In risk assessment, a threshold model implies what about effects at low doses?

• A. Risk occurs only at high doses, with no risk at very low doses.
• B. The risk is proportional to dose with no threshold.
• C. No effect below a certain dose.
• D. The relationship is random and unpredictable.

Answer: (C)

A threshold model means there is a dose level below which no adverse effects occur because the body's repair and defense systems can cope with the exposure. Only when exposure surpasses that threshold do adverse effects begin to appear, and they typically increase with higher doses.

So at low doses, you would expect no effect, which is captured by the statement that there is no effect below a certain dose. This contrasts with, for example, a linear no-threshold view where any amount of exposure carries some risk, or notions of randomness where the relationship isn’t predictable.