There are three primary sources of indoor radon:
The understanding of these sources and mechanisms that bring radon in doors has significantly advanced over 20-30 years. For example, in the WHO Handbook on Indoor Radon, A Public Health Perspective the World Health Organization (WHO) indicates that in the 1950’s there was concern about radon in water and attention was focused on health effects from ingesting the water. Later, it was determined that the primary health risk of radon in water was from the inhalation of radon decay products released from dissolved radon during the off gassing of steam.
In the 1970s, emanation of radon from alum shale in building materials was found to be a problem in some areas due to its enhanced levels of radium. However, WHO indicates that by 1978, houses were identified where the indoor radon concentrations were not associated with well water transport or emanation from building materials. Subsequently, soil gas infiltration became recognized as the most important source of indoor radon. Other sources, including building materials and well water, are of less importance in most circumstances.
Driving forces, sometimes called transport mechanisms, and pathways determine the distance that radon can travel from the source to the indoor air and the strength at which it collects in the building. For example, if it takes the soil gas four days to reach the house, approximately half of the radon will remain since radon has a half-life of about 3.8 days. If it takes 8 days, one quarter of the original radon will reach the house. The level of radon concentration collects in indoor air will greatly depend on the nearby soil and bedrock tends to be the dominant source of elevated radon levels in a building.