Radioactive Waste 323DoseFigure 16-3. Linear-quadratic dose-response relationship.Chronic effects resulting from long-term exposure to low doses of ionizing radi-
ation may include both somatic and genetic effects that may occur because ionizing
radiation damages the genetic material of the cell. Our knowledge of both somatic and
genetic effects of low-dose ionizing radiation is based on animal studies and a very
limited number of human epidemiological studies: studies of occupational exposure,
the Japanese Atomic Bomb Survivors’ Life Study, and studies of effects of therapeu-
tic radiation treatment. Based on extrapolation from these studies, the dose-response
relationship has been estimated to be linear at low doses and quadratic at higher doses
(Fig. 16-3). Much of the low-dose part of the curve is below any range of experiment.
As with all carcinogens, the absence of a threshold has been assumed. The extrapolated
linear dose-response relationship is often referred to as the linear-nonthreshold (LNT)
theory of radiation health effect.
Reevaluation of human epidemiological studies indicates that there may be a
threshold below which there is no chronic effect. That is, there is growing evidence
that the LNT theory may be incorrect. A study of cancer incidence in Japanese atom
bomb survivors showed that excess cancer incidence among survivors exposed to less
than 0.02 Gy (2 rad) was actually lower than in a population that had no exposure
above background? Similarly, a study of oral and laryngeal cancers in radium watch
dial painters showed a clear threshold below which no effects due to radium exposure
were observed? There has been no similar verification of the LNT theory. Revision of
4S. Hattori, “State of Research and Radiation Hormesis in Japan,” J. Occupational Med. Toxicol. 3,
203-217, 1994.5R. G Thomas, “The US Radium Luminisers: A Case for a Policy of ‘Below Regulatory Concern,’ ”
J. Radiol. Protection, 14, 141-153, 1994.