Standard measure of the toxicity of the surrounding medium that will kill half of the sample population of a specific test-animal in a specified period through exposure via inhalation (respiration). LC50 is measured in micrograms (or milligrams) of the material per liter, or parts per million (ppm), of air or water; lower the amount, more toxic the material. Used in the comparison of toxicities, LC50 values cannot be directly extrapolated from one specie to the other or to humans. Also called median lethal concentration or population critical concentration 50. Written also as LC50.