Swedish Radiation Protection Institute
Dear Nuclear Engineering International
“The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has just approved its UNSCEAR 2000 Report. The report contains extensive scientific reviews and assessments on natural background exposures; medical and occupational radiation exposures; review of radiation-associated cancer risks; DNA repair and effects at low radiation doses; effects of radiation in combination with other agents; and exposures and effects of the Chernobyl accident.
The primary source of epidemiological information remains the survivors of the atomic bombings of Hiroshima and Nagasaki. Based on the available epidemiological data, the Committee has derived risk estimates for radiation-induced cancer. There is no convincing scientific evidence that cancer risk from radiation disappears at very low doses. The dose response for solid tumours appears to be a simple function of increasing dose, the simplest being a linear one. For leukaemia a linear-quadratic function provides the best fit to the data.The Committee estimates that cancer mortality risks are 4-6 % per sievert at low to moderate doses, which are similar to its previous estimates.
There is still controversy at doses below 100mSv. Future studies in epidemiology will not resolve the uncertainties surrounding the effects in humans of low-dose radiation. The statistical power is insufficient, and the absence of a statistically observable effect at low doses cannot be equated with the absence of risk. Mechanistic considerations of radiation response must therefore be relied on for risk evaluation at lower doses, and there is a continued need for data from cellular/molecular studies on the biological mechanisms.
In its 1994 Report, the Committee considered the more recent studies on adaptive responses (hormesis) to radiation in cells and organisms. This type of effect is likely to be linked to an increased capacity for DNA repair. The degree of response depends on the stage of the cell cycle, and cells in phases prior to cell division are more likely to show the response than cells in other stages. The adaptive responses are thus not a generally occurring phenomenon; they require carefully controlled experimental conditions to elicit the behaviour, and may therefore be part of a common mechanism involving cellular response to damage.
It is generally more difficult to demonstrate adaptive responses in the whole organism. UNSCEAR judges that adaptive responses, manifested by improved repair of cellular damage that take place under specific conditions in cellular studies, probably do not completely eliminate residual damage in cells that may result in malignant transformation. The Committee stated in its 1994 Report that “At this stage it would be premature to draw any conclusions for radiological protection purposes.” The linear no threshold model is therefore not invalidated by hormesis.