Assumption of linearity in dose--effect relationships.

As a basis for establishing radiation protection standards, a substantial amount of quantitative information is now available on the frequency with which malignant diseases are induced in man by moderately high doses of radiation. Such estimates can now be made not only for irradiation of the whole body but also for exposure of a number of body organs individually. The frequency with which cancers might follow the much lower doses involved in occupational or environmental exposure to radiation, however, cannot be derived from any available epidemiological surveys. It can at present only be inferred by the (probably pessimistic) assumption that the frequency of such effects is linearly proportional to the size of dose received, even down to the lowest doses. Increasing information as to the probable form of the actual dose--effect relationship for radiation is indicating the extent to which the use of this "linear hypothesis" may overestimate the risk of low doses as inferred from the observed risk of higher doses. A linear hypothesis has been used in the same way for estimating the likely frequency of harm from low doses of chemical substances which have defined harmful effects at high dose. The appropriateness of this procedure depends critically upon the way in which chemical pollutants, or the relevant products of their metabolism in the body, are likely to become distributed through body tissues and cause the relevant harmful effects on cells.