Which one of the following is used to express the dose of a chemical that will kill 50% of the test organisms?

• A. LD50
• B. LC50
• C. LD95
• D. LC80

Answer: (B)

The key idea is the distinction between dose and concentration. LD50 is the amount of chemical taken in per unit body weight that kills 50% of the test organisms. It answers “how much substance did the organism receive per kilogram of body weight to reach 50% mortality?” and is typically expressed as mg of chemical per kg of body weight.

LC50, by contrast, refers to the concentration of the chemical in the environment (air, water, etc.) that causes 50% mortality, not the dose the organism receives. So LC50 is about environmental concentration, while LD50 is about the actual dose.

LD95 and LC80 describe different thresholds (95% lethal dose and 80% lethal concentration) and don’t match the “50%” criterion asked for in the question.

So when the question asks for the measure that expresses the dose killing 50% of organisms, the term used is LD50.