Standard measure of surrounding medium toxicity. Half of the sample population (50%) of a specific test-animal in a specified period die from exposure via ingestion, skin contact, or injection.. As a toxicity comparison, LD50 values have no directly correlation from one specie to the other or to humans. LD50 measurement is micrograms or milligrams of material per kilogram of the test animal's body weight. The lower the amount, the more toxic the material. Also known as median lethal dose 50. Also seen as LD50.