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In toxicology, the lethal dose (LD) is an indication of the lethal toxicity 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.
Median lethal dose (LD50)
The median lethal dose, LD50 (abbreviation for "lethal dose, 50%"), LC50 (lethal concentration, 50%) or LCt50 (lethal concentration and time) of a toxin, radiation, or pathogen is the dose required to kill half the members of a tested population after a specified test duration. LD50 figures are frequently used as a general indicator of a substance's acute toxicity. A lower LD50 is indicative of increased toxicity.
Lowest lethal dose
The lowest lethal dose (LDLo) is the least amount of drug that can produce death in a given animal species under controlled conditions.
Median lethal concentration
For gases and aerosols, lethal concentration (given in 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 term incipient lethal level is used to describe a LC50 value that is independent of time.
Lowest lethal concentration
The LCLo is the lowest concentration of a chemical, given over a period of time, that results in the fatality of an individual animal. LCLo is typically for an acute (<24 hour) exposure.
The lethal dose is different from organism to another. A compound/material that is extremely toxic to one organism, can be harmless to another (paracetamol e.g.).