Lethal dose
A lethal dose is an indication of the lethality
Lethality is a term designating the ability of a weapon to kill. Most often this term is used when referring to chemical weapons, biological weapons, or their chemical components. The use of this term denotes the ability of these weapons to kill, but also the possibility that they may not kill...

 of a given substance or type of radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

. Because resistance varies from one individual to another, the 'lethal dose' represents a dose (usually recorded as dose per kilogram of subject body weight) at which a given percentage of subjects will die. The LD may be based on the standard person
Standard person
The standard person or reference person is a theoretical individual that has perfectly "normal" characteristics. This model is used for much research into radiation safety. For many years, the standard person was called reference man because the work assumed a healthy, young adult male...

 concept, a theoretical individual that has perfectly "normal" characteristics, and thus not apply to all sub-populations.

LD values for humans are best estimated by extrapolating results from human cell culture
Cell culture
Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...

s. One outdated form of extrapolation involves measuring LD on animals like mice or dogs, converting to dosage per kilogram of biomass, and extrapolating to human norms. The degree of error from animal-extrapolated LD values is very large. The biology of test animals differs in important aspects to that of humans. For instance, mouse tissue is approximately fifty times less responsive than human tissue to the venom of the Sydney funnel-web spider
Sydney funnel-web spider
The Sydney funnel-web spider, Atrax robustus, is an Australian funnel-web spider usually found within a radius of Sydney, New South Wales, Australia.-Description:...

. The square-cube law
Square-cube law
The square-cube law is a principle, drawn from the mathematics of proportion, that is applied in engineering and biomechanics. It was first demonstrated in 1638 in Galileo's Two New Sciences...

also complicates the scaling relationships involved.
Researchers are now shifting away from animal-based LD measurements. The U.S. Food and Drug Administration has begun to approve more reliable non-animal methods in response to research cruelty concerns and the lack of validity/sensitivity of animal tests as they relate to humans.
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