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

• A. The smaller the LD50, the more toxic the substance
• B. The larger the LD50, the more toxic the substance
• C. LD50 reflects environmental persistence
• D. LD50 is unrelated to toxicity

Answer: (A)

Acute toxicity is inversely related to LD50. LD50 is the dose that kills 50% of a test population, usually expressed per unit body weight. If only a small amount is enough to reach that lethal threshold, the substance is highly toxic because minimal exposure can be fatal. If a substance requires a much larger dose to reach that threshold, it is less toxic acutely. Note that LD50 measures short-term, lethal effects under specific test conditions and exposure routes; it does not capture environmental persistence or long-term or non-lethal effects. So the statement that a smaller LD50 means a more toxic substance best describes the relationship.