Which of the following best describes the relationship between LD50 value and toxicity?

• A. A lower LD50 value indicates higher toxicity.
• B. A lower LD50 value indicates lower toxicity.
• C. A lower LD50 value indicates more chemical mass needed to kill.
• D. A lower LD50 value has no relation to toxicity.

Answer: (A)

Toxicity is about how much of a substance is needed to cause harm, and LD50 is the dose that will kill 50% of a test population. A smaller LD50 means only a small amount is required to reach that lethal dose, so the substance is more toxic on a per-mass basis. That’s why the statement that a lower LD50 value indicates higher toxicity is the best description. If the LD50 is larger, you’d need more of the chemical to reach the lethal effect, which means lower toxicity. The ideas that a lower LD50 indicates lower toxicity, or that it means more mass is needed to kill, don’t fit this relationship. And LD50 is a direct measure of acute toxicity, so it does relate to how toxic a substance is.