Non-cancer health effects from exposure to ionizing radiation

Human health effects due to exposure to ionizing radiation are usually divided into two categories: (a) Deterministic effects, i.e. effects which occur shortly after acute exposure to high doses above certain thresholds. Deterministic effects do not occur as a result of prolonged exposure to low doses. (b) Stochastic effects, i.e. an increase in the cancer incidence and mortality rates, assumed to have no dose threshold. The latency periods of stochastic effects range from a few years to decades. Our knowledge about stochastic effects is based primarily on the epidemiological studies on the Hiroshima and Nagasaki A-bombs survivors - the "Life Span Study" (LSS).

However, recent findings of the same research program show a significant increase in the total mortality from non-cancer diseases in the exposed population: between 1950-2003, 353 excess non-cancer deaths cases were found, with an excess relative risk (ERR) of 0.14/Gy. For comparison, during the same period, the number of excess cases of fatal solid cancers was 527 (ERR = 0.47/Gy).

Moreover, combined results of the LSS and the "Adult Health Study" (AHS - a subgroup of the LSS cohort) show excess morbidity and mortality rates of specific non-cancer diseases: among those that show a statistically significant increase are cardiovascular diseases, stroke, digestive diseases, respiratory diseases and some non-malignant tumors. Furthermore, for some of the effects, a significant dose-response association was found. Those findings are noteworthy, as some of those diseases are common among the general population, and even a slight increase in their ERR correspond to a considerable increase in the general morbidity. Some of those findings are supported by other epidemiological studies as well.

These non-cancer health effects (herein - NCHE) cannot be considered as either stochastic or deterministic. While it was previously thought that all non-cancer effects are deterministic, and occur shortly after acute exposure to high doses (several Gy), it appears that the NCHE demonstrate a similar pattern to stochastic effects: they may appear a long time after cumulative exposure to considerable lower doses than was previously thought. For example, the current estimated threshold for cardiovascular diseases is 0.5 Gy (or even less), with a latency period of many years. The term "tissue reaction" was proposed by the ICRP to encompass both the recently detected NCHE and the well-known deterministic effects.

To summarize, the discovery of the NCHE seems to requires a fundamental modification of the concept of ionizing radiation interaction with biological tissue, and might require some revisions of the radiation protection system. For example, a British HPA report from 2010 suggests that retrospective health assessments of individuals exposed to 0.5 Gy or more should evaluate also the possible excess risk of cardiovascular disease.