Of course, radon is in the natural outdoor air as well, but it is diluted by all the air that is available. The Environmental Protection Agency (EPA) did a national survey in the United States and determine that the average indoor air concentration ( picocuries of radon per liter of air) is about three times higher than the average outdoor air concentration ( picocuries per liter). The average indoor air concentration is three times less than the recommended limit ( picocuries per liter). Even though, on average, we could expect our house to have about picocuries per liter of radon, the EPA estimates that nearly 1 out of every 15 homes in the United States has elevated radon levels.
In the USA, 858 million tonnes of coal was used in 2013 for electricity production. With an average content of ppm uranium and ppm thorium, US coal-fired electricity generation in that year gave rise to 1100 tonnes of uranium and 2700 tonnes of thorium in coal ash. In Victoria, Australia, some 65 million tonnes of brown coal is burned annually for electricity production. This contains about ppm uranium and - ppm thorium, hence about 100 tonnes of uranium and 200 tonnes of thorium is buried in landfill each year in the Latrobe Valley.