How dangerous is exposure to low-dose radiation?

Protracted exposure to radiation can increase the risk of cancer and leukaemia even at low doses. This assumption is supported by the so-called INWORKS study. Based on theoretical considerations, international radiation protection systems assume that there exists no safe threshold for radiation exposure.

However, there has been little evidence on radiation exposures below 100 millisieverts from observational studies up to now. This may be because any potential increase in risk associated with such low doses of radiation would be extremely small. Only a large-scale study can establish proof of such a small increase in risk.

Importance of the study

The INWORKS study has a very large sample size, involving data from more than 300,000 nuclear workers from three countries. The data cover a period of up to 60 years. The risk of death from cancer or leukaemia in nuclear workers showed a statistically significant increase with increasing radiation exposure.

A statistically significant excess risk of cancer and leukaemia was found even at doses below 100 millisieverts for which it is difficult to provide scientific proof of potential radiation effects. However, in the case of leukaemia this applies only for a special subtype of the disease (chronic myeloid leukaemia) and only for radiation doses from 50 to 100 millisieverts.

For doses below 50 millisieverts, direct proof of excess risk of leukaemia was not provided even by this large-scale study. Nevertheless, the widely cited study underlines that further research into this field is important for dealing with existing uncertainties.

Comparison with radiation exposure of the general public and occupationally exposed persons

Radiation exposures of the public due to discharges from nuclear power plants or releases from intermediate storage facilities for irradiated fuel elements account for a few thousandths of one millisievert at maximum. These radiation exposures therefore are far below 50 millisieverts even if added up over many years.

Medical applications of radioactive substances and ionising radiation make by far the most important contribution to radiation exposure of the public from man-made sources of radiation, reaching an average of two millisieverts per year.

Compared to the general public, radiation exposure of staff members in nuclear power plants and other occupationally exposed persons is higher, but total exposures exceeding 50 millisieverts within occupational life are rare even for these latter.

Implications of the study for radiation protection

Knowledge about the health risks associated with ionising radiation has mainly been based on the results of the studies on the Japanese atomic bomb survivors up to now. Radiation exposure of these latter was due to a single release of a very large amount of radioactivity.

However, situations associated with low radiation exposures over a long period of time are those relevant to radiation protection. Even the statements on the effects of such radiation exposures have been derived from the A-bomb studies up to now. Calculations like these are subject to large uncertainty.

The nuclear worker study provides direct evidence for the cancer and leukaemia risk of persons exposed to relatively low radiation over a long period of time.

BfS does not expect amendments to international radiation protection recommendations

It is remarkable that the risk estimates from the two studies are almost in agreement. The results of the study on nuclear workers, therefore, are opposed to the common assumption that exposure to low doses distributed over a long period would mean less risk than single exposure to radiation at the same level.

The BfS does not expect amendments to the recommendations of the International Commission on Radiological Protection, since large uncertainties of present knowledge have been taken into account in these recommendations to be on the safe side. However, further research is needed in this field in the opinion of the BfS.

Limitations to the study

The study is an observational study examining the relationship between external radiation exposure and the risk to die from cancer or leukaemia. Although the researchers attempted to consider the influence of other risk factors, such as

• social standing, or
• additional radiation exposure due to the intake of radioactive substances into the body,

this is only possible to a certain degree. The study did not include other possible risk factors such as the workers’

• medical radiation exposure,
• natural radiation exposure from the environment, or
• occupational exposure to benzol.

It remains to be seen which results will be provided by future studies in this field.

Research into the risks of protracted, low-dose radiation is a main area of European radiation protection research. Therefore, important insights are expected in this field within the next few years.

References

Richardson, D. B., et al. (2015). "Risk of cancer from occupational exposure to ionising radiation: retrospective cohort study of workers in France, the United Kingdom, and the United States (INWORKS)." BMJ 351: h5359.

Leuraud, K., et al. (2015). "Ionising radiation and risk of death from leukaemia and lymphoma in radiation-monitored workers (INWORKS): an international cohort study." The Lancet Haematology 2(7): e276-e281.