Large study of nuclear workers shows that even tiny doses slightly boost risk of leukaemia.
For decades, researchers have been trying to quantify the risks of very low doses of ionizing radiation — the kind that might be received from a medical scan, or from living within a few tens of kilometres of the damaged Fukushima nuclear reactors in Japan. So small are the effects on health — if they exist at all — that they seem barely possible to detect. A landmark international study has now provided the strongest support yet for the idea that long-term exposure to low-dose radiation increases the risk of leukaemia, although the rise is only minuscule (K. Leuraud et al. Lancet Haematol. http://doi.org/5s4; 2015).
The finding will not change existing guidelines on exposure limits for workers in the nuclear and medical industries, because those policies already assume that each additional exposure to low-dose radiation brings with it a slight increase in risk of cancer. But it scuppers the popular idea that there might be a threshold dose below which radiation is harmless — and provides scientists with some hard numbers to quantify the risks of everyday exposures.
Ionizing radiation — the kind that can pull electrons from atoms and molecules and break DNA bonds — has long been known to raise the risks of cancer; the higher the accumulated dose, the greater the damage. But it has proved extremely difficult to determine whether this relationship holds at low doses, because any increase in risk is so small that to detect it requires studies of large numbers of people for whom the dose received is known. A study of more than 300,000 nuclear-industry workers in France, the United States and the United Kingdom, all of whom wore dosimeter badges, has provided exactly these data. A consortium of researchers coordinated by the International Agency for Research on Cancer (IARC) in Lyon, France, examined causes of death in the workers (one-fifth of whom had died by the time of the study) and correlated this with exposure records, some of which went back 60 years.
The workers received on average just 1.1 millisieverts (mSv) per year above background radiation, which itself is about 2–3 mSv per year from sources such as cosmic rays and radon. The study confirmed that the risk of leukaemia does rise proportionately with higher doses, but also showed that this linear relationship is present at extremely low levels of radiation. (Other blood cancers also tended to rise with radiation doses, but the associations were not statistically significant.) The results were published on 21 June.
The data also challenge an ICRP assumption that accumulated low-dose exposure gives a lower risk of leukaemia than does a single exposure to the same total dose (based on the idea that the body has time to recover if the assault comes in tiny, spread-out doses). But such details are unlikely to change the overall ICRP recommendations, which are deliberately conservative, says Thomas Jung, from Germany’s Federal Office for Radiation Protection in Munich.
Epidemiological studies suggest that radiation exposure has health effects beyond cancer. The IARC-led consortium is now looking at the effect on solid cancers, and also on diseases such as heart attack and stroke. Other studies are under way to study the long-term impact of low-dose radiation on different cohorts. One, the Epi-CT study, is recruiting one million people from nine European countries who had CT scans as children; its analysis will be complete by 2017. In another, the Helmholtz Center Munich is analysing heart tissue from workers who died in the Mayak uranium mines in the South Urals, Russia.