Answer to Question #9503 Submitted to "Ask the Experts"

I've come to understand that anything over 0.1–0.2 Sv increases risk of cancer. My question is, does the increased risk happen when you're exposed to 0.1–0.2 Sv in one sitting or when exposed to it over a lifetime? The reason I ask is if a person lives for 60 years with exposure to an average of 0.36 rem a year, that would be over 0.2 Sv. Wouldn't that put everyone at a slightly higher risk of cancer when they reach 60? Or does the understanding of 0.1–0.2 Sv mean at one sitting of radiation at that amount?

Your question involves the still-controversial issue of whether low doses or doses delivered at low dose rates of radiation cause cancer and, if so, whether the frequency per unit dose is the same as what has been observed at high doses and high dose rates. There is experimental evidence, and general agreement among experts in radiation effects, that there is a substantial difference in the degree of effectiveness of high doses of low LET radiation (x rays, gamma rays, electrons, beta radiation) delivered at high dose rates compared to the same doses delivered at low dose rates. For this reason, it has been recommended that a dose and dose rate effectiveness factor (DDREF) of at least two be used to reduce the high dose-rate effects to predict what might be expected at low dose rates.

There have not been any radiation-induced cancers observed in human beings at doses less than about 0.1 Sv. This 0.1 Sv is taken to be a delivered dose in excess of the natural background dose. The Health Physics Society issued a Position Statement in which it recommends no estimates of risks should be made for doses less than 0.1 Sv delivered over a lifetime or for doses of less than 0.05 Sv delivered within a one-year period.

We should note that these recommendations do not mean that the Society is stating that no effects occur below the cited dose levels, but only that there is sufficient uncertainty about the degree of such effects that we are not able to make any meaningful quantitative estimates. The possibility of zero effects does exist, but this also cannot presently be confirmed.

The doses cited as limiting values in these position statements are to be interpreted as doses beyond what has accrued from natural radiation background. If the 0.1–0.2 Sv that you cite were delivered over a short duration—for example, in a radiation accident—it would be acceptable to project excess cancer risk for the exposed individual(s), and it may be appropriate not to apply the DDREF that is used at low dose rates. However, if the cited dose is from normal background exposure, there is no intent by the HPS, or other expert groups that I am aware of, that we should attempt to quantify the degree of risk from this background exposure.

Background radiation has been with us since we emerged on the planet. We are who we are despite, or some think at least partly because of, the ionizing radiation background that we have experienced. Whether the background radiation has had an overall negative, positive, or nil effect on man is unknown. Background levels vary quite widely with geographical location, and studies of populations exposed to considerably higher background-radiation levels than what we experience in the United States have not demonstrated any elevated incidence of radiation-induced disease. In any case, with the exception of radon exposure, there is not much we can do practically to reduce our background doses. The normal incidence of cancer in a population includes disease induced by whatever provoking or causative agents are available. If background radiation is one of these, we are currently unable to resolve it, and any estimations of excess cancers that we make should be based on doses delivered in excess of the usual radiation background.

I hope this is useful to you.

George Chabot, PhD, CHP