WASHINGTON, D.C. -- Smokers who are exposed to radon appear to be at even greater risk for lung cancer, because the effects of smoking and radon are more powerful when the two factors are combined, says a new report by a committee of the National Research Council. Indoor radon contributes to about 12 percent of lung cancer deaths each year in the United States.
The report, Health Effects of Exposure to Radon (BEIR VI), is the sixth in a series by the Research Council on the biological effects of ionizing radiation. The report examines data from 11 major studies of underground miners exposed to radon, and new epidemiological data on lung cancer in the general population. Based on these studies, which provide substantially more information than was available for the Research Council's 1988 and 1991 reports on the health effects of radon, the committee developed two models to estimate the number of radon-related lung cancer deaths in the general population. Depending on which model is used, indoor radon contributes to 15,400 or 21,800 of the estimated 157,400 lung cancer deaths each year in the United States, the committee said. The majority of radon-related deaths are among smokers; perhaps 1,200 or 2,900 are among non-smokers.
"Radon -- particularly in combination with smoking -- poses an important public health risk, and it should be recognized as such," said committee chair Jonathan Samet, professor and chair of the department of epidemiology, Johns Hopkins University School of Hygiene and Public Health, Baltimore. "Reducing radon in homes could prevent some lung cancers in this country. In fact, radon reduction may benefit smokers more than non-smokers because of the strong combined effects of smoking and radon."
The Environmental Protection Agency (EPA) recommends that radon levels in homes should be reduced to at least 4 picocuries per liter of air. Some 6 percent of U.S. homes have levels of radon above that. If the radon in these homes were reduced to meet the EPA guideline, the committee said, then about one-third of the radon-related lung cancer deaths that occur each year could be prevented. Although the majority of preventable deaths would be among smokers, perhaps 1,000 non-smokers also would avoid lung cancer.
Effects of Radon
Radon is produced from the radioactive decay of uranium that occurs naturally in rocks and soil. Outside air contains very low levels of radon, but indoors the gas builds up to higher concentrations. Although radon is chemically inert and electrically uncharged, it also is radioactive, which means that radon atoms in the air can spontaneously decay, or change to other atoms. When the resulting atoms are formed, they are electrically charged and can attach themselves to tiny dust particles in the air. The radiation given off by inhaled particles cannot travel far enough to reach cells in organs other than the lung, so it is likely that lung cancer is the only significant health hazard posed by radon. Many studies have linked the high incidence of lung cancer in underground miners with radon exposure, leading to concerns that exposure to the gas in homes could cause cancer. Most homes have far less radon than underground mines, but some do contain comparable amounts.
The committee found that for both smokers and non-smokers, the risk of developing lung cancer from radon is proportionate to the amount of exposure to radon. For example, doubling the exposure doubles the risk, and cutting the exposure in half cuts the risk in half. Radon might pose some risk even at very low levels, the committee said. No evidence exists that shows a threshold of exposure below which levels are harmless. Biological data suggest that most cancers originate from damage to a single cell. Even a very small amount of radon can produce alpha particles that penetrate cells, causing irreparable damage.
Uncertainties in Estimates
Problems in converting radon risks from mines to homes were underscored by the committee. Miners inhale many other substances on the job, making it difficult to separate the effects of radon from other factors that may influence lung cancer rates. Moreover, the miner population studied was all male.
The most direct way to assess the effects of radon in homes would be to compare the levels of radon exposure among people who have lung cancer with those who do not. Such studies have not yet provided a definitive answer, the committee said, because levels of radon exposure in most homes are extremely small, making it difficult to estimate the risks accurately. In addition, the combined effects of radon and smoking create a greater risk for people, but the joint effect could not be precisely defined by the committee. These uncertainties do not change the committee's findings that lung cancer in the general population, including in smokers, can be reduced by limiting exposure to radon.
Data should become stronger as molecular and cellular evidence accumulates, as the health of miners continues to be monitored, and as studies are done to measure directly the effects of radon exposure in the general population.
The study was funded by the Environmental Protection Agency. The National Research Council is the principal operating arm of the National Academy of Sciences and the National Academy of Engineering. It is a private, non-profit institution that provides independent advice on science and technology issues under a congressional charter. A committee roster follows.