Indoor bioaerosol
Encyclopedia
Bioaerosols are natural or artificial particle
s of biological (microbial, plant, or animal) origin suspended in the air. These particles are also referred to as organic dust. Bioaerosols may consist of bacteria
, fungi
(and spore
s and cell fragments of fungi), virus
es, microbial toxins
, pollen
, plant fibers, etc. Size of bioaerosol particles varies from below 1 µm to 100 µm in aerodynamic diameter; viable bioaerosol particles can be suspended in air as single cells or aggregates of microorganism
as small as 1-10 µm in size. Since bioaerosols are potentially related to various human health effects and the indoor environment provides a unique exposure situation, concerns about indoor bioaerosols have increased over the last decade.
s may originate from outdoor air and indoor reservoirs. Although outdoor bioaerosols cannot easily migrate into large buildings with complex ventilation systems
, certain categories of outdoor bioaerosols (i.e., fungal spores) do serve as major sources for indoor bioaerosols in naturally ventilated buildings at specific periods of time (i.e., growing seasons for fungi).
Major indoor sources for bioaerosols at residential homes include human occupants, pet
s, house dust, organic waste, as well as the heating, ventilation and air-conditioning (HVAC
) system. Several studies have identified human activities as an important source for indoor bioaerosols. Human bodies can generate bioaerosols directly through activities like talking, sneezing, and coughing, while other residential activities (i.e., washing, flushing toilet, sweeping floor) can generate bioaerosols indirectly. Since microorganisms can accumulate and grow on dust
particles, house dust is a potential source of bioaerosols. In a study by Wouters et al., they investigated the effects of indoor storage of organic household waste on microbial contamination among 99 households in the Netherlands in the summer of 1997, and indicated that “increased microbial contaminant levels in homes are associated with indoor storage of separated organic waste”, which might elevate “the risk of bioaerosol-related respiratory symptoms in susceptible people”. However, the analysis by Wouters et al. was based on the collected samples of settled house dust, which might not serve as a strong indicator for bioaerosols suspended in the air. Other materials in residential buildings, such as food stuffs, house plants, textile
s, wood material and furniture stuffing can also become bioaerosol sources when water content is appropriate for microorganisms to grow.
For non-residential buildings, some specific indoor environments, such as hospital
s, wastewater treatment
plants, compost
ing facilities, certain biotechnical laboratories, have been revealed to have bioaerosol sources related to their particular environmental characteristics.
, characteristics of air ventilation systems, seasonal variation, temperature, and chemical composition of the air. Other factors, such as the type of home, building material
, geographical factors, do not seem to have significant impacts on respirable fungi and bacteria (important constituents of bioaerosols). Relative humidity is one of the most widely studied influencing factors for indoor bioaerosols. Concentrations of two categories of bioaerosols, endotoxin and airborne fungi, are both positively related to indoor relative humidity (higher concentration associated with higher relative humidity). Relative humidity also affects the infectivity
of airborne viruses. Regarding the characterization of air ventilation system, increased use of central air conditioning is found to be associated with lower fungal bioaerosol concentration.
, Legionnaires' disease and different forms of bacterial pneumonia
, coccidioidomycosis
, influenza
, measles
, and gastrointestinal illness. Bioaerosols are also associated with some noninfectious airway diseases, such as allergies and asthma. As a known component of indoor bioaerosol, β(1→3)-glucan (cell wall components of most fungi) is proposed to be the causative agent of mold
-induced nonallergic inflammatory reactions. It is reported that 25%-30% of allergenic asthma
cases in industrialized countries are induced by fungi, which has been the focus of concerns about human exposure to airborne microorganisms in recent years.
Some other human diseases and symptoms have been proposed to be associated with indoor bioaerosol, but no deterministic conclusions could be drawn due to the insufficiency of evidence. One example is the well-know sick building syndrome
(SBS). SBS refers to non-specific complaints, such as upper-respiratory irritative symptoms, headache
s, fatigue, and rash
, which cannot be related to an identifiable cause but are building related. Over the last two decades, there have been many studies indicating association of indoor bioaerosol with sick building syndrome. However, most of the related studies based their conclusions on statistical correlation between concentrations of certain types of bioaerosols and incidence of complaints, which has various drawbacks methodologically. For example, some studies have a small sample size, which critically undermines the validity of speculations based on the statistical results. Also, many studies were not able to exclude the influences of other factors beside bioaerosol in their analysis, which makes the statistical correlation theoretically inappropriate to support association of SBS with bioaerosols. Additional studies revealed that bioaerosol is unlikely to be the cause of SBS. Recent epidemiological and toxicological studies continued to suggest a possible link between bioaerosol exposure and sick building syndrome, but methodological limitations remained in these studies.
The ability of bioaerosols to cause human disease depend not only on their chemical composition and biological characteristics, but also on the quantity of bioaerosol inhaled and their size distribution, which determines the site of bioaerosol deposition to human respiratory system
s. Bioaerosols larger than 10 µm in aerodynamic diameter are generally blocked by the nasal region of the respiratory tract
, those between 5-10 µm mainly deposit in the upper respiratory system and usually induce symptoms like allergic rhinitis
, and particles with aerodynamic diameter less than 5 µm can reach the alveoli and hence lead to serious illnesses such as allergic alveolitis
.
Because of the confirmed and potential adverse health effects associated with indoor bioaerosol, some concentration limits for total number of bioaerosol particles are recommended by different agencies and organizations as follow: 1000 CFUs/m3 (National Institute of Occupational Safety and Health (NIOSH)), 1000 CFUs/m3 (American Conference of Governmental Industrial Hygienists
(ACGIH)) with the culturable count for total bacteria not exceeding 500 CFUs/m3. Note that for certain types of indoor bioaerosols, it might be hard to establish a specific concentration limit or acceptance level, due to the differences on sampling and analysis method.
, bioaerosols need to be captured from the air first. Different air sampling techniques have been used to realize the goal of capturing indoor bioaerosols.. Important characteristics of bioaerosol sampling include: representativeness of sampling, sampler performance, and compatibility with subsequent analysis. Long-term sampler theoretically has a better representiveness of sampling than short-term sampler, but may not have a good temporary resolution. Performance of samplers (i.e., limit of detection and upper limit of range) has a significant impact on the reliability of results. Different characterizations of samplers can also limit the possibilities for further analysis (identification and quantification). Major bioaerosol sampler types and their possible subsequent analysis are summarized in Table 1. A frequently used sampler in previous studies is the Andersen impactor.
Certain limitations exist for commonly used bioaerosol samplers. For most of the samplers, nonbiological environmental particles such as dust must be separated from bioaerosols prior to detection. The diluted nature of bioaerosol in the air also poses challenges to samplers. While total microorganism concentrations are on the order of 106/cm3 or greater, bioaerosol concentrations are commonly less than 1/cm3, and often less than 1/m3 in the case of infectious aerosols. Moreover, many commercially available bioaerosol samplers haven not been investigated on their collection efficiencies for particles with different aerodynamic diameters, which makes it impossible to get the size-resolved bioaerosol information.
Major culture-independent identification/quantification methods adopted in previous bioaerosol studies include polymerase chain reaction (PCR), quantitative polymerase chain reaction (qPCR), microarray (PhyloChip), fluorescent in situ hybridization (FISH), flow cytometry and solid-phase cytometry, immunoassay (i.e., enzyme-linked immunosorbent assay (ELISA)). The well-known PCR is a powerful tool in identifying and even quantifying the biological origin of bioaerosols. PCR alone cannot accomplish all the tasks related to bioaerosol detection; instead it usually serves as the preparation tool for subsequent processes like DNA sequencing, microarray, and community fingerprinting techniques. A typical procedure for PCR-based bioaerosol analysis is shown in Figure 1.
Molecular biological methods for bioaerosol are significantly faster and more sensitive than conventional culture-based methods, and they are also able to reveal a larger diversity of microbes. Targeting the variation in the 16S rRNA gene, a microarray (PhyloChip) was used to conduct comprehensive identification of both bacterial and archaeal organisms in bioaerosols. New U.S. EPA methods have been developed to utilize qPCR to characterize indoor environment for fungal spores. In a study by Lange et al., FISH method successfully identified eubacteria in samples of complex native bioaerosols in swine barns. Nonetheless, molecular biological tools have limitations. Since PCR methods target DNA, viability of cells could not be confirmed in some cases. When qPCR technique is used for bioaerosol detection, standard curves need to be developed to calibrate final results. One study indicated that “curves used for quantification by qPCR needs to be prepared using the same environmental matrix and procedures as handling of the environmental sample in question” and that “reliance on the standard curves generated with cultured bacterial suspension (a traditional approach) may lead to substantial underestimation of microorganism quantities in environmental samples”. Microarray techniques also face the challenge of natural sequence diversity and potential cross-hybridization in complex environmental bioaerosols).
Particle
A particle is, generally, a small localized object to which can be ascribed physical properties. It may also refer to:In chemistry:* Colloidal particle, part of a one-phase system of two or more components where the particles aren't individually visible.In physics:* Subatomic particle, which may be...
s of biological (microbial, plant, or animal) origin suspended in the air. These particles are also referred to as organic dust. Bioaerosols may consist 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...
, fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...
(and spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s and cell fragments of fungi), virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...
es, microbial toxins
Microbial toxins
Microbial toxins are toxins produced by microorganisms, including bacteria, viruses and fungi. Microbial toxins are important virulence determinants responsible for microbial pathogenicity and/or evasion of the host immune response. Some bacterial toxins, such as Botulinum neurotoxins, are the most...
, pollen
Pollen
Pollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...
, plant fibers, etc. Size of bioaerosol particles varies from below 1 µm to 100 µm in aerodynamic diameter; viable bioaerosol particles can be suspended in air as single cells or aggregates of microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
as small as 1-10 µm in size. Since bioaerosols are potentially related to various human health effects and the indoor environment provides a unique exposure situation, concerns about indoor bioaerosols have increased over the last decade.
Sources for indoor environments
Indoor bioaerosolBioaerosol
A bioaerosol is a suspension of airborne particles that contain living organisms or were released from living organisms. These particles are very small and range in size from less than one micrometer to one hundred micrometers . Bioaerosols react to air currents and move quickly or slowly...
s may originate from outdoor air and indoor reservoirs. Although outdoor bioaerosols cannot easily migrate into large buildings with complex ventilation systems
Ventilation (architecture)
Ventilating is the process of "changing" or replacing air in any space to provide high indoor air quality...
, certain categories of outdoor bioaerosols (i.e., fungal spores) do serve as major sources for indoor bioaerosols in naturally ventilated buildings at specific periods of time (i.e., growing seasons for fungi).
Major indoor sources for bioaerosols at residential homes include human occupants, pet
Pet
A pet is a household animal kept for companionship and a person's enjoyment, as opposed to wild animals or to livestock, laboratory animals, working animals or sport animals, which are kept for economic or productive reasons. The most popular pets are noted for their loyal or playful...
s, house dust, organic waste, as well as the heating, ventilation and air-conditioning (HVAC
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
) system. Several studies have identified human activities as an important source for indoor bioaerosols. Human bodies can generate bioaerosols directly through activities like talking, sneezing, and coughing, while other residential activities (i.e., washing, flushing toilet, sweeping floor) can generate bioaerosols indirectly. Since microorganisms can accumulate and grow on dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
particles, house dust is a potential source of bioaerosols. In a study by Wouters et al., they investigated the effects of indoor storage of organic household waste on microbial contamination among 99 households in the Netherlands in the summer of 1997, and indicated that “increased microbial contaminant levels in homes are associated with indoor storage of separated organic waste”, which might elevate “the risk of bioaerosol-related respiratory symptoms in susceptible people”. However, the analysis by Wouters et al. was based on the collected samples of settled house dust, which might not serve as a strong indicator for bioaerosols suspended in the air. Other materials in residential buildings, such as food stuffs, house plants, textile
Textile
A textile or cloth is a flexible woven material consisting of a network of natural or artificial fibres often referred to as thread or yarn. Yarn is produced by spinning raw fibres of wool, flax, cotton, or other material to produce long strands...
s, wood material and furniture stuffing can also become bioaerosol sources when water content is appropriate for microorganisms to grow.
For non-residential buildings, some specific indoor environments, such as hospital
Hospital
A hospital is a health care institution providing patient treatment by specialized staff and equipment. Hospitals often, but not always, provide for inpatient care or longer-term patient stays....
s, wastewater treatment
Wastewater Treatment
Wastewater treatment may refer to:* Sewage treatment* Industrial wastewater treatment...
plants, compost
Compost
Compost is organic matter that has been decomposed and recycled as a fertilizer and soil amendment. Compost is a key ingredient in organic farming. At its most essential, the process of composting requires simply piling up waste outdoors and waiting for the materials to break down from anywhere...
ing facilities, certain biotechnical laboratories, have been revealed to have bioaerosol sources related to their particular environmental characteristics.
Factors influencing indoor bioaerosol generation
According to previous studies, major indoor environmental factors influencing bioaerosol concentration include relative humidityHumidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
, characteristics of air ventilation systems, seasonal variation, temperature, and chemical composition of the air. Other factors, such as the type of home, building material
Building material
Building material is any material which is used for a construction purpose. Many naturally occurring substances, such as clay, sand, wood and rocks, even twigs and leaves have been used to construct buildings. Apart from naturally occurring materials, many man-made products are in use, some more...
, geographical factors, do not seem to have significant impacts on respirable fungi and bacteria (important constituents of bioaerosols). Relative humidity is one of the most widely studied influencing factors for indoor bioaerosols. Concentrations of two categories of bioaerosols, endotoxin and airborne fungi, are both positively related to indoor relative humidity (higher concentration associated with higher relative humidity). Relative humidity also affects the infectivity
Infectivity
In epidemiology, infectivity refers to the ability of a pathogen to establish an infection. More specifically, infectivity is a pathogen's capacity for horizontal transmission that is, how frequently it spreads among hosts that are not in a parent-child relationship...
of airborne viruses. Regarding the characterization of air ventilation system, increased use of central air conditioning is found to be associated with lower fungal bioaerosol concentration.
Human Health Effects
Adverse health effects/diseases related to indoor bioaerosol exposure can be divided into two categories: those confirmed to be associated with bioaerosol and those suspected but not confirmed to be associated with bioaerosol. Bioaerosols have been revealed to cause certain human diseases, such as tuberculosisTuberculosis
Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...
, Legionnaires' disease and different forms of bacterial pneumonia
Bacterial pneumonia
Bacterial pneumonia is a type of pneumonia caused by bacterial infection.-Sign and symptoms:*Fever*Rigors*Cough*Dyspnea*Chest pain*Pneumococcal pneumonia can cause Hemoptysis-Gram positive:...
, coccidioidomycosis
Coccidioidomycosis
Coccidioidomycosis is a fungal disease caused by Coccidioides immitis or C. posadasii. It is endemic in certain parts of Arizona, California, Nevada, New Mexico, Texas, Utah and northwestern Mexico.C...
, influenza
Influenza
Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...
, measles
Measles
Measles, also known as rubeola or morbilli, is an infection of the respiratory system caused by a virus, specifically a paramyxovirus of the genus Morbillivirus. Morbilliviruses, like other paramyxoviruses, are enveloped, single-stranded, negative-sense RNA viruses...
, and gastrointestinal illness. Bioaerosols are also associated with some noninfectious airway diseases, such as allergies and asthma. As a known component of indoor bioaerosol, β(1→3)-glucan (cell wall components of most fungi) is proposed to be the causative agent of mold
Mold
Molds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...
-induced nonallergic inflammatory reactions. It is reported that 25%-30% of allergenic asthma
Asthma
Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...
cases in industrialized countries are induced by fungi, which has been the focus of concerns about human exposure to airborne microorganisms in recent years.
Some other human diseases and symptoms have been proposed to be associated with indoor bioaerosol, but no deterministic conclusions could be drawn due to the insufficiency of evidence. One example is the well-know sick building syndrome
Sick building syndrome
Sick building syndrome is a combination of ailments associated with an individual's place of work or residence. A 1984 World Health Organization report into the syndrome suggested up to 30% of new and remodeled buildings worldwide may be linked to symptoms of SBS...
(SBS). SBS refers to non-specific complaints, such as upper-respiratory irritative symptoms, headache
Headache
A headache or cephalalgia is pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. The brain tissue itself is not sensitive to pain because it lacks pain receptors. Rather, the pain is caused by disturbance of the...
s, fatigue, and rash
Rash
A rash is a change of the skin which affects its color, appearance or texture. A rash may be localized in one part of the body, or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, chapped, dry, cracked or blistered, swell and may be painful. The causes, and...
, which cannot be related to an identifiable cause but are building related. Over the last two decades, there have been many studies indicating association of indoor bioaerosol with sick building syndrome. However, most of the related studies based their conclusions on statistical correlation between concentrations of certain types of bioaerosols and incidence of complaints, which has various drawbacks methodologically. For example, some studies have a small sample size, which critically undermines the validity of speculations based on the statistical results. Also, many studies were not able to exclude the influences of other factors beside bioaerosol in their analysis, which makes the statistical correlation theoretically inappropriate to support association of SBS with bioaerosols. Additional studies revealed that bioaerosol is unlikely to be the cause of SBS. Recent epidemiological and toxicological studies continued to suggest a possible link between bioaerosol exposure and sick building syndrome, but methodological limitations remained in these studies.
The ability of bioaerosols to cause human disease depend not only on their chemical composition and biological characteristics, but also on the quantity of bioaerosol inhaled and their size distribution, which determines the site of bioaerosol deposition to human 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...
s. Bioaerosols larger than 10 µm in aerodynamic diameter are generally blocked by the nasal region of the respiratory tract
Respiratory tract
In humans the respiratory tract is the part of the anatomy involved with the process of respiration.The respiratory tract is divided into 3 segments:*Upper respiratory tract: nose and nasal passages, paranasal sinuses, and throat or pharynx...
, those between 5-10 µm mainly deposit in the upper respiratory system and usually induce symptoms like allergic rhinitis
Rhinitis
Rhinitis , commonly known as a stuffy nose, is the medical term describing irritation and inflammation of some internal areas of the nose. The primary symptom of rhinitis is nasal dripping. It is caused by chronic or acute inflammation of the mucous membrane of the nose due to viruses, bacteria or...
, and particles with aerodynamic diameter less than 5 µm can reach the alveoli and hence lead to serious illnesses such as allergic alveolitis
Alveolitis
Alveolitis can refer to two inflammatory conditions. It can refer to inflammation of the alveoli in the lungs, or the dental alveolus in the jaw.Alveolus in the jaw is also known as dry socket...
.
Because of the confirmed and potential adverse health effects associated with indoor bioaerosol, some concentration limits for total number of bioaerosol particles are recommended by different agencies and organizations as follow: 1000 CFUs/m3 (National Institute of Occupational Safety and Health (NIOSH)), 1000 CFUs/m3 (American Conference of Governmental Industrial Hygienists
American Conference of Governmental Industrial Hygienists
The American Conference of Governmental Industrial Hygienists is a professional association of industrial hygienists and practitioners of related professions, with headquarters in Cincinnati, Ohio...
(ACGIH)) with the culturable count for total bacteria not exceeding 500 CFUs/m3. Note that for certain types of indoor bioaerosols, it might be hard to establish a specific concentration limit or acceptance level, due to the differences on sampling and analysis method.
Bioaerosol sampling techniques
To enable subsequent identification and quantificationQuantification
Quantification has several distinct senses. In mathematics and empirical science, it is the act of counting and measuring that maps human sense observations and experiences into members of some set of numbers. Quantification in this sense is fundamental to the scientific method.In logic,...
, bioaerosols need to be captured from the air first. Different air sampling techniques have been used to realize the goal of capturing indoor bioaerosols.. Important characteristics of bioaerosol sampling include: representativeness of sampling, sampler performance, and compatibility with subsequent analysis. Long-term sampler theoretically has a better representiveness of sampling than short-term sampler, but may not have a good temporary resolution. Performance of samplers (i.e., limit of detection and upper limit of range) has a significant impact on the reliability of results. Different characterizations of samplers can also limit the possibilities for further analysis (identification and quantification). Major bioaerosol sampler types and their possible subsequent analysis are summarized in Table 1. A frequently used sampler in previous studies is the Andersen impactor.
| Sampler | Example of Device | Possible Subsequent Analysis |
|---|---|---|
| Impactors and Sieve Samplers | Anderson impactor; SAS; Burkard sampler | Cultivation; Microscopic analysis |
| Impingers | AGI-30; Shipe sampler; Midget, multi-stage and micro-impingers | Cultivation; Microscopic analysis; Biochemical analysis; Immunoassays |
| Centrifugal Samplers | RCS; Aerojet cyclone | Cultivation; Microscopic analysis; Biochemical analysis; Immunoassays |
| Filter Cassette | Glass fiber; Teflon filters; Polycarbonate | Cultivation; Microscopic analysis; Biochemical analysis; Immunoassays |
Certain limitations exist for commonly used bioaerosol samplers. For most of the samplers, nonbiological environmental particles such as dust must be separated from bioaerosols prior to detection. The diluted nature of bioaerosol in the air also poses challenges to samplers. While total microorganism concentrations are on the order of 106/cm3 or greater, bioaerosol concentrations are commonly less than 1/cm3, and often less than 1/m3 in the case of infectious aerosols. Moreover, many commercially available bioaerosol samplers haven not been investigated on their collection efficiencies for particles with different aerodynamic diameters, which makes it impossible to get the size-resolved bioaerosol information.
Identification/quantification methods
In previous research on indoor bioaerosol in residential environments, microorganisms have been quantified by conventional culture-based techniques, in which colony forming units (CFU) on selective media are counted. Cultivating methods have several disadvantages. Culture-based methods are known to underestimate environmental microbial diversity, based on the fact that only a small percentage of microbes can be cultivated in the laboratory. This underestimation is likely to be signified for the quantification of bioaerosol, since colony counts of airborne microbes are typically quite different from direct counts. Culture-based methods also need relatively long incubation times (over 24 hours) and are labor intensive. Consequently, culture-based methods are no longer suitable for effective and rapid identification and quantification of bioaerosol, and non-culture based methods, such as immunoassays, molecular biological tests, and optical, and electrical methods, have been developing over the past few decades.Major culture-independent identification/quantification methods adopted in previous bioaerosol studies include polymerase chain reaction (PCR), quantitative polymerase chain reaction (qPCR), microarray (PhyloChip), fluorescent in situ hybridization (FISH), flow cytometry and solid-phase cytometry, immunoassay (i.e., enzyme-linked immunosorbent assay (ELISA)). The well-known PCR is a powerful tool in identifying and even quantifying the biological origin of bioaerosols. PCR alone cannot accomplish all the tasks related to bioaerosol detection; instead it usually serves as the preparation tool for subsequent processes like DNA sequencing, microarray, and community fingerprinting techniques. A typical procedure for PCR-based bioaerosol analysis is shown in Figure 1.
Molecular biological methods for bioaerosol are significantly faster and more sensitive than conventional culture-based methods, and they are also able to reveal a larger diversity of microbes. Targeting the variation in the 16S rRNA gene, a microarray (PhyloChip) was used to conduct comprehensive identification of both bacterial and archaeal organisms in bioaerosols. New U.S. EPA methods have been developed to utilize qPCR to characterize indoor environment for fungal spores. In a study by Lange et al., FISH method successfully identified eubacteria in samples of complex native bioaerosols in swine barns. Nonetheless, molecular biological tools have limitations. Since PCR methods target DNA, viability of cells could not be confirmed in some cases. When qPCR technique is used for bioaerosol detection, standard curves need to be developed to calibrate final results. One study indicated that “curves used for quantification by qPCR needs to be prepared using the same environmental matrix and procedures as handling of the environmental sample in question” and that “reliance on the standard curves generated with cultured bacterial suspension (a traditional approach) may lead to substantial underestimation of microorganism quantities in environmental samples”. Microarray techniques also face the challenge of natural sequence diversity and potential cross-hybridization in complex environmental bioaerosols).
Concentration Levels in Different Geographical Regions
Concentration levels of indoor bioaerosols in different regions of the world recorded in published literatures are summarized as Table 2.| Geographical Region | Study Period | Sampling/Survey Size | Average Concentration Level (CFU/m3) | Major Microbes Present | References |
|---|---|---|---|---|---|
| Midwestern area, USA | April-September, 1991 | 27 (noncomplaint homes) | Viable bacteria: 970; Culturable fungi: 1200. | N/A | |
| Taipei area, Taiwan | July 1996 | 40 daycare centers (DC), 69 office buildings (OB), 22 homes (H) | Bacteria: 7651(DC), 1502(OB), 2907(H); Fungi: 854(DC), 195(OB), 695(H). | N/A | |
| 25 states of USA | 1994-1998 | 100 large office buildings | Total bacteria (average): 101.9; Total bacteria (90th percentile): 175. | Mesophilic bacteris | |
| Upper Silesia, Poland | 1996-1998 | 70 dewellings | Bacterial aerosol in homes: 1000; Bacterial aerosol in offices: 100. | Micrococcus spp; Staphylococcus epidermidis | |
| The city of Boston, USA | May 1997-May 1998 | 21 offices | Fungi: 42.05 (Standard deviation=69.60) | N/A | |
| Hong Kong, China | About 1 week | 2 offices | Highest bacteria concentration: 2912; Highest fungi concentration: 3852. | Cladosporium; Penicillium | |
| The city of Daegu, Republic of Korea | June 2003-August 2004 | 41 bars, 41 internet cafes, 44 classrooms, 20 homes | Total bacteria and total fungi: 10-1000. | N/A |