What is toxicity?

• A. Harm from too much of a substance
• B. The benefit of a substance at a safe level
• C. Harm from too little of a substance
• D. A measure of heat stress

Answer: (A)

Toxicity means the harm a substance can cause when exposure is too high or prolonged. Every substance has a level at which it starts to become toxic; at low levels it may be harmless or even beneficial, but as the dose increases, harmful effects appear. This dose–response idea explains why something essential in small amounts can be toxic in large amounts. Toxicity is about damage to the body, not about benefits or adequate levels away from harm.

That’s why stating harm from too much of a substance is the best answer: it captures the core idea that excessive exposure leads to adverse effects. The notion of benefit at a safe level describes safety or positive effects, not toxicity; harm from too little describes deficiency rather than toxicity; and a measure of heat stress is unrelated to toxic effects.