What does a lower LD-50 imply about a substance's toxicity?

• A. The substance is less toxic.
• B. The substance is more toxic.
• C. The substance has a wider safety margin.
• D. The substance is non-toxic.

Answer: (B)

LD-50 is the amount of a substance required to kill 50% of the test population. When that amount is small, it means the chemical is highly potent at causing death, so it’s more toxic. For example, a substance with an LD-50 of 5 mg/kg needs only a tiny dose to reach lethal effects, whereas one with 500 mg/kg would require a much larger dose to be deadly, indicating lower toxicity. A higher LD-50 means a larger dose is needed, which corresponds to a wider safety margin. Non-toxic substances are typically associated with very high or undefined LD-50 values, since they don’t cause death within the tested range.