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Radon is the invisible, radioactive atomic gas that results from radioactive decay of some forms of uranium that may be found in rock formations beneath buildings or in certain building materials themselves. Radon is most likely the most pervasive serious hazard for indoor air in the United States and Europe, and is probably responsible for tens of thousands of lung cancer deaths per year. There are relatively simple tests for radon gas, but these tests are not commonly done, even in areas of known systematic hazards. Radon is a very heavy gas and thus will tend to accumulate at the floor level. Building materials can be a significant source of radon, but very little testing is done for stone, rock, or tile products brought into building sites. The half-life for radon is 3.8 days, indicating that once the source is removed, the hazard will be greatly reduced within a few weeks. However, annually, thousands of people go to radon-contaminated mines for purposeful exposure to help with the symptoms of arthritis without any serious health effects.^[28]

Radon presents significant risks, since it is a colorless and odorless gas, and therefore not readily detectable by a human. The radiation decay products ionize genetic material, causing mutations that sometimes turn cancerous. Radon exposure is the second major cause of lung cancer after smoking.^[28] Radon gas levels vary by locality and the composition of the underlying soil and rocks. For example, in areas such as Cornwall in the UK, which has granite as substrata, radon gas is a major problem, and buildings have to be force-ventilated with fans to lower radon gas concentrations. The United States Environmental Protection Agency (EPA) estimates that one in 15 homes in the United States has radon levels above the recommended guideline of 4 picocuries per liter (pCi/L) (148 Bq/m³).^[47] Iowa has the highest average radon concentration in the United States; studies performed there have demonstrated a 50% increased lung cancer risk with prolonged radon exposure above the EPA's action level of 4 pCi/L.^[60][61]

Radon is a terrestrial source of radiation of particular concern because — although on average it is very rare — this intensely radioactive element can be found in high concentrations in many areas of the world, where it represents a significant health hazard. Radon is a decay product of uranium, which is relatively common in the earth's crust, but generally concentrated in ore-bearing rocks scattered around the world. Radon seeps out of these ores into the atmosphere or into ground water, and in these localities it can accumulate within dwellings and expose humans to high concentrations. The widespread construction of well insulated and sealed homes in the northern industrialized world has led to radon becoming the primary source of background radiation in some localities in northern North America and Europe. Some of these areas, including Cornwall and Aberdeenshire in the United Kingdom have high enough natural radiation levels that nuclear licensed sites cannot be built there — the sites would already exceed legal radiation limits before they opened, and the natural topsoil and rock would all have to be disposed of as low-level nuclear waste.^[62]

Radiation exposure from radon is indirect. Radon has a short half-life (4 days) and decays into other solid particulate radium-series radioactive nuclides. These radioactive particles are inhaled and remain lodged in the lungs, causing continued exposure. People in affected localities can receive up to 10 mSv per year background radiation.^[62] Radon is thus the second leading cause of lung cancer after smoking, and accounts for 15,000 to 22,000 cancer deaths per year in the US alone.^[63] The general population is exposed to small amounts of polonium as a radon daughter in indoor air; the isotopes ²¹⁴Po and ²¹⁸Po are thought to cause the majority^[64] of the estimated 15,000–22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.^[65]

The general effects of radon to the human body are caused by its radioactivity and consequent risk of radiation-induced cancer. As an inert gas, radon has a low solubility in body fluids, which leads to a uniform distribution of the gas throughout the body. Radon gas and its solid decay products are carcinogens. The greatest health risks come from exposure to the inhaled solid radon gas decay products that are produced during its radioactive decay. Two of these decay products, polonium-218 and 214, present a significant radiologic hazard.^[66] Once the radioactive decay products are inhaled into the lung, they undergo further radioactive decay, releasing small bursts of energy in the form of alpha particles that can either cause DNA breaks or create free radicals.^[66]

It is not known whether radon can cause health effects in other organs besides the lungs. The effects of radon, if found in food or drinking water, are unknown.

The largest single source of radiation exposure to the general public is naturally-occurring radon gas, which comprises approximately 55% of the annual background dose. The largest natural contributor to public radiation dose is radon, a naturally occurring, radioactive gas found in soil and rock.^[67] If the gas is inhaled, some of the radon particles may attach to the inner lining of the lung. These particles continue to decay, emitting alpha particles which can damage cells in the lung tissue.^[47] The death of Marie Curie at age 66 from leukemia was likely caused by prolonged exposure to high doses of ionizing radiation. Its discoverer, Curie worked extensively with radium, which decays into radon,^[68] along with other radioactive materials that emit beta and gamma rays.