Which statement best describes how LD50 values relate to toxicity when comparing two substances under identical test conditions?

• A. The substance with the higher LD50 is more poisonous.
• B. The substance with the lower LD50 is more poisonous.
• C. LD50 values can be used to compare acute toxicity for the same exposure route.
• D. LD50 values do not reveal anything about toxicity.

Answer: (C)

LD50 is a measure of acute toxicity that indicates the dose needed to kill 50% of test animals, and it only makes sense to compare it when every factor is held the same—same species, same exposure route, and the same testing conditions. A lower LD50 means the substance is more toxic because a smaller amount is required to reach that lethal threshold. So, when two substances are tested under identical conditions, you can rank their acute toxicity by comparing their LD50 values. The best statement is that LD50 values can be used to compare acute toxicity for the same exposure route.

Keep in mind this reflects only acute toxicity for a specific route of exposure. It doesn’t tell you about chronic effects, carcinogenicity, or toxicity via other routes, and LD50 values can vary with species and other test conditions. For example, if one substance has an LD50 of 50 mg/kg and another has 200 mg/kg under the same test setup, the first is more acutely toxic for that route.