International Commission for Protection Against Environmental Mutagens and Carcinogens. Dosimetry of genotoxic agents and dose-response relationships of their effects.

Dose-response relationships and determination of dose of mutagens and carcinogens are summarized and discussed on the basis of conceptual and kinetic aspects. Different dose definitions may be referred to steps in the chain of events from exposure (or emission) to observed effects. A system is applied to show the influence of various processes on the kinetics of the transfers between consecutive steps. The same system illustrates processes influenced by protraction and fractionation of dose, synergists, comutagens/cocarcinogens, heritable factors, etc. The response at a given dose is expected to depend on the product of consecutive transfer functions. An application of general rules of chemical kinetics shows that when a chemical is introduced at a sufficiently low level, all processes affecting the transfers and therefore the transfer functions themselves become first-order, provided the induction status of enzymes and the cell-division rate remain constant. Under the same conditions, dose-response relationships are expected to be linear, i.e. without "safe" thresholds. However, present knowledge of the kinetics of repair at low levels of DNA damage and of the kinetics of induction of repair functions is not enough complete to be decisive. These considerations and the fact that observed dose-response data in some cases indicate the existence of thresholds but in others appear able to reject the threshold hypothesis lead to the conclusion that, generally, dose-response curves are most probably linear down to dose zero. However, certain mutagens/carcinogens give rise to lesions repaired so effectively that quasi-thresholds appear in certain subpopulations or organs.