Radon is a worldwide health risk in homes. Most radon-induced lung cancers occur from low- and medium-dose exposures in people's homes. Radon is the second most important cause of lung cancer after smoking in many countries.

• Lung cancer kills thousands of Americans every year. Smoking, radon, and secondhand smoke are the leading causes of lung cancer. Although lung cancer can be treated, the survival rate is one of the lowest among all those with cancer. From the time of diagnosis, between 11 and 15% of those afflicted will live beyond five years, depending upon demographic factors. In many cases, lung cancer can be prevented.
   
• Smoking is the leading cause of lung cancer. Smoking causes an estimated 159,000 cancer deaths in the U.S. every year (American Cancer Society, 2014).  And the rate among women is rising. On January 11, 1964, Dr. Luther L. Terry, then U.S. Surgeon General, issued the first warning on the link between smoking and lung cancer. Lung cancer now surpasses breast cancer as the number one cause of death among women. A smoker who is also exposed to radon has a much higher risk of lung cancer.
   
• Radon is the number-one cause of lung cancer among non-smokers, according to EPA estimates.  Overall, radon is the second-leading cause of lung cancer. Radon is responsible for about 21,000 lung cancer deaths every year. About 2,900 of these deaths occur among people who have never smoked. On January 13, 2005, Dr. Richard H. Carmona, the U.S. Surgeon General, issued a national health advisory on radon. 
   
• Secondhand smoke is the third-leading cause of lung cancer and responsible for an estimated 3,000 lung cancer deaths every year. Smoking affects non-smokers by exposing them to secondhand smoke.  Exposure to secondhand smoke can have serious consequences for children’s health, including asthma attacks, effects on the respiratory tract (e.g., bronchitis, pneumonia), and possibly causing ear infections.

The following websites provide a wide range of comprehensive information about lung cancer, prevention, and treatment:

• American Cancer Society: www.cancer.org 
• American Lung Association: http://www.lungusa.org/
• National Cancer Institute: http://www.nci.nih.gov/ 
• Vanderbilt-Ingram Cancer Center: http://www.vicc.org/
• Memorial Sloan-Kettering Cancer Center: http://www.mskcc.org/mskcc/html/44.cfm

Studies Find Direct Evidence Linking Radon in Homes to Lung Cancer

(2005) Studies show definitive evidence of an association between residential radon exposure and lung cancer. Two studies, a North American study and a European study, both combined data from several previous residential studies. These two studies go a step beyond earlier findings — they confirm the radon health risks predicted by occupational studies of underground miners who breathed radon for a period of years. Early in the debate about radon-related risks, some researchers questioned whether occupational studies could be used to calculate risks from exposure to radon in the home environment. “These findings effectively end any doubts about the risks to Americans of having radon in their homes,” said Tom Kelly, Former Director of EPA’s Indoor Environments Division. “We know that radon is a carcinogen. This research confirms that breathing low levels of radon can lead to lung cancer.”

• Abstract of the pooling of North American Residential Radon studies (PDF)
  
• Abstract of the pooling of the European Residential Radon studies (PDF)

World Health Organization Launches International Radon Project

(2009) The World Health Organization (WHO) says radon causes up to 15% of lung cancers worldwide. In an effort to reduce the rate of lung cancer around the world, the World Health Organization (WHO) launched an international radon project to help countries increase awareness, collect data and encourage action to reduce radon-related risks. The U.S. EPA is one of several government agencies and countries supporting this initiative and is encouraged by WHO’s attention to this important public health issue. "Radon poses an easily reducible health risk to populations all over the world, but has not up to now received widespread attention," said Dr. Michael Repacholi, coordinator of WHO’s Radiation and Environmental Health Unit. He went on to say, "Radon in our homes is the main source of exposure to ionizing radiation, and accounts for 50% of the public’s exposure to naturally occurring sources of radiation in many countries."

Surgeon General Releases National Health Advisory on Radon

(January 13, 2005) U.S. Surgeon General Richard H. Carmona issues a health advisory warning Americans about the health risk from exposure to radon in indoor air. The Chief Physician urged Americans to test their homes to find out how much radon they might be breathing in. Dr. Carmona also stressed the need to remedy the problem as soon as possible when the radon level is 4 pCi/L or more, noting that more than 20,000 Americans die of radon-related lung cancer each year.

How does radon induce cancer?

If inhaled, radon decay products (polonium-218 and polonium-214, solid form), unattached or attached to the surface of aerosols, dusts and smoke particles become deeply lodged in the lungs, where they can radiate and penetrate the cells of mucous membranes, bronchi, and other pulmonary tissues. The ionizing radiation energy affecting the bronchial epithelial cells is believed to initiate the process of carcinogenesis. Although radon-related lung cancers are mainly seen in the upper airways, radon increases the incidence of all histological types of lung cancer, including small-cell carcinoma, Aden carcinoma, and squamous cell carcinoma. Lung cancer due to inhalation of radon decay products constitutes the only known risk associated with radon. In studies done on miners, variables such as age, duration of exposure, time since initiation of exposure, and especially the use of tobacco have been found to influence individual risk. In fact, the use of tobacco multiplies the risk of radon-induced lung cancer enormously.

What is the evidence?

More is known about the health risk of radon exposure to humans than about most other human carcinogens. This knowledge is based on extensive epidemiological studies of thousands of underground miners, carried out over more than 50 years worldwide, including on miners in the United States and Canada. In addition to the data on miners, experimental exposures on laboratory animals confirm that radon and its decay products can cause lung cancer.

Human Studies and Animal Studies

Research on lung cancer mortality in miners exposed to radon progeny is substantial and consistent. Studies of thousands of miners, some with follow-up periods of 30 years or more, have been conducted in metal, fluorspar, shale, and uranium mines in the United States, Canada, Australia, China, and Europe. These studies have consistently shown an increase in the occurrence of lung cancer with exposure to radon decay products, despite differences in study populations and methodologies.

The miner studies detailed the following findings:

•    At equal cumulative exposures, low exposures in the range of the EPA's 4 pCi/L action level over longer periods produced a greater risk of lung cancer than high exposures over short periods.

•    Increased lung cancer risk with radon exposure has been observed even after controlling for, or in the absence of, other exposure risks, such as asbestos, silica, diesel fumes, arsenic, chromium, nickel, and ore dust.

•    Non-smoking miners exposed to radon have been observed to have an increased risk of lung cancer.

Animal experiments conducted by the U.S. Department of Energy's Office of Energy Research, as well as those conducted in France, have confirmed the carcinogenicity of radon, and have provided insight into the nature of the exposure-response relationship, as well as the modifying effects of the exposure rates.

To date, these animal studies have produced several relevant findings for humans:

•    Health effects observed in animals exposed to radon and radon decay products include lung carcinomas, pulmonary fibrosis, emphysema, and a shortening of lifespan.

•    The incidence of respiratory tract tumors grew with an increase in cumulative exposure coupled with a decrease in rate of exposure.

•    Increased incidence of respiratory tract tumors was observed in rats at cumulative exposures as low as 20 WLM.

•    Exposure to ore dust or diesel fumes simultaneously with radon did not increase the incidence of lung tumors above that produced by radon progeny exposures alone.

•    Lifetime lung-tumor risk coefficients that have been observed in animals are similar to the lifetime lung-cancer risk coefficients observed in human studies.

•    In a study of rats exposed to radon progeny and uranium ore dust simultaneously, it was observed that the risk of lung cancer was elevated at exposure levels similar to those found in homes. The risk decreased in proportion to the decrease in exposure to radon progeny.

In 1988, a panel of experts convened by WHO's International Agency for Research on Cancer unanimously agreed that there is sufficient evidence to conclude that radon causes cancer in humans and in laboratory animals. Scientific committees assembled by the National Academy of Sciences (NAS), the International Commission on Radiological Protection (ICRP), and the National Council on Radiation Protection and Measurement (NCRP) also have reviewed the available data and agreed that radon exposure causes human lung cancer.

Recognizing that radon is a significant public health risk, scientific and professional organizations such as the American Medical Association, the American Lung Association, and the National Medical Association have developed programs to reduce the health risks of radon. The National Institute for Occupational Safety and Health (NIOSH) reviewed the epidemiological data and recommended that the annual radon progeny exposure limit for the mining industry be lowered.

Is occupational exposure to radon comparable to residential exposure?

Because questions have been raised about the appropriateness of using the epidemiological studies of underground miners as a basis for estimating the risk radon poses to the general population, the EPA commissioned the NAS to investigate the difference between underground miners and members of the general public in the doses they receive per unit-exposure due to inhaled radon progeny.

The NAS report, published in 1991, concluded that it is reasonable to extrapolate from the miner data to a residential situation, but that the effective doses per unit of exposure for people in their homes are approximately 30% less than for the miners. In its analysis, the NAS considered variables such as the amount and types of dust to which the radon decay particles would attach, the breathing rates of working miners compared to that of people at home, and the presence of women and children in the homes.

The EPA has adjusted its residential risk estimates accordingly. The result is still considerable: it now estimates that approximately 14,000 lung cancer deaths in the U.S. annually are due to residential radon exposures. As more data are gathered, the risk estimates may be adjusted further. Enough statistical evidence exists now, however, to state with certainty that each year in the United States, thousands of deaths due to preventable lung cancer are attributable to indoor residential exposure to radon.

More information is needed to answer important questions about radon's effect on women and children — two groups not included in the occupational studies. Although children have been reported to be at greater risk than adults for developing certain types of cancer from radiation, there is no current or conclusive evidence that radon exposure puts children at a greater risk. Some studies on miners and on animals indicate that, for the same total exposure, a lower exposure over a longer period is more hazardous than brief, high exposures. These findings increase concerns about residential radon exposures. Epidemiological control studies are underway in the U.S. and in Europe, the pooled results of which should enhance the understanding of the risk of residential exposure to radon.

What about smoking and radon exposure?

Some people ask whether the lung cancer deaths attributed to radon exposure actually may be the result of smoking. A 1989 study by researchers from NIOSH, the Centers for Disease Control, the Harvard School of Public Health, and the University of California at Davis demonstrated a greatly increased risk of lung cancer in non-smoking uranium miners exposed to high radon concentrations. Compared to typical non-smoking populations, these miners had nine to 12 times the risk of developing lung cancer.

Evidence from some of the epidemiological studies of American underground uranium miners indicates that radon exposure and smoking may have a synergistic relationship. Either smoking or radon exposure can independently increase the risk of lung cancer; however, exposure to both greatly enhances that risk.

Your chances of getting lung cancer from radon depend mostly on:

•    how much radon is in your home;
•    the amount of time you spend in your home; and
•    whether you are a smoker, or have ever smoked.