Radon (Rn) is a naturally occurring colorless, odorless, tasteless radioactive gas that occurs in different forms with the same atomic number but different atomic mass, called isotopes.

As radon undergoes radioactive decay, radiation is released predominantly by high-energy alpha particle emissions, which are the source of health concerns.

Radon is measured in terms of its activity (curies or becquerels). Both the curie (Ci) and the becquerel (Bq) tell us how much a radioactive material decays every second (1 Ci = 37 billion Bq = 37 billion decays per second).

Radon isotopes are formed naturally through the radioactive decay of uranium or thorium.

Uranium and thorium (solids) are found in rocks, soil, air, and water. Uranium and thorium decay to other elements such as radium (a solid), which in turn decays into radon gas.

Uranium and thorium have been present since the earth was formed and have very long half-lives (4.5 billion years for uranium and 14 billion years for thorium). The half-life is the time it takes for half of the atoms of a radionuclide to undergo radioactive decay and change it into a different isotope. Uranium, thorium, radium, and thus radon, will continue to exist indefinitely at about the same levels as they do now.

Radon has no commercial uses.

The most common radon isotope is radon-222 (²²²Rn), which is part of the uranium decay chain.

An atom of ²²²Rn gives off an alpha particle (which is the size of a helium atom without electrons), transforming into an atom of polonium-218 (²¹⁸Po), which later gives off an alpha particle of its own, transforming into an atom of radioactive lead (²¹⁴Pb).  This process is called radioactive decay and radon decay products are called radon progeny or radon daughters.  The final step in the radioactive decay of radon progeny results in the formation of an atom of stable lead which is not radioactive.

The half-life of ²²²Rn is 3.82 days.  Some of the radon decay products have the following half-lives:  ²¹⁸Po is 3.05 minutes; ²¹⁴Pb is 26.8 minutes; and ²¹⁰Pb is 22.3 years.

Radon gas in the rocks and soil can move to air, groundwater, and surface water.

Decay products of ²²²Rn, such as ²¹⁸Po and ²¹⁴Pb, are solids that can attach to particles in the air and be transported this way in the atmosphere.  They can be deposited on land or water by settling or by rain.

Radon will undergo radioactive decay in the environment.

Since radon progeny are often attached to dust, you are exposed to them primarily by breathing them in. They are present in nearly all air. Depending on the size of the particles, the radioactive particulates can deposit in your lungs and impart a radiation dose to the lung tissue.

Background levels of radon in outdoor air are generally quite low, but can vary based on location and the underlying soil geology.

In indoor locations, such as homes, schools, or office buildings, levels of radon and radon progeny are generally higher than outdoor levels and may be particularly high in some buildings, especially in newer construction that is more energy-efficient.

Cracks in the foundation or basement of your home may allow increased amounts of radon to move into your home. Also, radon is heavier than ambient air, and therefore, the concentrations of radon are generally higher in the lower levels or basement of the homes.

You may be exposed to radon and radon progeny by coming into contact with radon-contaminated surface or groundwater or by drinking water from wells that contain radon.

Radon in water can become airborne; it is estimated that 1/1,000th of the radon in water may become airborne during indoor activities that use water.

Radon and its radioactive progeny can enter your body when you breathe them in or swallow them.

Most of the inhaled radon gas is breathed out again.

Some of the radon progeny, both unattached and attached to dust, may remain in your lungs and undergo radioactive decay. The radiation released during this process passes into lung tissue and can cause lung damage.

Some of the radon that you swallow with drinking water passes through the walls of your stomach and intestine.

After radon enters your blood stream most of the radon quickly moves to the lungs where you breathe most of it out.

Radon that is not breathed out goes to other organs and fat tissue where it may remain and undergo decay.

Many scientists believe that long-term exposure to elevated levels of radon and radon progeny in air increases your chance of getting lung cancer.

The greater your exposure to radon, the greater your chance of developing lung cancer.

Smoking cigarettes greatly increases your chance of developing lung cancer if you are exposed to radon and radon progeny at the same levels as people who do not smoke. Because tobacco is naturally sticky, many of the radon decay products actually stick to tobacco products. Therefore, when tobacco is smoked or otherwise used, these radon products may also enter your system. Breathing in other substances that cause lung cancer may also increase your chance of developing lung cancer from exposure to radon progeny.

Smaller lungs and faster breathing rates in children may result in higher estimated radiation doses to the lungs of children relative to adults. However, limited information from children employed as miners in China do not provide evidence of increased susceptibility to the effects of exposure to radon.

Indoor radon levels can be reduced by methods that include the sealing of surfaces between the ground and the building and installation of ventilation systems that route air from materials under the building to outdoor air. Certified radon mitigation experts can be located by contacting your state health or environmental program.

Radon in human tissues is not detectable by routine medical testing.

Some radon decay products can be detected in urine and in lung and bone tissue. Tests for these products are not generally available to the public and are of limited value since they cannot be used to accurately determine how much radon you were exposed to, nor can they be used to predict whether you will develop harmful health effects.

EPA recommends fixing your home if measured indoor levels of radon are 4 or more pCi per liter (pCi/L) of air. EPA also notes that radon levels less than 4 pCi/L still pose a health risk and can be reduced in many cases. The EPA recommends using a certified radon mitigation specialist if indoor radon levels need to be reduced to ensure that appropriate methods are used to reduce radon levels.

The Mine Safety and Health Administration (MSHA) has adopted an exposure limit of 4 Working Level Months (WLM) per year for people who work in mines (WLMs basically combine the concentration of radon in mine air with length of exposure inside the mine).

The Nuclear Regulatory Commission published a table of allowable exposure to radon by workers and allowable releases of radon to the environment by its licensees.

In the 1990s, EPA introduced a proposal to limit levels of radon in drinking water. As of August 2008 the proposal had not been adopted as a regulation.