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Am Fam Physician. 2000;62(5):950-951

Family physicians often must counsel patients about preventive measures that may help prevent disease or prolong life. Lung cancer continues to be the leading cause of cancer deaths. While we know that cigarette smoking causes lung cancer, many physicians are not aware of the hidden threat posed by naturally occurring radon gas.

Results of several large epidemiologic studies of uranium miners exposed to radon,1 along with experimental data,2 have led the U.S. Surgeon General to warn that radon is the second leading cause of lung cancer.3 The World Health Organization and the National Academy of Sciences agree that radon is a known human carcinogen.3

Radon gas is a byproduct of naturally occurring uranium decay. Odorless, tasteless and invisible, it seeps into homes through cracks in foundations, pores in walls, floor drains or any areas exposed to the ground. Differences in pressure create a vacuum effect between buildings and the ground that draws radon into homes, where it can accumulate undiluted by outdoor air. The amount of radon in the soil beneath the house and the design of the house itself contribute to the variation of indoor radon levels. Knowing the radon level in a home is not possible without testing. Levels can vary widely, even between houses directly adjacent to each other.

Radon may cause cancer in smokers and nonsmokers, but the risk to smokers is much greater. The risk of developing lung cancer is proportionate to the amount of exposure (Table 1), with higher levels posing a higher risk. It is estimated that radon causes up to 30,000 deaths per year in the United States.3

Radon levelSmokers*Never-smokers
2 pCi per L151
4 pCi per L292
8 pCi per L573
10 pCi per L714
20 pCi per L1358

Since 1986, the U.S. Environmental Protection Agency (EPA) and the Surgeon General have recommended that all homes be tested for radon. The American Medical Association supports this recommendation. The average indoor level of radon in United States homes is 1.25 picocuries per L (pCi per L). Millions of homes in the United States are estimated to have a high radon level. If the measured level of radon in a home is greater than 4 pCi per L, it is recommended that a radon mitigation system be installed. A mitigation system vents radon from beneath the house to the outside of the home. Installation typically costs from $500 to $2,500.

Radon is a known carcinogen and may pose a serious health threat. Physicians may be the only science professionals that patients contact. It may not be cost effective to install a mitigation system in every home, but patients should be educated about the potential dangers of radon so that they are able to make informed decisions. Smokers should be made aware that they are at particularly high risk.

Of course, smoking cessation remains the most effective way to reduce the incidence of lung cancer. More information about radon can be found at the EPA Web site ( In addition, the EPA will supply a free booklet codeveloped by the American Medical Association entitled, “Radon: The Health Threat with a Simple Solution: A Physician's Guide.”3

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