Radon is harmful to health and increases the risk of lung cancer. It is a radioactive, colorless, odorless, and tasteless noble gas. Scientists and medical experts have studied radon to understand its effects on health. Radon forms naturally as a decay product of radium. It is one of the densest gases that remains a gas under normal conditions and is considered a health hazard because of its radioactivity. The most stable form of radon is radon-222, which has a half-life of 3.8 days. Because of its high radioactivity, chemists have studied radon less than other elements, but some compounds of radon are known.

Radon-222 is part of the uranium decay chain, which is the process of uranium-238 breaking down into lead-206. Uranium has existed since Earth was formed, and its most common isotope has a very long half-life of 4.5 billion years. This means uranium and radon will remain in the environment for millions of years at similar levels.

Radon is the main source of public exposure to ionizing radiation. It is often the largest contributor to a person's background radiation dose and varies widely depending on location. Radon gas from natural sources can build up in buildings, especially in enclosed spaces like attics and basements. It can also be found in some spring and hot spring waters.

According to a 2003 report by the United States Environmental Protection Agency, studies show a clear link between high radon levels and lung cancer. The report estimates that 21,000 lung cancer deaths in the U.S. each year are caused by radon exposure, making it the second leading cause of lung cancer after smoking. In areas where radon levels are higher, it is considered a major indoor air pollutant.

Occurrence

Radon levels in the air are usually measured in becquerels per cubic meter (Bq/m³), which is a unit used in science. For example, typical indoor radon levels are about 100 Bq/m³, and outdoor levels are usually between 10 and 20 Bq/m³. In the United States, radon is often measured in picocuries per liter (pCi/L), with 1 pCi/L equal to 37 Bq/m³.

The mining industry uses a different system to measure radon exposure. They use the working level (WL) and working level months (WLM). One WL is the amount of radon decay products in 1 liter of air that releases 1.3 × 10⁶ MeV of energy. This is equal to 2.08 × 10⁶ joules per cubic meter (J/m³). The SI unit for cumulative exposure is joule-hours per cubic meter (J·h/m³). One WLM is equal to 3.6 × 10⁶ J·h/m³. If someone is exposed to 1 WL for 170 hours (about one month of work), this equals 1 WLM.

A cumulative exposure of 1 WLM is similar to living one year in an area with radon levels of 230 Bq/m³.

Radon (Rn) in the air decays into lead (Pb) and other radioactive elements. The amount of lead in the air can be measured, and this depends on weather conditions.

Radon levels in natural environments are too low to be detected by chemical methods. For example, a high radon level of 1000 Bq/m³ is equal to 0.17 picograms per cubic meter. On average, the atmosphere contains about 6 × 10⁻⁵ radon atoms per air molecule, or about 150 radon atoms in each milliliter of air. The total radon in Earth’s atmosphere at any time comes from the decay of uranium and radium, which are constantly replaced. Radon levels vary widely depending on location. In open air, radon ranges from 1 to 100 Bq/m³, with even lower levels (0.1 Bq/m³) above oceans. In caves, mines, or poorly ventilated homes, levels can rise to 20–2000 Bq/m³.

In mining areas, radon levels can be much higher. Ventilation rules in uranium mines aim to keep radon below the "working level" and under 3 WL (546 pCi/L or 20.2 kBq/m³) 95% of the time. In unventilated places like the Gastein Healing Gallery, radon levels average 43 kBq/m³ (about 1.2 nCi/L) and can reach as high as 160 kBq/m³ (about 4.3 nCi/L).

Radon naturally comes from the ground and some building materials, especially in areas with uranium or thorium, such as regions with granite or shale soils. Each square mile of soil contains about 0.035 ounces of radium (0.4 grams per square kilometer), which releases small amounts of radon into the air. Sand used in concrete is a major source of radon in buildings.

Globally, about 2,400 million curies (91 TBq) of radon are released from soil each year. Radon is a noble gas, so it moves freely through soil and may collect in caves or water. Radon has a short half-life (four days for Rn), so its levels drop quickly as it moves away from the source.

Radon levels in the air change with the seasons and weather. For example, radon can build up during meteorological inversions when there is little wind.

Because radon levels in the air are very low, radon-rich water exposed to air loses radon quickly through evaporation. Groundwater often has higher radon levels than surface water because radon is continuously produced by the decay of radium in rocks. Similarly, the saturated parts of soil (where water fills the spaces) have more radon than the unsaturated parts. Some springs, including hot springs, contain significant radon. Towns like Boulder, Montana; Misasa; Bad Kreuznach; and Japan have radium-rich springs that emit radon. To be called radon mineral water, it must have at least 2 nCi/L (7 Bq/L). Radon levels in Merano and Lurisia (Italy) reach 2,000 and 4,000 Bq/L, respectively.

Radon is also found in some petroleum. Because radon has similar pressure and temperature properties to propane, oil refineries can accidentally collect radon in pipes used for propane. Residues from oil and gas production often contain radium and its decay products. Radon decays into solid radioisotopes that form coatings inside pipes. In oil processing plants, areas where propane is handled are often more contaminated because radon and propane have similar boiling points.

Indoor radon levels are usually around 100 Bq/m³, but construction and ventilation affect these levels. Radon concentrations can change by up to two times in an hour and vary greatly between rooms in the same building.

Radon levels are not evenly distributed. Higher concentrations have a much greater impact on average levels. Indoor radon levels are often described using a lognormal distribution, meaning the geometric mean is used to estimate the "average" level in an area.

Radon levels in some European countries range from less than 10 Bq/m³ to over 100 Bq/m³. Studies show that the geometric standard deviation of radon levels is between 2 and 3, meaning that 2 to 3% of homes have radon levels more than 100 times the average.

In 1984, a man named Stanley Watras, a construction engineer, triggered radiation monitors at a nuclear power plant despite being decontaminated each day. This led to the discovery of radon levels of 100,000 Bq/m³ (2.7 nCi/L) in his home’s basement. Living there was compared to smoking 135 packs of cigarettes daily and increased his family’s lung cancer risk by 13–1

Health effects

High levels of radon in mines, where concentrations can reach 1,000,000 Bq/m³, have been linked to health problems. In 1530, Paracelsus described a wasting disease in miners called "mala metallorum." At that time, radon and radiation were not yet understood, but Georg Agricola, a mineralogist, suggested improving mine ventilation to reduce the illness, known as "Bergsucht." In 1879, Herting and Hesse identified the disease as lung cancer in miners from Schneeberg, Saxony, Germany. Because uranium ore like pitchblende was mined in the Ore Mountains, it is likely that radon exposure from uranium caused the lung cancer.

Radon is especially dangerous in uranium mining. Studies of uranium miners from the 1940s and 1950s found higher rates of lung cancer. Processing uranium ore can also release radon, especially from uncovered waste piles and tailing ponds. Modern techniques, such as better ventilation, radiation monitoring, and in-situ leaching, have reduced radon exposure in mines.

The first major studies on radon and health focused on uranium mining in Bohemia and the Southwestern United States during the Cold War. Uranium mines often have high radon levels because radon forms from uranium decay. In the 1950s, many miners in the Four Corners region developed lung cancer and other illnesses due to radon exposure. Native American and Mormon miners had higher rates of lung cancer than usual, possibly because they normally had lower rates. Safety measures like ventilation were not widely used or enforced during this time.

Studies show that uranium miners exposed to 50 to 150 picocuries of radon per liter of air (2000–6000 Bq/m³) for about 10 years had higher lung cancer rates. Even exposures below 50 WLM led to increased lung cancer deaths. However, results vary between studies, and the reasons are unclear. Differences may be due to errors in measuring exposure, population differences, or other factors like silica dust, smoking, or work conditions. Many miners smoked and inhaled mine dust, making it hard to separate the effects of radon from other causes.

Ventilation and other measures have reduced radon levels in active mines, bringing exposure levels closer to those in homes. This has lowered the risk of radon-related cancer but remains a concern for current and former miners. Detecting radon's effects is harder now because exposure levels are much lower than in the past.

Radon and dust levels in mines depend on ventilation, making it hard to prove radon alone causes cancer. High dust levels from poor ventilation could also contribute to lung cancer.

Radon-222 is classified as a human carcinogen by the International Agency for Research on Cancer. In 2009, the World Health Organization recommended a radon reference level of 100 Bq/m³, urging better radon measurement and prevention in homes. Studies of miners show higher lung cancer rates, but smoking and dust are major confounding factors. While most regulatory agencies agree radon and its decay products are carcinogenic, some recent studies suggest lower cancer risks at certain exposure levels. A meta-analysis of multiple studies, however, shows a consistent increase in lung cancer risk with higher radon levels.

Radon exposure mainly occurs through inhalation. The health risk comes from radon's decay products, which emit radiation. Radon itself is not the main danger, but its decay products can damage lung cells, leading to cancer. Radon has a short half-life of 3.8 days and decays into radioactive particles like polonium-218 and 214. These particles can attach to lung tissue or dust, emitting alpha and gamma radiation that harms DNA and may cause mutations. Radioactive particles can also enter the bloodstream, potentially affecting other parts of the body.

Studies on domestic exposure

Radon is the largest natural source of radiation exposure for people. It is a radioactive gas that forms naturally in soil and rock. Radon contributes about 55% of the yearly radiation dose people receive from natural sources. Radon levels depend on where a person lives and the type of soil and rock beneath the ground.

In the 1980s, scientists discovered that radon can cause cancer, based on studies of miners who had high exposure to the gas. While radon can be dangerous, some people visit radon-contaminated mines intentionally to help with arthritis symptoms. These people usually do not experience serious health problems.

Radon is a major source of background radiation from Earth. Even though it is rare, when it is present, it can be found in very high amounts. In some areas, such as parts of Cornwall and Aberdeenshire, natural radon levels are so high that nuclear power plants cannot be built there. The natural soil and rock in these areas would already exceed legal radiation limits before the plants even started operating. People living in these areas may receive up to 10 mSv of radiation each year from natural sources.

This situation created a health challenge: what are the health effects of radon levels (about 100 Bq/m³) found in some homes?

When people are exposed to a substance that can cause cancer, it is difficult to determine if the cancer was caused by that substance. Lung cancer can occur naturally, and there is no difference between cancer caused by radon and cancer caused by other factors, such as smoking. Because cancer takes years to develop, it is hard to know a person’s past exposure to radon. Scientists can only study the health effects of radon using theories and statistical data.

There are three main types of studies used to examine health risks:

• Cohort studies follow groups of people with known exposure levels over time. These studies are reliable but expensive and require large groups. They are best for studying strong effects, such as those from very high radon exposure.
• Case-control studies compare people who have a disease (cases) with people who do not (controls) to find possible causes. These studies are useful for identifying factors that may increase risk but can be influenced by other variables.
• Ecological studies compare health outcomes and environmental factors across large populations. These studies are easy to conduct but are less reliable because they can be affected by other factors.

For a health risk to be accepted as proven, both theory and observation must support the same conclusion. Even if a statistical link between a factor and a health effect is found, it must be explained by a theory, and the theory must be confirmed by observations.

Cohort studies are not practical for studying radon exposure in homes. Because the health effects of low radon levels are small, studying them would require very large groups of people, such as entire countries.

Several ecological studies have been done to examine possible links between cancer and radon levels in areas with higher-than-average radon. Results from these studies have been mixed, with some showing a link, some showing no link, and others showing weak links.

The most direct way to study the risks of radon in homes is through case-control studies. However, these studies have not provided a clear answer. This is partly because the risk from low radon levels in homes is likely very small and because it is hard to estimate a person’s lifetime radon exposure. It is also clear that smoking causes far more lung cancer than radon.

Studies on radon and lung cancer have found that the risk of lung cancer increases with higher radon exposure. There is no evidence of a safe level (a threshold) for radon exposure, even at very low levels. This supports the idea that the risk of lung cancer increases proportionally with radon exposure.

A detailed case-control study led by R. William Field and others found that prolonged exposure to radon at the EPA’s action level of 4 pCi/L (about 230 Bq/m³) increased the risk of lung cancer by about 50%. This study was strengthened by the fact that Iowa, where the study took place, has high radon levels and a stable population. The study found a slightly increased risk for people exposed to radon levels above 17 WLM (about 230 Bq/m³).

A ten-year case-control study in Worcester County, Massachusetts, found that people exposed to low radon levels (0–150 Bq/m³) had a 60% lower risk of lung cancer. This supports the idea of radiation hormesis, where low levels of radiation may reduce cancer risk. However, this study was not based on the entire population, and errors in estimating past radon exposure could not be ruled out. Other studies, such as the "Iowa Radon Lung Cancer Study," have not found evidence of radiation hormesis.

Intentional exposure

"Radon therapy" is when people intentionally breathe in or swallow radon. Studies show that breathing large amounts of radon increases the risk of lung cancer.

In the late 1900s and early 2000s, some "health mines" were created in Basin, Montana. These places attracted people who believed that being near radioactive mine water and radon could help with health issues like arthritis. This practice is controversial because high levels of radiation can harm the body. Some doctors claim that radon has long-term benefits, but no proper scientific tests have been done to support these claims. A study about this practice is questionable because it left out patients who needed cortisone injections due to worsened arthritis during treatment. The study also assumed that 60 patients represented all patients and did not track whether patients used pain medications like ibuprofen or naproxen. The study suggested that radon helps by entering the skin.

Radioactive water baths have been used since 1906 in Jáchymov, Czech Republic. They were also used in Bad Gastein, Austria, before radon was discovered. Radium-rich water is used in traditional hot springs in Misasa, Tottori Prefecture, Japan. Drinking radon water is practiced in Bad Brambach, Germany. Inhaling radon is done in places like Gasteiner-Heilstollen, Austria; Kowary, Poland; and Boulder, Montana, United States. In the United States and Europe, "radon spas" allow people to sit in areas with high radon levels, believing that small amounts of radiation can improve energy.

Radon has been made for medical use, such as in radiation therapy. However, it is mostly replaced by other radioactive materials created in particle accelerators and nuclear reactors. Radon is sometimes placed in tiny seeds made of gold or glass to treat cancer. These seeds are filled with radon from a radium source and then cut into short sections. The gold keeps radon inside and blocks certain types of radiation, allowing gamma rays to escape. These gamma rays destroy diseased tissue. The seeds can have activity levels between 2 and 200 MBq. Gamma rays come from radon and its first decay products, including polonium, lead, and bismuth.

Radon and its decay products break down quickly. After 42 days, radon's radioactivity is about 1/2,000 of its original level. At this point, the main remaining activity comes from lead, which lasts 2,000 times longer than radon. Lead's decay products, bismuth and polonium, make up 0.03% of the original activity in the seed.

Health policies

The Federal Radon Action Plan, or FRAP, was created in 2010 and started in 2011. It was led by the U.S. Environmental Protection Agency with help from several U.S. government departments, including Health and Human Services, Agriculture, Defense, Energy, Housing and Urban Development, Interior, Veterans Affairs, and the General Services Administration. FRAP aimed to stop cancer caused by radon by increasing radon testing, reducing high radon levels, building homes that resist radon, and meeting goals set in Healthy People 2020. FRAP identified challenges, such as people not knowing about radon dangers, thinking mitigation was too costly, and limited access to testing. To address these, FRAP focused on showing the importance of testing and mitigation, offering rewards for testing and mitigation, and growing the radon services industry. Representatives from each group made specific promises and set timelines to complete tasks, meeting regularly. FRAP ended in 2016 when the National Radon Action Plan, or NRAP, took over. In its final report, FRAP completed 88% of its goals. By 2014, it had achieved the highest rates of radon mitigation and new construction mitigation in the United States. FRAP reported that at least 1.6 million homes, schools, and childcare facilities benefited directly from its efforts.

The National Radon Action Plan, or NRAP, was created in 2014 and started in 2015. It is led by the American Lung Association with help from groups such as the American Association of Radon Scientists and Technologists, American Society of Home Inspectors, Cancer Survivors Against Radon, Children's Environmental Health Network, Citizens for Radioactive Radon Reduction, Conference of Radiation Control Program Directors, Environmental Law Institute, National Center for Healthy Housing, U.S. Environmental Protection Agency, U.S. Department of Health and Human Services, and U.S. Department of Housing and Urban Development. NRAP continues FRAP’s work to stop cancer caused by radon by increasing testing, reducing high radon levels, and building radon-resistant homes. NRAP also aims to reduce radon risk in 5 million homes and save 3,200 lives by 2020. To reach these goals, groups have created plans to make radon risk reduction a standard practice in housing, offer support and rewards for testing and mitigation, promote certified radon services, and grow the industry. They also want to raise public awareness about radon risks and the importance of reducing them. NRAP is currently working to carry out these plans, test methods, and partner with organizations to achieve its goals.

Policies and scientific modelling worldwide

The only information about how radon exposure affects health comes from studies of miners who were exposed to very high levels of radon. Research on people who survived the atomic bombings of Hiroshima and Nagasaki is less helpful because their exposure to radon was different—it was not long-term or localized, and the radiation involved was not the same type (alpha rays). Even miners with lower exposure had similar levels to people living in homes with high radon. However, miners on average had about 30 times more total exposure over time. Smoking is also a major factor in miner studies, making it harder to separate the effects of radon alone. From these studies, it is clear that radon is a health risk when levels are above 1,000 Bq/m³. In the 1980s, scientists knew little about how radon exposure relates to health risks, both in theory and through data.

Since the 1980s, scientists have studied radon’s effects using both medical research and biology. Progress has been made in understanding how cancer starts, but no clear model has been confirmed to predict health risks. Scientists now know that the relationship between radon exposure and health is very complex and likely not a straight line. Some models suggest that low doses of radon may cause less harm than higher doses. In medical studies, no clear conclusion has been reached, but it is possible that very low radon levels might not cause harm.

Using models that estimate health risks, scientists can predict how many people might be affected by radon in homes. This helps guide public health policies. According to the BEIR VI model, most radon-related deaths occur at low exposure levels because most people live in homes with radon levels between 0 and 200 Bq/m³. Reducing radon in homes with average or higher levels could help lower overall exposure. However, the relationship between low exposure and health risks is uncertain. If a safe level of exposure exists (a threshold), the real health risks might only affect homes with very high radon levels, like those seen in mines. If radon at low levels actually helps protect against cancer (a theory called hormesis), reducing low radon exposure could increase cancer risk. Because the effects of low doses are unclear, choosing a model to estimate risks is very debated.

Since no clear data exists for typical home radon levels, health risks are often estimated by comparing radon’s effects in homes to those seen in miners. This assumption is that cancer risk increases directly with exposure. This was the basis for the BEIR IV model in the 1980s. The linear no-threshold model, which assumes no safe level of exposure, has been used in later reports like BEIR VI and BEIR VII because no better model has been found.

It is important to note that radon levels in homes and their health effects at low levels are not fully understood. Scientists must estimate health risks based on models, as radon-related deaths from homes cannot be directly observed. These estimates depend heavily on the model used.

According to these models, radon is the second leading cause of lung cancer after smoking. In Iowa, the highest average radon levels in the U.S., studies show a 50% higher risk of lung cancer with long-term exposure above the EPA’s action level of 4 pCi/L. Based on studies by the National Academy of Sciences, radon is estimated to cause between 15,000 and 22,000 cancer deaths in the U.S. each year. The EPA says radon is the leading cause of lung cancer among non-smokers. People are exposed to small amounts of polonium, a radon decay product, in indoor air. These isotopes are thought to cause most of the estimated 15,000–22,000 lung cancer deaths linked to radon in the U.S. The U.S. Surgeon General reports that over 20,000 Americans die each year from radon-related lung cancer.

In the United Kingdom, radon is the second most common cause of lung cancer deaths after smoking. Models estimate that 83.9% of deaths are from smoking, 1.0% from radon alone, and 5.5% from a combination of radon and smoking.

The World Health Organization recommends a radon level of 100 Bq/m³ (2.7 pCi/L) as a reference. The European Union suggests taking action when radon levels reach 400 Bq/m³ (11 pCi/L) in older homes and 200 Bq/m³ (5 pCi/L) in newer homes. After studies in North America and Europe, Canada lowered its action level to 200 Bq/m³ (5 pCi/L). The EPA strongly recommends fixing homes with radon levels above 148 Bq/m³ (4 pCi/L) and encourages action at 74 Bq/m³ (2 pCi/L).

The EPA advises testing all homes for radon. If levels are below 4 pCi/L (160 Bq/m³), no action is needed. For levels of 20 pCi/L (800 Bq/m³) or higher, steps should be taken to reduce radon. For example, opening windows daily can reduce radon levels by about 7