Classification of harmful radiation-induced effects on human health for radiological protection purposes: history and concepts

The classification of harmful radiation-induced effects into 'stochastic' and 'deterministic' or 'cancer/heritable effects' and 'tissue reactions' has played a significant role in the recommendations of the International Commission on Radiological Protection (ICRP) over a number of decades. More recently, however, there has been a debate regarding the adequacy of the current scheme for classification of health effects with current scientific evidence. While these scientific aspects are discussed in greater detail in a parallel article, we will emphasise the practical importance of the classification itself here. The setting of dose limits is quite different for 'deterministic' and 'stochastic' effects. In the first case, provided reliable data regarding the threshold doses for radiation effects on a particular tissue are available, the dose limit is supposed to avoid harm altogether. With 'stochastic effects', the expected risks at the dose limit are supposed to be 'tolerable'. Below the dose limit, optimisation must be aimed for based on the 'prudent' assumption that the stochastic risks linearly depend on the dose and that there is no threshold. If the distinction between the two kinds of harmful effects is drawn into question, possible consequences for the system of radiological protection need to be considered, especially in the context of its aims as they currently apply, namely, 'to manage and control exposures to ionising radiation so that deterministic effects are prevented, and the risks of stochastic effects are reduced to the extent reasonably achievable'. It may be necessary, moreover, to discuss if and how the terminology used by ICRP (and others), in particular the terms 'deterministic effects' and 'tissue reactions', should continue to be used for the classification of radiation effects.