Radon concentrations found in natural environments are much too low to be detected by chemical means: for example, a 1000 Bq/m (relatively high) concentration corresponds to 0.17 picogram per cubic meter. The average concentration of radon in the atmosphere is about 6 atoms of radon for each molecule in the air, or about 150 atoms in each mL of air. The entire radon activity of the Earth's atmosphere at any one time is due to some tens of grams of radon, constantly being replaced by decay of larger amounts of radium and uranium. Its concentration can vary greatly from place to place. In the open air, it ranges from 1 to 100 Bq/m, even less (0.1 Bq/m) above the ocean. In caves, aerated mines, or poorly ventilated dwellings, its concentration can climb to 20–2,000 Bq/m.
In mining contexts, radon concentrations can be much higher. Ventilation regulations try to maintain concentrations in uranium mines under the "working level", and under 3 WL (546 pCi  per liter of air; 20.2 kBq/m measured from 1976 to 1985) 95 percent of the time.
The concentration in the air at the (unventilated) Gastein Healing Gallery averages 43 kBq/m (about 1.2 nCi/L) with maximal value of 160 kBq/m (about 4.3 nCi/L).
Radon emanates naturally from the ground and from some building materials all over the world, wherever there are traces of uranium or thorium, and particularly in regions with soils containing granite or shale, which have a higher concentration of uranium. In every 1 square mile of surface soil, the first  (of depth) contains about 0.035 oz of radium (0.4 g per km) which releases radon in small amounts to the atmosphere. Sand used in making concrete is the major source of radon in buildings.
On a global scale, it is estimated that 2,400 million curies (91 TBq) of radon are released from soil annually. Not all granitic regions are prone to high emissions of radon. Being an unreactive noble gas, it usually migrates freely through faults and fragmented soils, and may accumulate in caves or water. Due to its very small half-life (four days for ), its concentration decreases very quickly when the distance from the production area increases.
Its atmospheric concentration varies greatly depending on the season and conditions. For instance, it has been shown to accumulate in the air if there is a meteorological inversion and little wind.
Because atmospheric radon concentrations are very low, radon-rich water exposed to air continually loses radon by volatilization. Hence, ground water generally has higher concentrations of  than surface water, because the radon is continuously replenished by radioactive decay of  present in rocks. Likewise, the saturated zone of a soil frequently has a higher radon content than the unsaturated zone because of diffusional losses to the atmosphere. As a below-ground source of water, some springs—including hot springs—contain significant amounts of radon. The towns of Boulder, Montana; Misasa; Bad Kreuznach, and the country of Japan have radium-rich springs which emit radon. To be classified as a radon mineral water, radon concentration must be above a minimum of 2 nCi/L (7  Bq/L). The activity of radon mineral water reaches 2,000 Bq/L in Merano and 4,000 Bq/L in the village of Lurisia (Ligurian Alps, Italy).
Radon is also found in some petroleum. Because radon has a similar pressure and temperature curve to propane, and oil refineries separate petrochemicals based on their boiling points, the piping carrying freshly separated propane in oil refineries can become partially radioactive due to radon decay particles. Residues from the oil and gas industry often contain radium and its daughters. The sulfate scale from an oil well can be radium rich, while the water, oil, and gas from a well often contains radon. The radon decays to form solid radioisotopes which form coatings on the inside of pipework. In an oil processing plant, the area of the plant where propane is processed is often one of the more contaminated areas, because radon has a similar boiling point to propane.