What does LD-50 represent in toxicology?

• A. The dose required to kill 50% of a test population.
• B. The dose that causes 50% of population to experience symptoms.
• C. The dose that kills all test subjects.
• D. The dose that is safe for 50% of population.

Answer: (A)

LD-50, or median lethal dose, is the amount of a chemical that would kill 50% of a defined test population under specified conditions. It’s a standard measure of acute toxicity used to compare how potent different substances are. The value is typically expressed as milligrams of substance per kilogram of body weight (mg/kg) and depends on factors like the route of exposure (oral, dermal, inhalation), the species tested, age, and health. A lower LD-50 means the substance is more toxic because a smaller dose causes death in half the population. LD-50 comes from dose–response data analyzed statistically to estimate lethality, not the presence or absence of symptoms alone. It doesn’t imply safety for any portion of humans or guarantee that more than 50% would die; it’s a comparative, probabilistic measure rooted in animal testing, and modern toxicology often uses alternative metrics for ethical and practical reasons.