LD50 refers to which toxicity measure?

• A. Lethal dose for 50% of test animals
• B. Lethal concentration for 50% of test animals
• C. Dose that kills all of a group of test animals
• D. Dose that produces nonlethal effects in 50% of animals

Answer: (A)

LD50 is the median lethal dose—the amount of substance needed to kill 50% of the test animals under specific conditions. It’s typically expressed as a dose per body weight (for example, mg of substance per kg of body weight) and is derived from a dose–response curve that shows mortality at different doses. This measure reflects how potent a substance is in causing acute toxicity: a lower LD50 means greater potency because a smaller dose can be fatal to half the population. It depends on the route of exposure (oral, dermal, inhalation) and other factors like species, sex, and age, so different LD50 values can exist for the same chemical. It’s not a lethal concentration in air or water (that would be LC50), and it doesn’t describe nonlethal effects.