Which of the following means there is no safe level of radiation and the response to the radiation is directly proportional to the dose received?

• A. Nonlinear-nonthreshold
• B. Nonlinear-threshold
• C. Linear-threshold
• D. Linear-nonthreshold

Answer: (D)

The main idea here is how risk from radiation scales with dose. The linear-nonthreshold model says there is no safe level of radiation—any amount carries some risk—and that risk increases in direct proportion to the dose. In other words, twice the dose means roughly twice the risk, and the risk-versus-dose line passes through the origin (zero dose, zero risk). This contrasts with a threshold model, which would imply no harm below a certain dose, or a nonlinear model, where the risk doesn't increase in a straight 1:1 slope. The linear-nonthreshold view is the standard for estimating cancer risk and guiding protection measures because it reflects the idea that no dose is completely risk-free and that protection should minimize exposure as much as reasonably achievable.