Lethal dose
A
lethal dose 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 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, the point where 50% of test subjects exposed would die, in the units of mg/kg body weight. For gases and aerosols,
lethal concentration 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 One outdated form of extrapolation involves measuring LD on animals like mice or dogs, converting to dosage per kilogram of biomass, and extrapolating to human norms.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.