What does LD50 mean?

• A. Lethal dose that is lethal to 50% of a group of test animals after a single dose
• B. Lethal dose that kills 50% after multiple dosing
• C. Lethal concentration affecting 50% of population after exposure
• D. Lethal dose observed in humans after one year

Answer: (A)

LD50 means the median lethal dose: the amount of a substance that, when given as a single exposure, is expected to cause death in 50% of a defined group of test animals within a standard observation period. This single-dose measure captures acute toxicity and is used to compare how toxic different substances are—the smaller the LD50, the more toxic the substance. It is a dose, not a concentration, and it reflects a one-time exposure rather than effects from multiple dosing or long-term use. The other descriptions don’t fit LD50: multiple dosing refers to cumulative effects, lethal concentration describes a concentration rather than a dose, and long-term human observation isn’t how LD50 is determined.