LC50 is defined as

• A. The amount of a chemical that kills 50% of the animals in a test population
• B. A measure of chronic toxicity after long-term exposure
• C. The time it takes for 50% of organisms to die after exposure
• D. The concentration of a substance needed to kill 50% of the organisms within a specified period of time

Answer: (D)

LC50 is a measure of acute toxicity expressed as the concentration of a substance in the exposure medium that kills 50% of the test organisms within a defined period. This makes it a concentration-based metric tied to a specific exposure duration, not the total amount administered. It also isn’t about the time to death (that would be LT50) or about chronic, long-term effects. In practice, LC50 values are reported for chosen test time points (such as 24, 48, or 96 hours) to compare how toxic different substances are to the organisms tested.