Typical domestic exposures are of around 100Bq/m indoors, but specifics of construction and ventilation strongly affect levels of accumulation; a further complication for risk assessment is that concentrations in a single location may differ by a factor of two over an hour, and concentrations can vary greatly even between two adjoining rooms in the same structure.
The distribution of radon concentrations is highly skewed: the larger concentrations have a disproportionately greater weight. Indoor radon concentration is usually assumed to follow a lognormal distribution on a given territory. Thus, the geometric mean is generally used to estimate the "average" radon concentration in an area.
The mean concentration ranges from less than 10 Bq/m to over 100 Bq/m in some European countries. Typical geometric standard deviations found in studies range between 2 and 3, meaning (given the 68–95–99.7 rule) that the radon concentration is expected to be more than a hundred times the mean concentration for 2 to 3% of the cases.
The so-called "Watras incident" in 1984 is named for American construction engineer Stanley Watras, an employee at the Limerick nuclear power plant in the United States, who triggered radiation monitors while leaving work over several days—even though the plant had not yet been fueled, and despite Watras being decontaminated and sent home "clean" each evening. This pointed to a source of contamination outside the power plant, which turned out to be radon levels of 100,000 Bq/m (2.7 nCi/L) in the basement of his home. He was told that living in the home was the equivalent of smoking 135 packs of cigarettes a day, and he and his family had increased their risk of developing lung cancer by 13 or 14 percent. The incident dramatized the fact that radon levels in particular dwellings can occasionally be orders of magnitude higher than typical. Radon soon became a standard homeowner concern, though typical domestic exposures are two to three orders of magnitude lower (100 Bq/m, or 2.5 pCi/L), making individual testing essential to assessment of radon risk in any particular dwelling.
Radon exists in every U.S. state, and about 6% of American houses have elevated levels. The highest average radon concentrations in the United States are found in Iowa and in the Appalachian Mountain areas in southeastern Pennsylvania. Some of the highest readings have been recorded in Mallow, County Cork, Ireland. Iowa has the highest average radon concentrations in the United States due to significant glaciation that ground the granitic rocks from the Canadian Shield and deposited it as soils making up the rich Iowa farmland. Many cities within the state, such as Iowa City, have passed requirements for radon-resistant construction in new homes. In a few locations, uranium tailings have been used for landfills and were subsequently built on, resulting in possible increased exposure to radon.