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Focus : Undermining our lives? - Radon seeps into the homes of millions of people around the world. Its effects may have been seriously exaggerated

点击量:   时间:2017-12-08 01:01:02

By Fred Pearce BERNARD COHEN has stirred up a hornet’s nest. He has completed the biggest ever study into what is supposedly the most important cause of lung cancer other than smoking—radon gas. His investigation, which drew on data from almost half a million homes throughout the US, suggests that below a certain level the risk of contracting lung cancer from radon is zero. But far from rejoicing at his findings, researchers and health officials on both sides of the Atlantic are rubbishing them. “His conclusions are absolute nonsense,” says Sarah Darby of Britain’s Imperial Cancer Research Fund, who is investigating radon in Cornwall. “I don’t care how big the study was, the method is just inappropriate,” says Jonathan Samet of Johns Hopkins University, Baltimore. Samet chaired the latest review of radon research, published by the US’s National Research Council last month, which estimates that the gas causes 18 000 deaths in the US every year, 12 per cent of all lung cancer deaths. It roundly rejects Cohen’s work. Yet it also provides evidence from other studies that, as Cohen maintains, there may be a threshold below which radon has no measurable effect—and that health officials are unnecessarily worrying millions of people exposed to radon. Radon is a radioactive decay product of uranium. A colourless and odourless gas, it seeps out of rocks such as granite, dispersing in the air unless it becomes trapped inside mines or well-insulated homes. The gas swiftly decays into radioactive polonium atoms, which can stick to dust particles—including cigarette smoke. These in turn can become stuck deep in the lungs, irradiating them and causing cancer. It has long been accepted that miners exposed to high doses of radon often die from lung cancer. A study of former miners at Radium Hill in South Australia found that men exposed to radon levels 30 times the US government’s safety limit had a threefold increase in the risk of contracting lung cancer. In the early 1970s researchers quantified the risks to miners and extrapolated them to reflect the lower doses experienced by millions of people in their homes. By the 1980s, radon was seen in many countries as the second biggest cause of lung cancer after smoking. In Britain, the National Radiological Protection Board estimates that 2500 people die annually from radon exposure. Yet researchers have struggled to find signs of this theoretical death toll in real statistics. Cornwall, for example, has by far the worst radon problem in Britain but significantly less lung cancer than the national average. Cohen, a radiation physicist at the University of Pittsburgh, set out a decade ago to find such evidence. By 1995 he had collected data on radon in houses from 1729 American counties, covering 90 per cent of the population, and compared the average radon levels found in each county with the death rates from lung cancer. Surprisingly, he found no evidence linking increased risk of lung cancer with increased radon levels. After intense analysis of the data and several research papers—the most recent will be published shortly in the journal Health Physics—he still maintains his findings stand up. He has accounted for more than 500 variables that could have confused the figures, from age to rainfall to health-care provision. Most importantly, he has attempted to correct for smoking. His critics claim that even a small difference in the prevalence of smoking across the US might swamp any trend in lung cancers caused by radon. People in urban areas, for instance, smoke more but have less radon in their homes. But he has found no explanation other than the obvious—that there is no link between lung cancer and the low radon levels in most houses. So why are his findings being dismissed so readily? The problem is that his is an “ecological”, rather than an epidemiological, study. In epidemiology, researchers gather data on individuals’ exposure to radon and any potential confounding variables, such as smoking, and then look for patterns between them and the incidence of cancer. Cohen’s study is much cruder because it deals only in county-wide averages. John Boice, former head of radiation epidemiology at the National Cancer Institute in the US, says: “A county may contain hundreds of thousands of people and yet his study will have one data point for smoking and radon exposure. There is a great possibility of a mismatch. It’s not Cohen’s fault. He has done the best he can with the data.” Cohen accepts that his use of averages is a “serious problem”. But he insists it does not invalidate his data. On the crucial question of smoking, he says: “Even if there were a perfect negative correlation between radon and smoking prevalence, this could explain only half of the discrepancy. What I have found has still got to be explained, especially given the size of the study. Epidemiologists can’t just ignore this.” He says his study “clearly shows that currently used analyses grossly exaggerate the cancer risk from low-level radiation”. In his war with the epidemiologists Cohen has supporters, especially among physicists. Richard Wilson, at Harvard, says: “I don’t believe that the confounding effect of smoking can easily explain Cohen’s findings. Nobody has a viable explanation except that the linear extrapolation of risk doesn’t work at low levels. But epidemiologists close ranks.” The radon debate is hampered by the fact that few people are exposed to high levels of the gas, while tens of millions are exposed to low levels. Most of the estimates of death rates at low exposure are based on extrapolating from the risks at high exposure, measured mostly in miners. So while decrying Cohen’s study for its methodology, epidemiologists are trying to support their extrapolations with real data from studies of individuals exposed to radon at household levels. So far, the results have been mixed. Boice says that of eight studies that have now been completed, only one—from Sweden—has found a statistically significant link between radon in the home and lung cancer. It suggested that a house with the American maximum recommended radon level—150 becquerels per cubic metre, which is exceeded in 6 per cent of homes in the US—increased the lifetime risk of lung cancer by 15 per cent. But two years ago, a study of more than a thousand homes of lung cancer victims in Finland found no evidence that these kinds of levels increased the risk of lung cancer (This Week, 27 July 1996, p 8). To date, says Boice, “the data are consistent with a very low risk from radon at low levels—but they are also consistent with no risk at all”. Weeding out the much greater effects of smoking, he says, remains the Achilles heel of all studies into the health effects of radon. The problem is magnified by the suspicion that smoking itself worsens the risk from radon. This uncertainty is one reason why the National Research Council’s recent report halves its previous estimate of the number of radon deaths among nonsmoking Americans to 2500 a year. The next contribution to the debate will be Darby’s study in Cornwall, due to be published this summer. It has tracked individual histories of health and radon exposure for 35 years. Nobody involved is revealing the contents, but Boice says: