What does LD50 refer to in toxicology?

• A. The dose required to kill 100 percent of test animals
• B. The dose required to kill 50 percent of test animals
• C. The concentration required to kill 50 percent
• D. The time to kill 50 percent

Answer: (B)

LD50 is the median lethal dose—the amount of a substance that will kill 50% of a defined test population under specified conditions. It’s a dose, typically expressed as milligrams of substance per kilogram of body weight, and it depends on the route of exposure (oral, dermal, inhalation) and the species tested, with a standard observation period after dosing. This measure helps compare acute toxicity: a substance with a lower LD50 is more acutely toxic because a smaller amount is enough to cause death in half the population. It does not refer to the time to death, nor to exposure concentration, and it is not the dose required to kill all individuals (LD100).