What does a high LD-50 indicate about a substance's toxicity?

• A. The substance is less toxic because it takes a larger dose to be deadly.
• B. The substance is more toxic because a small dose can be deadly.
• C. LD-50 measures the dose that kills all test animals.
• D. LD-50 has no relation to toxicity.

Answer: (A)

LD50 is the amount of a substance needed to kill 50% of a test population, so it measures how potent a toxin is in acute exposure. A high LD50 means you have to give a lot of the substance before half the organisms die, which means the substance is less toxic for short-term exposure. Conversely, a low LD50 means only a small amount is enough to cause death in half the population, indicating higher acute toxicity. So the statement that the substance is less toxic because a larger dose is required is the best description.

The other ideas don’t fit because a small dose causing death would indicate high toxicity, not low; LD50 is not the dose that kills everyone (that would be LD100); and LD50 is indeed related to toxicity as a standard measure of acute toxicity.