The standards used for radiation protection around the world today are primarily based on an extrapolation of risk estimates derived from studies of the survivors of atomic bombings in Hiroshima and Nagasaki in August 1945. The main issue of concern however is the protection of the general public and of occupationally exposed workers, who generally receive much lower doses, protracted over many years.

For Dr Elisabeth Cardis, of the IARC Radiation Group, "There has been a controversy for decades about the use of data on A-bomb survivors for setting standards for the protection of the general public and radiation workers. There was therefore a need for a direct assessment of the carcinogenic effects of low-dose exposures to evaluate the adequacy of these standards."

IARC–led international cohort study conducted to confirm these standards

To this effect, a retrospective cohort study was coordinated by the WHO’s International Agency for Research on Cancer to estimate the risk of cancer death, including leukaemia, after low-level exposure to high energy photon (gamma-ray) radiation in a worldwide population of over 400,000 nuclear industry workers in 15 countries(1). Individual real-time measurements of external radiation dose were available for all workers. This is the largest study of nuclear industry workers ever carried out.

Nuclear industry workers scrutinized

This study was restricted only to workers who wore a radiation dosimeter or badge, and who worked for at least one year in the nuclear industry in one of 15 countries. Included were persons who worked in nuclear power plants, in nuclear research or waste management, or in the production of nuclear fuel, isotopes or weapons. Workers who might have had substantial neutron or internal (for example, plutonium) exposure were excluded because these exposures may have been poorly measured in the past.

Most of the workers were men (90%), and the average total workplace dose was about 19 millisievert (mSv, which corresponds to 1.9 rem) per worker. Only about 6% of the international cohort was found to be deceased, with a total of 6519 deaths from cancers other than leukaemia and 196 deaths from leukaemia other than chronic lymphocytic leukaemia (CLL).

Risk estimates per level of radiation dose were calculated for deaths from all cancers excluding leukaemia and from leukaemia excluding chronic lymphocytic leukaemia, the main causes of death on which radiation protection standards are based.


Small but significant radiation related increase in cancer mortality

The risk estimates from this study are statistically similar to those of the atomic bomb survivor data. Overall, based on the risks found in this study, 1 to 2% of deaths from cancer (including leukaemia) among the workers studied may have been caused by radiation exposure. Many of the subjects in this study worked in the early years of the industry when doses tended to be higher than they are today, however. Only a small proportion of cancer deaths would therefore be expected to occur from low-dose chronic exposures to X- and gamma- radiation among current nuclear workers and in the general population.

The excess relative risk (ERR) for cancers other than leukaemia was statistically significantly elevated (0.97 per Sv, 95% Confidence Interval, 0.14-1.97). Analyses of smoking and non-smoking related causes of death indicate that, although confounding by smoking may be present, it is unlikely to explain all of this increased risk. The excess relative risk for leukaemia excluding chronic lymphocytic leukaemia was 1.93 per Sv (95% Confidence Interval, <0-8.47).

Conclusion

The main result from this study is that there appears to be a small increase in cancer risk even at the low doses and dose-rates typically received by nuclear workers in this study.

Said Dr Boyle, Director of the IARC: "These results provide the most precise and comprehensive direct estimates of cancer risk after protracted exposure to low doses of ionising radiation; they strengthen the scientific basis of radiation protection standards for environmental, occupational, and medical diagnostic exposures. They support current evidence on the carcinogenic potential of ionising radiation yet provide reassurance concerning the likely impact of ionising radiation on the global cancer burden".


(1) The study included workers from Australia, Belgium, Canada, Finland, France, Hungary, Japan, South Korea, Lithuania, Slovak Republic, Spain, Sweden, Switzerland, the UK, and the US.

Risk of cancer after low doses of ionising radiation - retrospective cohort study in 15 countries, British Medical Journal Online First, accessible at:
http://bmj.bmjjournals.com/onlinefirst_date.shtml.