Threshold is defined as the smallest dose capable of eliciting a response.

• A. The largest dose with no response.
• B. The average dose causing response in half the population.
• C. The smallest dose capable of eliciting a response.
• D. A dose always causing a response.

Answer: (C)

Understanding thresholds in dose-response terms: the threshold is the point at which a first detectable effect appears. Below this dose, there’s no observable response; once you cross the threshold, responses begin and typically increase with higher doses. This concept helps distinguish exposure that has no effect from exposure that starts to produce measurable effects, which is essential for setting safe exposure limits. It’s different from the ED50, which is the dose that produces a response in 50% of the population, and from a dose that always causes a response, which describes consistency rather than the onset of effect.