Hiroshima study shows higher risks of low-level radiation

 作者:怀堠     |      日期:2019-03-04 08:20:02
By CHRIS VAUGHAN in WASHINGTON THE RISK of developing cancer from exposure to low levels of radiation from X-rays or gamma-rays is three to four times as high as previously thought. And fetuses exposed to low levels of radiation are at greater risk of mental retardation than earlier estimates had suggested. These are the findings of a committee of the National Research Council in the US, whose latest report was published just before Christmas. The report is based on new models for cancer risk, revised estimates of how much radiation the survivors of the nuclear explosions at Hiroshima and Nagasaki were exposed to, and new epidemiological data on cancers and other problems that afflicted those survivors. The committee’s report is the fifth in a series on the biological effects of ionising radiation. The last was issued in 1980. The 17 scientists on the current committee found that the number of tumours increased in direct proportion to the size of the dose of low-level radiation – in other words, there is no thresholdbeneath which the effects of radiation can be disregarded. In contrast, the members of the committee responsible for the 1980 report favoured a ‘linear-quadratic’ model, in which the risk of cancer rises more rapidly at high doses than at low doses. However, the committee was deeply split over this conclusion at the time and five of its members issued a dissenting statement. According to the new committee’s report, the risk that tumours will develop from low doses of radiation is three times what was previously thought. In the case of leukaemias, the linear-quadratic model still fits best, but the estimated risk of contracting leukaemia from low doses of radiation was raised by a factor of four. The scientists increased their general estimates of risk based on data from Hiroshima and Nagasaki because there was much less neutron radiation than previously thought in the attacks on the two cities. Therefore, the current incidence of cancer among survivors is the result of lower radiation doses than had been assumed. The scientists also abandoned the idea that it is possible to assign some ‘absolute’ scale for the risk of cancer resulting from exposure to low levels of radiation. Risk depends in part on age, and children exposed to a given dose of radiation are at twice the adult risk of contracting cancer. Children who survived the nuclear explosions in Japan are just now reaching the age where cancer is most common, so there may be many more cancers in the future than current estimates suggest, said the committee’s chairman, Arthur Upton. If this trend were to be borne out, it would cause scientists to raise their estimates of risk from low doses of radiation still further, he added. ‘Because over half the survivors of the atomic bomb explosions are still alive, it is very difficult to extrapolate into the future,’ Upton said. The report is basically in agreement with a UN report on the same subject issued in January 1988. But members of the National Research Council’s committee say that their research is more complete, because they used original data instead of previously published data. The International Commission on Radiological Protection, which is studying both reports, is due to publish its revised recommendations on permitted radiation dose limits later this year. According to existing ICRP advice,