one's breath longer after hyperventilating than without hyperventilating. This carries the risk that unconsciousness may result before the need to breathe becomes overwhelming, which is why hyperventilation is particularly dangerous before free diving.

See also

  • Bosch reaction
    Bosch reaction
    The Bosch reaction is a chemical reaction between carbon dioxide and hydrogen that produces elemental carbon , water and a 10% return of invested heat...

  • Bottled gas
    Bottled gas
    Bottled gas is a term used for substances which are gaseous at standard temperature and pressure and have been compressed and stored in carbon steel, stainless steel, aluminum, or composite bottles known as gas cylinders.-Gas state in cylinders:...

  • Carbogen
    Carbogen
    Carbogen, also called Meduna's Mixture after its inventor Ladislas Meduna, is a mixture of carbon dioxide and oxygen gas. Meduna's original formula was 30% CO2 and 70% oxygen, but the term carbogen can refer to any mixture of these two gases, from 1.5% to 50% CO2.- Mechanism :When carbogen is...

  • Carbon cycle
    Carbon cycle
    The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

  • Carbon dioxide (data page)
    Carbon dioxide (data page)
    - Material Safety Data Sheet : The handling of this chemical may incur notable safety precautions. It is highly recommended that you seek the Material Safety Datasheet for this chemical from a reliable source such as , and follow its directions...

  • Carbon dioxide sensor
  • Carbon dioxide sink
    Carbon dioxide sink
    A carbon sink is a natural or artificial reservoir that accumulates and stores some carbon-containing chemical compound for an indefinite period. The process by which carbon sinks remove carbon dioxide from the atmosphere is known as carbon sequestration...

  • Carbon monoxide
    Carbon monoxide
    Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

  • CO2 sequestration
  • CO2 degassing in Lake Nyos
    Lake Nyos
    Lake Nyos is a crater lake in the Northwest Region of Cameroon, located about northwest of Yaoundé. Nyos is a deep lake high on the flank of an inactive volcano in the Oku volcanic plain along the Cameroon line of volcanic activity...

  • Dry ice
    Dry ice
    Dry ice, sometimes referred to as "Cardice" or as "card ice" , is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue...

  • EcoCute
    EcoCute
    The EcoCute is an energy efficient electric heat pump, water heating and supply system that uses heat extracted from the air to heat water for domestic, industrial and commercial use. Instead of the more conventional ammonia or haloalkane gases, EcoCute uses supercritical carbon dioxide as a...

     – As refrigerants
  • Emission standard
    Emission standard
    Emission standards are requirements that set specific limits to the amount of pollutants that can be released into the environment. Many emissions standards focus on regulating pollutants released by automobiles and other powered vehicles but they can also regulate emissions from industry, power...

    s
  • Global warming
    Global warming
    Global warming refers to the rising average temperature of Earth's atmosphere and oceans and its projected continuation. In the last 100 years, Earth's average surface temperature increased by about with about two thirds of the increase occurring over just the last three decades...

  • Greenhouse gas
    Greenhouse gas
    A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

  • Industrial gas
    Industrial gas
    Industrial gas is a group of gases that are commercially manufactured and sold for uses in other applications. These gases are mainly used in an industrial processes, such as steelmaking, oil refining, medical applications, fertilizer, semiconductors, etc.,...

  • Kaya identity
    Kaya identity
    The Kaya identity is an equation relating factors that determine the level of human impact on climate, in the form of emissions of the greenhouse gas carbon dioxide. It states that total emission level can be expressed as the product of four inputs: population, GDP per capita, energy use per unit...

  • Ocean acidification
    Ocean acidification
    Ocean acidification is the name given to the ongoing decrease in the pH and increase in acidity of the Earth's oceans, caused by the uptake of anthropogenic carbon dioxide from the atmosphere....

  • List of least carbon efficient power stations


Further reading

  • Tyler Volk
    Tyler Volk
    Tyler Volk is an Associate Professor in the Department of Biology at New York University. Volk is an active proponent of the Gaia hypothesis. A 1989 study, co-authored by Volk, published in the journal Nature asserts that without the cooling effects of living things, Earth would be 80 degrees...

     (2008), CO2 Rising: The World's Greatest Environmental Challenge, The MIT Press, 223 pages, ISBN 978-0-262-22083-5. A short, balanced primer on CO2's role as a greenhouse gas
    Greenhouse gas
    A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...

    . Review at Environmental Health Perspectives
    Environmental Health Perspectives
    Environmental Health Perspectives is a peer-reviewed open-access medical journal published monthly by the National Institute of Environmental Health Sciences. According to the Journal Citation Reports, the journal has a 2009 impact factor of 6.191....

  • Shendell, Prill, Fisk, Apte1, Blake & Faulkner, Associations between classroom CO2 concentrations and student attendance in Washington and Idaho, Indoor Air 2004.
  • Seppanen, Fisk and Mendell, Association of Ventilation Rates and CO2 Concentrations with Health and Other Responses in Commercial and Institutional Buildings, Indoor Air 1999.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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