Radon is a naturally occurring radioactive gas. Inhalation of radon and its decay products can increase the risk of lung cancer. Based on occupational studies, it is estimated that between 6,000 to 36,000 Americans may die each year of radon-associated lung cancer. Smokers exposed to radon are at greatest risk, with risk for smoking and radon exposure exceeding the sum of risks from the individual factors.
Jay Lubin, Ph.D., of the National Cancer Institute, and John Boice, Ph.D., of the International Epidemiology Institute in Rockville, Md., used the statistical technique of meta-analysis to combine data from eight case-control studies in Canada, China, Finland, Sweden, and the United States that directly examined the effects of residential radon levels on lung cancer risks. Results are published in the Jan. 1 issue of the Journal of the National Cancer Institute.
The studies included a total of 4,263 lung cancer patients and 6,612 persons without lung cancer who served as controls. Individually, some of the studies found increased risk from household radon while other found no increased risk. The purpose of the meta-analysis was to evaluate these seeming inconsistences and to estimate an overall effect of residential radon.
Based on the eight studies combined, Lubin and Boice estimated that the risk of developing lung cancer increased 14 percent for a person living 30 years in a house with a radon level of 150 Becquerels per cubic meter (Bq/m3). This is approximately equal to 4 picocuries per liter, the level at which the U.S. Environmental Protection Agency recommends taking action to reduce radon in a house. About 6 percent of U.S. houses have radon levels at or above 150Bq/m3.
The meta-analysis findings are subject to uncertainty because they are based on combined data from studies done in different ways with inconsistent results. But the risk estimate derived from the meta-analysis is very close to the 13 percent increased risk that is estimated by extrapolating results from an analysis of pooled data from 11 underground miner studies to the lower levels of radon exposure in the household setting.
"Our results suggest that the current method of extrapolation from miner studies provides a reasonable estimate of the lung cancer risk for people exposed to elevated radon levels in their homes," Lubin said. "The fact that our risk calculations are consistent with the risk figures based on miner studies offers some assurance that estimates of household radon risk are not far off the mark."
The meta-analysis provides evidence against the connection of some scientists that the use of data from miner studies might underestimate the risks of household radon. That belief is based on the fact that household residents are exposed to relatively low radon levels for long periods of time, while miners are generally exposed to higher radon levels for shorter periods. For miners at least, there is evidence that the same total exposure received over a long period of time is riskier than if given over a shorter time. In addition, nearly all miners studied have been men , leading some to question the validity of applying miner data to women and children. But the new findings suggest that miner-based estimates appear to be on target despite those concerns, Lubin said.
The meta-analysis also provides no evidence for a threshold exposure, that is, an exposure so low that no risk would be present. Some geographic correlation analysis have purported such a notion, but the more epidemiologically sound case-control studies are consistent with miner estimates of risk based on linear extrapolation to residential radon levels.
Several case-control studies of household radon are in progress, and efforts are under way to pool the data from completed and ongoing studies. Pooling of individual data should provide more accurate results than this meta-analysis, in which summary results of each study are combined. But it will be at least several years before pooled results of household radon studies are available.
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