Radon

What Is Radon?

Radon-222 is a colorless, odorless, radioactive gas. It forms from the decay of naturally occurring uranium-238, which is found in soil and rock throughout the world. Radon is present both outdoors and indoors. Exposure in homes mostly results from radon-contaminated gas rising from the soil. This makes it an unusual indoor air pollutant in that it has a natural source. Exposure to radon is also a known cause of lung cancer in underground miners of uranium and other ores. Thus, its presence in indoor air has raised concern that it may also be a cause of lung cancer for the entire population.

Radon has a half-life of 3.8 days; that is, half of a given amount takes 3.8 days to decay. It breaks down into a series of solid elements called radon progeny. Several of these elements (such as polonium-218, polonium-214, and lead-214) emit radiation in the form of alpha particles. An alpha particle consists of 2 protons and 2 neutrons, like the nucleus of a helium atom, and carries a positive charge. Although alpha particles do not penetrate deeply into tissue (as gamma radiation can), they carry enough energy to permanently change DNA if they reach the nucleus of a cell. As a result, when inhaled radon progeny reach the lungs, the alpha particles can damage cells within the airways and thereby increase lung cancer risk.

How Are People Exposed to Radon?

A link between cancer and working in underground mines was suspected even before radon was identified as an element. In 1556, German scholar Georgius Agricola wrote in De re metallica about the high death rates of miners in the Carpathian Mountains of Eastern Europe. More than 300 years later, autopsy studies of miners in that region revealed a common cause of death to be chest tumors, which were later shown to be primary lung cancer.

In the early 20th century, high levels of radon were found in mines in Germany and Czechoslovakia (now the Czech Republic), and researchers believed that this exposure led to the miners’ lung cancer. In the 1950s, radiation scientists recognized that tiny particles of radon progeny and not radon gas delivered the radiation dose that caused the cancer. Epidemiologic studies of radon-exposed miners during the 1950s and 1960s confirmed the connection between radon exposure and lung cancer (NRC, 1998).

A recent concern is whether radon exposure might also cause lung cancer in the general population. Adults and children are exposed to radon in homes, commercial buildings, schools, and other places. The radon gas emitted by soil or rock enters the buildings through cracks in floors or walls; construction joints; or gaps in foundations around pipes, wires, or pumps. Without ventilation or another way of dissipating, radon can build up and reach rather high levels.

Within buildings, radon levels are usually highest in the basement. This level is closest to the soil from which the radon-containing gas diffuses. Therefore, people who spend much of their time in basement rooms at home or at work would have a greater risk for exposure.

Radon emitted into outdoor spaces generally disperses and does not reach high levels. Indoor exposure is usually far lower than the occupational exposure for miners. However, the range of levels in the most contaminated homes approaches levels found in mines and may exceed the exposure standards permitted for underground miners.

Radon levels in the air are measured by units of radioactivity per volume of air. The most common concentration measure used is picocuries per liter (pCi/L). Background outdoor levels of radon range from near 0 to over 2 pCi/L. The US Environmental Protection Agency (EPA) has set an action level of 4 pCi/L as an annual average for homes and schools, and the National Council of Radiation Protection recommends a limit of no more than 8 pCi/L indoors.

Radon concentration may also be expressed in units of becquerels per cubic meter (Bq/m3) or as a working level (WL), a unit applied previously to underground mines. Exposure to radon or radon progeny incorporates the time spent at different concentrations; one commonly used unit for exposure is the working level month, which refers to spending 1 working month of time at a concentration of 1 WL.

The level of personal exposure to radon varies, depending on the concentration at home and any occupational exposure. Higher levels of radon exposure are more likely for people who:

* work in some underground mines (not only uranium but also some other types), in caves, or in uranium processing factories
* come in contact with phosphate fertilizers, which have high levels of radium (the immediate precursor of radon)
* live near uranium mines, although few facilities are now operating

Radon exposure can occur through drinking water, but this type of exposure is minimal. As the radon moves from the water to air, it can be inhaled. Water that comes from deep, underground wells in rock with high radium concentrations may have high levels of radon, whereas surface water (drawn from lakes or rivers) usually has very low radon levels. For the most part, water does not contribute much to the overall exposure to radon.

Radon exposure can also occur from some building materials if they are made from radon-containing substances. Specific instances of higher levels have been linked to particular building materials. For example, in Sweden, wallboard made with phosphogypsum having a high concentration of radium led to elevated concentrations of radon. Such materials usually contribute significantly to radon exposure.

The potential of the soil to release radon and contaminate homes depends on the concentration of radium and the characteristics of the earth. As a result, radon levels vary greatly in different parts of the United States, even within small geographic regions. As shown by the US Geological Survey map (see Figure), an area in the mid-Atlantic states stretching from New York through Pennsylvania to Maryland and Virginia, as well as a broad stretch of the upper Midwest, has geological formations that yield higher radon levels. In contrast, radon levels are low in the Southeast as far west as Texas and along much of the West coast. However, elevated radon levels have been found in almost every state.

The EPA estimates that as many as 8 million homes in the United States have elevated radon levels (EPA, 1998). The EPA posts detailed geographic information on radon levels, including state maps, on its Web site at www.epa.gov/iaq/radon/zonemap.html. The US Geological Survey publishes a series of geologic radon potential books that provide detailed geographic information, also available at energy.cr.usgs.gov/radon/radonhome.html

Does Radon Cause Cancer?

Radon is among the best studied of environmental carcinogens. Through epidemiologic, animal, and laboratory studies, we’ve come to understand how inhaling radon progeny can deliver a radiation dose to the lungs and how alpha particles emitted by radon progeny can damage the cells. We’ve also learned more about the degree of lung cancer risk associated with radon exposure. This evidence is summarized in Health Effects of Exposure to Radon, the 1998 report of the National Research Council’s Committee on the Biological Effects of Ionizing Radiation (BEIR) VI, often called the BEIR VI report.

What Do Epidemiologic Studies Show?

The epidemiologic evidence on radon and lung cancer risk comes from (1) cohort studies of underground miners with rather high levels of radon exposure, (2) case-control studies that compare radon exposure of persons with lung cancer and appropriate controls from the general population, and (3) ecologic studies comparing lung cancer deaths or incidence across geographic areas with differing levels of radon exposure. The cohort studies provided the first evidence showing that radon is a cause of lung cancer. These studies have also been used for estimating the risk of radon exposure. Once the problem of indoor radon was widely recognized, the case-control and ecologic studies were initiated.

Scientists have studied many groups of underground miners exposed to radon, including uranium miners in Czechoslovakia, France, Canada, Australia, and the US; fluorspar miners in Canada; iron miners in Sweden; and tin miners in China. To estimate lung cancer risk, researchers gathered data from 11 cohort studies, including 68,000 miners with 2,700 deaths from lung cancer. They looked at how lung cancer risk varies with exposure to radon and other factors, including cigarette smoking. At all but the highest levels, risk increases as exposure increases. In those miners known to be nonsmokers, a direct relationship between lung cancer risk and exposure also exists. These studies of miners provide good information on the relationship between increasing radon exposure and lung cancer risk at high levels of exposure.

However, average radon concentrations in the indoor air of most homes are considerably lower than those in uranium mines. The EPA set recommended levels for maximum concentration of radon in homes based on extrapolation from high occupational exposures to the generally much lower residential exposures. Researchers have subsequently confirmed the reliability of these risk estimates by combining the results of multiple studies of indoor radon exposures. The observed risk of lung cancer from indoor radon exposures approximates that which was predicted by studies of uranium miners exposed at much higher doses.

In 1998, the Biological Effects of Ionizing Radiation (BEIR VI) Committee of the National Research Council estimated that between 10% and 14% of lung cancer deaths in the US could be attributed to radon. Most of the radon-related lung cancers occur among smokers. However, an estimated 2,100-2,900 of the 11,000 deaths from lung cancer among nonsmokers in the United States each year are estimated to be radon-related.

What Do Animal and Laboratory Studies Suggest?

Like the studies done with miners, experimental studies on animals have clearly shown a risk of lung cancer with exposure to radon. These animal studies revealed that breathing in radon and its progeny (chemical products of radon decay) significantly increased the incidence of respiratory tract tumors. Nonspecific effects on the lungs have also been reported. In laboratory studies using human cells, radon and its decay products induced chromosomal abnormalities and other signs of permanent cellular change.

What Do the Experts Say?

The National Toxicology Program (NTP) evaluates exposures that may be carcinogenic. Exposures that are thought to cause cancer are included in the Reports on Carcinogens, published every 2 years. Each exposure is assigned to 1 of 2 categories: "known to be human carcinogens" or "reasonably anticipated to be human carcinogens." The first category includes substances for which human studies (epidemiologic studies or experimental studies) provide "sufficient evidence" of carcinogenicity in humans. The second category includes substances for which there is limited evidence of carcinogenicity in humans or sufficient evidence of carcinogenicity in experimental animals. Using this scheme, the NTP classifies radon as a "known human carcinogen.”

The International Agency for Research on Cancer (IARC) also evaluates exposures that may be carcinogenic. IARC classifies exposures into 1 of 4 categories:

* Group 1 exposures are those "known to be carcinogenic to humans," usually based on "sufficient" human evidence but sometimes based on "sufficient" evidence in experimental animals and "strong" human evidence.

* Group 2 exposures are divided into 2 categories; Group 2A ("probably carcinogenic to humans") has stronger evidence, and Group 2B ("possibly carcinogenic to humans") has weaker evidence.

* Group 3 exposures are not considered classifiable because available evidence is limited or inadequate.

* Group 4 exposures are "probably not carcinogenic to humans," based on evidence suggesting lack of carcinogenicity in humans and in experimental animals.

IARC rates radon as "carcinogenic to humans" (Group 1).

The Environmental Protection Agency (EPA), through its Integrated Risk Information System, uses a classification scheme similar to that of IARC. It classifies exposures into 1 of 5 categories:

* (A) human carcinogen
* (B) probable human carcinogen
* (C) possible human carcinogen
* (D) not classifiable as to human carcinogenicity
* (E) evidence of noncarcinogenicity for humans

The EPA has not classified radon as to its carcinogenicity.

The Agency for Toxic Substances and Disease Registry (ATSDR) has concluded that radon is carcinogenic.

Does Radon Cause Any Other Health Problems?

There is convincing evidence that radon causes lung cancer. Some studies of miners further suggest a connection between radon exposure and nonmalignant (non-cancerous) respiratory disease, especially pulmonary fibrosis (scar tissue formation in the lungs that leads to shortness of breath). These effects appear primarily in miners with high levels of exposure. The patterns seen on radiographs are not typical of silicosis (a lung disease caused by overexposure to crystalline silica), which is also a problem in the uranium miners.

Some evidence has also linked radon exposure with malignancies other than lung cancer. An analysis of the 11 cohort studies of miners found an increase in leukemia and in cancers of the stomach and liver. Risk for these cancers in the miners does not increase with the level of exposure, and these associations do not appear to be causal. Ecologic studies have linked radon with elevations of cancers other than lung cancer but without any consistent pattern. A recent case-control study of acute lymphocytic leukemia did not show an increased risk linked to radon exposure. Because radon is absorbed mainly through inhalation, and because alpha particles penetrate tissues only superficially, effects on tissues other than lung tissue would not be expected.

What Should I Do If I’ve Been Exposed to Radon?

Concerned patients may ask their doctor about medical tests for radon exposure. At present, no test is sensitive enough to determine past levels of exposure. Researchers are evaluating some methods for estimating past exposure (for example, special counting of levels of radiation in the skull), but these lack the needed sensitivity for most exposures in the general population.

For people exposed to higher levels of radon, it is especially important to quit smoking. Evidence has shown that the combined effect of cigarette smoking and radon exposure can cause lung cancer. For miners, the synergy between smoking and radon exposure has resulted in extremely high health risks.

For those miners whose health was compromised by exposure to radon, the United States has established a national compensation approach through the Radiation Exposure Compensation Act, passed in 1990 (Radiation Exposure Compensation Act Committee) and amended in 2000. The act began with an apology to the uranium miners and offered compensation to miners for lung cancer and selected nonmalignant lung diseases, if selected criteria were met. Health care professionals should be aware of the availability of this compensation for eligible persons.

How Can I Avoid Exposure to Radon?

Radon exposure in homes can be assessed readily. You can purchase radon detection kits in hardware or home supply stores. Some common types of detectors include charcoal canisters, alpha track devices, electret ion chambers, continuous monitors, and charcoal liquid scintillation detectors. Do-it-yourself kits are placed in the home for several days or for as long as 3 months and then mailed to a laboratory for analysis. Alternatively, you can hire private contractors to perform this testing. Qualified contractors can be located through state radon offices, which are listed on the EPA web site at www.epa.gov/iaq/contacts.html.

The EPA recommends remediation if test results show radon levels above 4 pCi/L. This value refers to the annual average, and the short-term tests often used at the time of sale of a home tend to give measurements that are biased upward from the actual value if the EPA protocol is followed. If values exceed the guideline, you should perform a repeat measurement, generally using one of the longer-term devices if possible.

Remediation can be performed by the homeowner, depending on the approach, or by a commercial contractor. A variety of methods can be used to reduce radon levels in your home, such as sealing cracks in floors and walls or increasing ventilation through "sub-slab depressurization" using pipes and fans. Again, qualified contractors can be located through state radon offices, which are listed on the EPA web site at www.epa.gov/iaq/contacts.html.

Certain building materials may be more "radon tight" and may help reduce exposure in areas where radon levels are high. You can obtain further information from state radon offices or from qualified contractors.

The Bottom Line

Radon is a naturally occurring environmental carcinogen. It is estimated that radon exposure accounts for between 3,000 and 33,000 lung cancer deaths in the United States each year, with central estimates of 15,400 or 21,800 (depending on the model used). This makes radon the second-leading cause of lung cancer after cigarette smoking, although cigarettes account for far more cases than radon.

Most radon-induced cases of lung cancer occur in smokers, reflecting synergy between smoking and radon exposure. Quitting smoking is an essential part of prevention, but the estimates of radon-caused lung cancer are also substantial.

Radon-induced lung cancer can be prevented by reducing radon levels in homes and other buildings. Approximately one-third of radon-induced lung cancer could be avoided if homes with radon concentrations exceeding 4 pCi/L (the EPA action level) underwent changes to reduce radon concentrations to below that level. However, eliminating all radon exposure is not possible.

Additional Resources

National Organizations and Web Sites

In addition to the American Cancer Society, other sources of patient information and support include*:

Environmental Protection Agency (EPA)
Internet Address: http://www.epa.gov/iaq/radon/
Useful EPA publications:
Radon: A Physician’s Guide (http://www.epa.gov/iaq/radon/pubs/physic.html)
A Citizen’s Guide to Radon: The Guide to Protecting Yourself and Your Family from Radon (http://www.epa.gov/iaq/radon/pubs/citguide.html)

Federal Radiation Exposure Compensation Program
Information available at: http://www.usdoj.gov/civil (click "Radiation Exposure Compensation Program" from the menu)

United States Geological Survey
Internet Address: http://www.usgs.gov/
General background on radon ("Radon in Earth, Air, and Water") and
maps of radon exposure potential available at:
http://energy.cr.usgs.gov/radon/radonhome.html

*Inclusion on this list does not imply endorsement by the American Cancer Society

Other Publications*

Axelson O. Cancer risk from exposure to radon in homes. Environ Health Perspect. 1995;103 :37-43.

Boice JD, Lubin JH. Occupational and environmental radiation and cancer. Cancer Causes Control 1997;8:309-322.

Lubin JH, Boice JD Jr, Edling C, et al. Lung cancer in radon-exposed miners and estimation of risk from indoor exposure. J Natl Cancer Inst. 1995;87:817-827.

Samet J. Radon and lung cancer. J Natl Cancer Inst. 1989;81:745-757.