Lethal dose of a toxic substance or radiation 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. LD₅₀ are expressed in mg or Bq per kg of species (animal, human), and can correspond to different type of contamination (inhalation, oral or dermal). European Commission (directive 2001/59/EC) proposed a classification into 4 categories.

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