High Levels of Natural Radiation: Radiation Doses and Health Effects

Abstract

Limits on exposure to artificial radiation have been pushed down over the past few decades, while at the same time we have become more aware of the potential for high doses from natural sources. If these two trends were graphed at the same scale they would have crossed years ago, leaving us now with significant numbers of people receiving natural radiation doses that dwarf the exposures of classified radiation workers, and which are distributed to all without favour, whether young, old or pregnant. The principal sources are gamma rays from the ground and building materials, internal irradiation from ingested radioactivity and lung doses from radon decay products. Unless you are an astronaut, doses from cosmic rays never reach the same heights. This book contains the proceedings of the fourth conference on high levels of natural radiation, held in China in 1996. An interesting overview is provided by Sohrabi, who proposes a four-level classification scheme for high natural radiation areas, depending on the annual effective dose. Under this scheme, areas would be classified as low (<5 mSv), medium (5-20 mSv), high (20-50 mSv) and very high (>50 mSv). He identifies three areas that are in the high category because of external and internal radiation: Ramsar in Iran, Lake Miri in Sudan and the Chavara-Neendakara coastal belt in Kerala, India. Radon takes four areas into the very high category: Umhausen in the Austrian Tyrol, Tuwa in India and two metal mining areas of the former East Germany. Doses from radon decay products can reach several sieverts in a year in some houses in Germany. It is necessary to emphasise that these are not organ doses, but effective doses. An important question arises from the recognition of these high background areas: what effects do they have on the human population? This question was considered at three sessions of the conference, on epidemiology, radiation hormesis and adaptive response and genetic effects. In epidemiology, radiation hormesis and adaptive response and genetic effects. In epidemiology, little new was contributed. Radon has already been clearly shown to be a cause of lung cancer in miners, and the conference was held before the publication of a meta-analysis of eight case-control studies of radon in the home which produced results in line with miner studies. Most of the epidemiological studies reported in this volume are area correlation studies on gamma-ray exposures, and show nothing significant. Papers on biology produced some interesting results. Chinese and Japanese studies show increasing numbers of dicentric and centric ring chromosome aberrations with age in high radiation areas, but not low areas. On the hormesis side, there is a report suggesting an increased immunity to virus infection in people in a high background area compared with those in a low one. The implication of these results for human communities is unclear. Nevertheless, the existence of areas with such high natural radiation levels poses a problem for the consistent application of radiological protection principles. Jon Miles The above review was first published in the NRPB Bulletin and is reproduced with the permission of the publishers.