Human exposure to high natural background radiation: what can it teach us about radiation risks?

Natural radiation is the major source of human exposure to ionising radiation, and its largest contributing component to effective dose arises from inhalation of (222)Rn and its radioactive progeny. However, despite extensive knowledge of radiation risks gained through epidemiologic investigations and mechanistic considerations, the health effects of chronic low-level radiation exposure are still poorly understood. The present paper reviews the possible contribution of studies of populations living in high natural background radiation (HNBR) areas (Guarapari, Brazil; Kerala, India; Ramsar, Iran; Yangjiang, China), including radon-prone areas, to low dose risk estimation. Much of the direct information about risk related to HNBR comes from case-control studies of radon and lung cancer, which provide convincing evidence of an association between long-term protracted radiation exposures in the general population and disease incidence. The success of these studies is mainly due to the careful organ dose reconstruction (with relatively high doses to the lung), and to the fact that large-scale collaborative studies have been conducted to maximise the statistical power and to ensure the systematic collection of information on potential confounding factors. In contrast, studies in other (non-radon) HNBR areas have provided little information, relying mainly on ecological designs and very rough effective dose categorisations. Recent steps taken in China and India to establish cohorts for follow-up and to conduct nested case-control studies may provide useful information about risks in the future, provided that careful organ dose reconstruction is possible and information is collected on potential confounding factors.

Long-terms effects of ionizing radiation

The fact that X-rays can induce mutation was established first in 1927. In the years following the mutagenic effects of some rays were described. When DNA was identified as the universal genetic material, it soon became obvious, and was confirmed by genetic evidence that the genetic material of all living beings is susceptible to radiation induced damage. When DNA is damaged by radiation enzymes within the cell nucleus attemp to repair that damage. If repair does not suceed or not correctly, the cell may die or may suffer changes in genetic information. It is thought that killing of cells is the basis for deterministic effect and that subtlechanges in information are important in the development of radiation-induced cancer or of genetic effects if these changes are induced in germ cells. This document describes aspects such as biological effects of ionizing radiation (deterministic radiation effects); stochastic radiation effect and quantification of risk; somatic effects; genetic effects of the ionizing radiation