A lethal dose (LD) is an indication of the lethality of a given substance or type of radiation. 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 concept, a theoretical individual that has perfectly "normal" characteristics, and thus not apply to all sub-populations.
Lethal doses are usually expressed as median lethal dose (LD50), the point where 50% of test subjects exposed would die, in the units of mg/kg body weight. For gases and aerosols, lethal concentration (mg/m³ or ppm, parts per million) is the analogous concept, although this also depends on the duration of exposure, which has to be included in the definition. The lowest known lethal dose, derived from an individual case of poisoning, is abbreviated LCLo.
LD values for humans are best estimated by extrapolating results from human cell cultures. One outdated form of extrapolation involves measuring LD on animals like mice or dogs, converting to dosage per kilogram of biomass, and extrapolating to human norms.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. The square-cube law 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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