WASHINGTON--Radon contributes to a significant number of lung cancer deaths each year, perhaps as many as 12% of those in the United States, a new National Research Council (NRC) report confirms. The threat is greatest for smokers, and reducing radon levels in all homes below the current recommended guideline could prevent about one-third of the lung cancer deaths associated with radon exposure.
"Radon, particularly in combination with smoking, poses an important public health risk and should be recognized as such," said Jonathan M. Samet, MD, professor of epidemiology, Johns Hopkins University School of Hygiene and Public Health, who led the NRC study.
In a nearly 4-year effort, a committee of experts examined 11 major studies involving a total of 68,000 underground miners exposed to radon, as well as new epidemiologic data on lung cancer in the general population.
Using two different models, its members estimated the number of radon-related lung cancer deaths at 15,400 (or 21,800 using the second model) of the total 157,400 US lung cancer deaths in 1995."The majority of radon-related deaths occur among smokers, but 2,100 (or 2,900 using the second model) of radon-related lung cancer deaths each year occur in nonsmokers," Dr. Samet said in releasing the report.
If radon levels in all American homes were reduced below 4 picocuries per liter of air, the guideline suggested by the Environmental Protection Agency (EPA), "we would estimate about 30% of attributable deaths could be eliminated," said Jay H. Lubin, PhD, of the National Cancer Institutes Biostatistics Branch.
Most of the deaths prevented would be among smokers, "but by reducing levels of radon in homes to the EPA level, 1,000 nonsmokers might avoid [fatal] lung cancer," Dr. Samet added.
Among both smokers and nonsmokers, the committee found the risk of developing lung cancer proportionate to the radon exposure. Doubling the exposure doubled the risk.
The committee said it found no evidence for a threshold effect below which radon posed no threat, although it could not totally rule out that possibility. "The lowest dose of radiation from radon exposure is a single alpha particle, which by itself may cause substantial cell damage," Dr. Samet said. "Thus, radon might pose some risk even at very low levels."
Dr. Lubin noted that the estimate of preventable lung cancers applied to people living in homes with radon exposures of 4 picocuries or more, well above any possible threshold. "So even if there was a threshold effect, in the practical realm of preventable lung cancer, there would be very little change," he said.
The report, titled "Health Effects of Exposure to Radon" and known also as BEIR VI, is the sixth in a long-running series of reports by the NRC addressing the issue of the biologic effects of ionizing radiation on human health. It was requested by the EPA, which develops guidelines for indoor radon, as a follow-up to a 1994 NRC assessment of the radioactive gas.
No Request for Guidelines
About 6% of US homes have radon levels in excess of the current EPA guideline. Dr. Samet said that the EPA did not ask the panel to make policy recommendations on radon, and it did not do so.
Daniel Krewski, PhD, of the Canadian governments Bureau of Chemical Hazards, said he hoped now that the information on radon risk has been "characterized as fully as possible, that the EPA will take a look at that information and try to develop guidelines in understandable terms."
The NRC study failed to find an association between radon and other health problems. Although some ecologic studies have suggested an increased risk of leukemia, other cancers, and pulmonary fibrosis, the committee found no evidence to support this in its analysis of the miner data. It concluded that these findings from the 68,000 miners could be reasonably extended to the general population.
Although the percentage of lung cancer deaths ascribed to radon remains essentially the same as in the BEIR IV report a decade ago and in the NRCs 1994 radon assessment, several committee members emphasized that the current report is backed by substantial new scientific evidence.
"We have a richer, much more robust data set available," said Roger McClellan, DVM, president of the Chemical Industry Institute of Toxicology, Research Triangle Park, NC. "All of this provides a much clearer exposition of what it is we know and what it is we do not know."
This stated, the committee acknowledged that its estimates remain subject to several important uncertainties. For example, the miners in the 11 studies inhaled many substances besides radon while working underground, and the majority of them smoked. "Disentangling the effects of radon in miners from these other factors is difficult," Dr. Samet said. "In addition, the miners were all male, while in residential settings, women and children also are exposed to radon."
The most direct route to determining indoor radons effects would be to compare levels of radon exposure among lung cancer patients with those who do not have the disease. However, such studies so far have failed to provide a definitive answer "because radon levels in most homes are extremely small, making it difficult to estimate the risk accurately," he said. Even so, he noted, "these uncertainties do not change the committees finding that lung cancer in the general population, including smokers, can be reduced by limiting radon exposure."