"Residential radon exposure is an important environmental risk factor for lung cancer," says lead study author R. William Field, Ph.D., a research scientist with the department of epidemiology at the University of Iowa's College of Public Health. The study confirms radon is the second most important risk factor for lung cancer after smoking.

Radon is an invisible, odorless gas released when uranium in soil decays. The amount of uranium in soil varies greatly from area to area within the United States. Radon can enter buildings through holes in the foundation made for plumbing, or through cracks in the foundation, according to a National Cancer Institute report on radon gas.

Radon exposure levels were compared among 413 Iowa women diagnosed with lung cancer and 614 Iowa women of the same ages without lung cancer. Radon levels in the women's homes were measured, and they gave detailed information about how much time they had spent in various areas of the home. All of the women had lived in the same homes for at least 20 years.

The researchers found women who spent the equivalent of 15 years in a home with radon levels of four picoCuries per liter (pCi/L) of air were 50 percent more likely to develop lung cancer than the average person. Risk increased with rising radon levels. Four pCi/L is the level at which the Environmental Protection Agency (EPA) suggests taking action to lower radon concentrations.

To put the risk from domestic radon exposure into perspective, Dr. Field notes that, "The lung cancer risk from radon exposure pales in comparison to the increase in lung cancer risk from smoking."

Nonsmoking women exposed to domestic radon had a 50 percent higher lung cancer risk compared to unexposed women.  However, women who smoked for six months or a total of 100 cigarettes or more had a 1,200 percent increase in lung cancer risk compared to unexposed nonsmokers.

The risk of lung cancer was 13 times higher among women who had ever smoked (for six months or a total of 100 cigarettes or more) than among those who never smoked. Among those with lung cancer, 86 percent had smoked, compared to 32 percent of those without lung cancer.

The Iowa team's findings are in agreement with other findings on the subject, says another expert in the field. "Radon is the second most important cause of lung cancer behind smoking," says Jonathon M. Samet, M.D., chairman of the department of epidemiology at Johns Hopkins University School of Hygiene and Public Health and a member of the American Cancer Society (ACS) Advisory Committee on Environmental Factors and Cancer.

The combination of radon exposure and smoking is synergistic, according to Dr. Samet. "The risk is larger than the risk expected from just adding the two risks together," he says.

The EPA recommend testing all homes below the third floor for radon. If high levels are found, the agency advises using a government-certified local contractor to take steps to lower the concentrations. According to the EPA, there are a variety of ways to lower radon levels in your home. In some homes, sealing cracks in floors and walls may help to lower radon concentrations. In others, simple systems using pipes and fans may be used.

More information on radon testing and correcting any problems can be accessed toll-free from the EPA at 1-800-SOS-RADON or by visiting the EPA Web site on radon.

The ACS estimates there will be 164,100 new cases of lung cancer diagnosed in the United States in the year 2000, and expects 156,900 deaths this year from lung cancer. Cigarette smoking is the cause of 87 percent of all lung cancers, according to the ACS. Exposure to high concentrations of radon is estimated to be the cause of about 10 percent of all lung cancers.