Guidance

Ionising radiation: estimation of cancer risk at low doses

Published 4 September 2008

The cancer-causing effects of ionising radiation at high doses have been known for many decades. Since then, there have been many large studies worldwide of cancer arising in people exposed to high and low doses. These studies include people exposed to the atomic bomb explosions in Japan, to fallout from nuclear weapons tests and during radiation accidents.

Information is also available from people irradiated for medical reasons, during their work or as a result of living in a region that has unusually high levels of radioactive radon gas or gamma radiation. From all of this scientific work published in peer-reviewed papers we know more about cancer risk after ionising radiation than for any other cancer-causing substance. However, because cancer is unfortunately a common disease with many causes it is extremely difficult to measure directly the small extra risk from ionising radiation when the doses are very low.

National and international organisations worldwide constantly discuss the best way to use the cancer information that we have to make estimates of the risks at the low doses that are received by the general public and workers. Special attention is given to the risks from man-made ionising radiation that can be controlled and regulated. As mentioned earlier, it is also important to take account of the risks from natural radiation, most of which cannot be controlled.

At present the estimate of cancer risk at low doses as recommended by Public Health England (PHE) for use in the UK predicts that a lifetime of exposure of the population to all sources of ionising radiation (natural plus man-made) could be responsible for an additional risk of fatal cancer of about 1% - this can be compared with a life-time risk of cancer of about 20% to 25% from all causes. The very small doses from non-medical, man-made radiation would be responsible for only a tiny fraction (about one-hundreth) of this 1% radiation risk. Therefore, compared with other known cancer risk factors in the population such as cigarette smoking, excessive exposure to sunlight and poor diet, the risk to the population from non-medical man-made radiation is generally agreed to be very small indeed.

It is the responsibility of PHE, to advise the UK government on cancer risk estimates and standards for radiation protection. At present there are only small differences in the risk estimates used by different countries world-wide for the protection of their populations - almost all countries follow the recommendations made by the independent International Commission on Radiological Protection (ICRP).

However, from time to time scientific advances make it necessary to consider an adjustment of these estimates and this can be done at a national as well as international level. To illustrate this, a review by NRPB in 1987 of newly published epidemiological studies led to a recommendation to increase the estimate of low dose cancer risk to be used in the UK. This was accepted by government and within a few years other national and international bodies came to similar conclusions. PHE, through its Advisory Group on Ionising Radiation (AGIR), continues to assess radiation cancer risks to the UK population. Most recently leukaemia risks have been reviewed, a sub-group of AGIR has been formed to examine solid cancer risks.

1. 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 2006 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 2 (higher or lower) for the projection of these risks to very low doses and low dose rates.

In 1993, NRPB, now PHE, 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 2006 review is the best recent example of international scientific agreement on cancer risks after ionising radiation exposure. PHE contributes to the scientific discussions and reviews of UNSCEAR and agrees with the conclusions that this international committee has reached.