Radon is a radioactive gas that forms naturally from the decay of the radioactive metals uranium, thorium, and radium in rocks and soil. Radon is the second leading cause of lung cancer in the United States.


Radon (Rn)—number 86 on the Periodic Table of the Elements—is a colorless, odorless, tasteless gas formed from the radioactive decay of heavier elements present in small amounts in almost all air and soil, as well as rocks, groundwater, and surface water. Uranium and thorium atoms undergo about a dozen decay steps, emitting radiation at each step and forming different elements with different radioactive properties. Radium and then radon are formed midway through these decay chains. As a noble gas, radon does not form chemical bonds, enabling it to travel freely from soil and rocks into air and water.

Outdoor air and river and lake water normally have only very low levels of radon; thus, most human exposure is from breathing in radon in the air that enters homes and buildings through cracks and gaps. Exposure also can occur by breathing in or swallowing radon-contaminated dust. Underground water, such as well water, also may have higher radon levels.


For hundreds of years before the discovery of radon gas in 1900, it was known that miners in some parts of the world had high death rates from lung disorders, but it was not until the 1950s and 1960s that studies revealed high rates of lung cancer in underground uranium miners. In the past, radon was used to treat cancer, diabetes, arthritis, and ulcers. It is still used in chemical reactions, for studying atmospheric transport and surface reactions, predicting earthquakes, detecting leaks, and searching for petroleum and uranium deposits.


Sources of radon and its progeny include:

Indoor radon levels depend on characteristics of the underlying soil and rock. Because indoor air pressure is usually lower than pressure in soil beneath and around foundations, most indoor radon is sucked inside from cracks and pores in the soil underneath buildings through the following:

Risk Factors

Everyone is exposed to some level of radon in air and water. The major risk to the general public is indoor air in homes and buildings built on soil that is high in uranium, thorium, and radium, especially well-insulated, tightly sealed homes. Radon levels are usually highest in crawl spaces, basements, and first floors close to the ground.

The risk of lung cancer depends on the degree of exposure to higher radon levels. Smokers exposed to radon are at highest risk, and the majority of radon-related lung cancers occur in smokers. Burning fuels that emit indoor particles—such as wood or coal—in homes with higher radon is also a risk factor for lung cancer.

Living or working underground in areas with high levels of uranium and radium in rocks is a major risk factor. Workplaces with higher radon-exposure risk include:


Radon levels vary greatly among regions of the United States and the world; levels can vary even within neighborhoods and in individual homes over the course of hours or days. According to the World Health Organization (WHO), average outdoor radon levels are 5–15 Becquerels (Bq)/m3 of air. In buildings such as homes, schools, and offices, levels range from 10–10,000 Bq/m3.

Elevated radon levels occur in every U.S. state. According to the Environmental Protection Agency (EPA), the average outdoor level is about 0.4 picocuries (pCi)/L (15 Bq/m3), and the average indoor level is about 1.3 pCi/L (48 Bq/m3). Almost 1 in every 15 U.S. and Canadian homes has elevated radon levels.

Causes and symptoms

Some radon and radon-progeny decays emit high-energy alpha particles that can lodge in the lining of the lungs, continuing to emit radiation that damages cells and potentially leading to lung cancer. High-level radon exposure can cause other diseases such as thickening of lung tissue and emphysema. However, it can take many years before the health effects of radon become evident.

Some studies have suggested that radon may be associated with other cancers, such as childhood and adult leukemias, but evidence is mixed. Since radon and its progeny enter the body primarily via inhalation, and the radiation travels only a short distance, it seems unlikely that tissues other than the lungs would be affected. Radon is nonirritating, and ingestion of radon progeny on dust particles or in contaminated groundwater has not been shown to be either toxic or carcinogenic.

Symptoms of lung cancer can include:


Radon exposure is diagnosed by radon levels in the air, since routine testing does not detect it in human tissues. Some radon progeny can be detected in urine or lung and bone tissue, but these tests neither indicate the amount of exposure nor predict future health effects. Certain specialized techniques can more precisely measure radon exposure over time. People exposed to high radon levels over long periods may need regular checkups and tests for signs of possible lung cancer.


Accidental exposure to very high radon levels may require protecting the individual from further exposure and possibly decontamination. Emergency basic or advanced life-support procedures may be required, including establishing an open airway, assisted ventilation, or oxygen administration. Treatment for shock, seizures, or coma may be necessary.

Radon-induced lung cancer or possible radon-induced skin cancer is treated as similar cancers from other causes. Radiation-induced cataracts have occurred following implantation of gold radon seeds near the eyes, and these are treated as similar types of cataracts.

Public health role and response

The World Health Organization (WHO) recommends that radon exposure be limited to 100 Bq)/m3 (2.7 pCi/L) of air; if that is not possible, levels should not exceed 300 Bq/m3 (8.1 pCi/L). Since 2009, WHO has recommended inclusion of radon prevention in building codes for new construction, mitigation to bring levels within standards, and development of better measurement protocols. The United States and many European countries routinely include protective measures in new buildings, and in some countries inclusion is mandatory.

The U.S. Occupational Safety and Health Administration, the Mine Safety and Health Administration, and the American Conference of Governmental Industrial Hygienists have an exposure limit of four working level months (WLMs) per year for workers exposed to radon progeny. WLMs are a combination of radon progeny concentration in mine air and the length of time spent inside the mine. Underground mines have mechanisms for lowering radon levels. The Nuclear Regulatory Commission also sets workplace radon exposure limits. The Radon Exposure Compensation Program provides lump-sum payments to uranium miners, millers, and transporters who develop lung cancer and certain other lung diseases.

Alpha particle—
A helium nucleus (two protons and two neutrons) emitted during radioactive decay.
Becquerel (Bq)—
A standard radiation unit defined as the radioactive disintegration or decay (transformation) of one atomic nucleus per second.
Destructive changes in the lungs that begin with breathlessness during exertion and progress to continuous shortness of breath.
The time required for the radioactivity of a specific isotope to decrease by one-half.
Picocurie (pCi)—
A measure of radioactivity equivalent to 2.2 disintegrations of radium per minute.
Radioactive decay—
Radioactivity; radiation, such as an alpha particle, that is emitted as an unstable atomic nucleus loses energy.
Radium (Ra)—
Element 88 on the Periodic Table that can decay (transform) into radon.
Radon progeny—
Radon daughters; radon decay products.
Thorium (Th)—
Element 90 on the Periodic Table that can decay (transform) into radon.
Uranium (U)—
Element 92 on the Periodic Table that can decay (transform) into radon.
Working level months (WLMs)—
A measure used for allowable workplace exposure to hazardous substances such as radon.

The Environmental Protection Agency (EPA) funds the National Radon Program Services and collaborates with the Centers for Disease Control and Prevention on public and professional radon education. However as of 2018, President Trump had proposed eliminating the EPA Indoor Radon Program and State Indoor Radon Grants.

All U.S. states have radon public-information programs. As of 2018, 39 states and the District of Columbia required disclosure, testing, and/or mitigation of radon levels for real-estate transactions. Four states required landlords to provide tenants with radon-testing information. Maine required landlords to test residential buildings every ten years and to fix any with high radon levels. Twenty-four states and the District of Columbia required licensing or certification of radon home-testing and mitigation professionals; some states required reporting of test results. Building codes in a few states and locales required radon-resistant construction.


No radon level is 100% safe. However, hundreds of thousands of homes are built each year in potentially high-radon areas without any controls, and millions of homes with high radon levels are not being fixed. It has been estimated that U.S. lung-cancer deaths from lung cancer could be reduced by 2%–4% by lowering home radon levels below 4.0 pCi/L. The U.S. Congress has set a long-term goal of indoor levels equal to outdoor levels. Although as of 2018 this was not achievable, most homes could be reduced to at least 2 pCi/L.


Although it is impossible to completely avoid radon exposure, testing and mitigation can prevent exposure to dangerous levels. The EPA has a map showing potentials for elevated indoor radon at https://www.epa.gov/radon/epa-map-radon-zones .


The EPA and the U.S. Surgeon General recommend radon testing of every home and office and of all homes before they are sold, since homes next to each other can have very different radon levels. Even homes built to radon-resistant standards should be tested, since it is easier and cheaper to further reduce levels in homes built to be radon-resistant. In addition, people should not smoke or allow smoking in their homes.


Improving ventilation or changing indoor/outdoor air-pressure differentials are generally effective for lowering radon. Passive mitigation systems can lower levels by more than 50%, whereas sophisticated systems can reduce levels by up to 99%. Mitigation can include:

Mitigation costs vary, but most homes can be fixed for about the cost of other common repairs. Homes should be retested after mitigation.

Although radon from soil is usually a much greater risk than from a water, private well water can be tested, and point-of-entry treatment can remove radon before it enters the home. Point-of-use removal at the tap is not effective in reducing radon released into the air from all water in the home.

New construction

New homes in the EPA Zone 1 should be built with radon-resistant features that can also reduce other soil gases, moisture problems, and energy costs. If radon levels are high after construction, a fan can be added.

Although techniques vary depending on the site and foundation, radon-resistant features include:

See also Cancer ; Sick building syndrome .



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American Cancer Society, 250 Williams St. NW, Atlanta, GA, 30303, (800) 227-2345, https://www.cancer.org .

Environmental Protection Agency, 1200 Pennsylvania Ave. NW, Washington, DC, 20460, https://www.epa.gov .

National Cancer Institute, BG 9609, MSC 9760, 9609 Medical Center Dr., Bethesda, MD, 20892-9760, (800) 4-CANCER (422-6237), https://www.cancer.gov .

National Institute of Environmental Health Sciences, PO Box 12233, MD K3-16, Research Triangle Park, NC, 27709-2233, (919) 541-3345, Fax: (301) 480-2978, webcenter@niehs.nih.gov, https://www.niehs.nih.gov .

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National Radon Safety Board, 14 Hayes St., Elmsford, NY, 10523, (914) 345-1168, Fax: (914) 345-1169, (866) 329-3474, info@nrsb.org, http://www.nrsb.org .

World Health Organization, Avenue Appia 20, 1211 Geneva, Switzerland, 27, 41 22 791 21 11, Fax: 41 22 791 31 11, http://www.who.int/en .

Margaret Alic, PhD

  This information is not a tool for self-diagnosis or a substitute for professional care.