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 inhalation or respiration. As a toxicity comparison, LC50 values have no directly correlation from one specie to the other or to humans. LC50 measurement is micrograms or milligrams of material per liter, or parts per million (ppm), of air or water. The lower the amount, the more toxic the material. Also known as median lethal concentration or population critical concentration 50. Also seen as LC50.