Incorporation of background in dose-response models.

Dose-response models are generally classified as either threshold-tolerance or mechanism models. These models are briefly reviewed and their impact upon low-dose estimation is discussed. The concept of independent versus additive backgrounds is explored. The mathematical implications are considered with the conclusion that additive background implies linearity and that the assumption of total independence of background is required to justify nonlinearity at low doses. This finding has important implications to low-dose extrapolations for the purpose of risk estimation.