A measure of the toxicity of a chemical substance or ionizing radiation that leads to death. It is usually expressed as the amount of chemical or ionizing radiation per unit body weight. The median lethal dose denoted as LD₅₀ represents the dose that results in the death of half (50 per cent) of the exposed animals within a population within a defined period of time such as within a two-week period. The subscript represents the statistically expected level of mortality. For example, LD₉₉ represents a lethal dose leading to the death of 99 per cent of those exposed. The lethal concentration is the concentration of an airborne chemical substance over a four-hour period that causes the death of exposed test animals. Where half of test animals in a population die, the lethal concentration is denoted as LC₅₀.