How Reliable Are Estimates on Cancer Risks at Low Doses?

There are a number of scientific uncertainties in making these estimates of cancer risk at low doses. In its most recent report in the year 2000 the highly respected United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) suggests that uncertainties in cancer risk estimates may be about twofold higher or lower for acute doses where cancer risk can be directly assessed and a further factor of two (higher or lower) for the projection of these risks to very low doses and low dose rates. In 1993, NRPB, now the HPA, had suggested similar degrees of uncertainty on radiation cancer risks to be applied in the UK. For external radiations where the energy enters the body from the outside there is good scientific evidence to support these estimates of uncertainty. However, for internal radiations where radioactive material is taken into the body via food, water or air, the uncertainties on risks may be greater. The main reason for this extra uncertainty is that some forms of internal radiation concentrate in different parts of the body and therefore the doses to different body organs are not the same. This uncertainty is in part dealt with by taking account of concentration effects and of differences between body organs in their sensitivity to radiation-induced cancer. These estimates on risk have also been supported by research on humans and experimental animals exposed to internal radiations.

In the light of all the available information, the argument that risks from low-dose man-made internal radiations have been greatly underestimated is scientifically very weak. First, the doses to the general public from man-made internal radiations represent a small fraction of internal radiation dose from naturally radioactive material. On this point, the studies on people exposed to internal radiation via radon gas entering the body through the lungs do not provide evidence of unexpectedly high cancer risk at low doses. Second, markedly higher than expected cancer risk has not been seen in people exposed throughout Europe to man-made radioactive fallout from nuclear testing in the 1950s and 1960s and from the Chernobyl accident. Third, the same conclusion is reached from research on populations in the former Soviet Union who received low-dose internal radiation as a result of radiation accidents and incidents. Many of these studies are continuing. They have different strengths and weaknesses but taken together they do not suggest that internal radiations, whether natural or man-made, have special properties that make them particularly dangerous. Furthermore, these studies do not indicate that the cancer risks from these internal radiations have been greatly underestimated.

There certainly are well publicised scientific reports which claim that risks of cancer after low dose radiation are much higher than those provided by international scientific consensus. Equally, other reports claim much lower risks at low doses and even no risk at all because of an assumed low-dose threshold for the process of cancer induction. It is usually the case that these extreme positions are taken using information from selected studies. By contrast, the international scientific consensus on radiation risk has been built over many years by large numbers of scientists working in different countries who have considered the strengths and weaknesses of all available evidence before reaching conclusions.

The UNSCEAR 2000 review is the best recent example of international scientific agreement on cancer risks after ionising radiation exposure. The HPA contributes to the scientific discussions and reviews of UNSCEAR and agrees with the conclusions that this international committee has reached.

Last reviewed: 4 September 2008