What are the Routes of Exposure to Radon?

Learning Objectives

Upon completion of this section, you will be able to

  • Identify the most important radon exposure route.

The average person in the United States receives an estimated 625 millirem/year dose from ionizing radiation. The largest percentage is from medical radiation (48%, 300 mrem), primarily due to the popularity of CT scans and nuclear medicine. This is followed by radon (37%, 228 mrem), which is the largest source of background radiation. While the dose from radon has remained the same over the years, the percentage that it represents has dropped from 55%, based on 1980s data, to 37% using 2006 data. Due to the increased use of certain medical procedures, this trend is expected to continue (NCRP 2009). The dose of ionizing radiation from radon comes from soil, water, natural gas, and building materials.

The primary pathway for human exposure to radon is inhalation from soil vapor intrusion into dwellings and buildings. Indoor radon levels can, however, also originate from water usage, outdoor air infiltration, and the presence of building materials containing radium (EPA 2003).

Dermal exposure is not considered an important exposure route.


The main source of inhalation exposure is radon gas that is released from the soil to trapped indoor air.

  • Radon is a gas, but its radon progeny are charged and often attached to dust.
  • Radon progeny are present in nearly all air.
  • Radon gas itself is breathed in and out without imparting much dose.
  • It is primarily the progeny-carrying dust particulates that deposit in the lungs and give a radiation dose to the lung tissue.

Background levels of radon in outdoor air are generally quite low and represent a goal for reducing indoor levels. But radon levels can vary based on location and soil geology.

In indoor locations, such as homes, schools, or office buildings, levels of radon and radon progeny are generally higher than are outdoor levels. This is especially true of newer construction that is more energy-efficient. In new construction, indoor radon levels may actually increase, due in part to decreased air entry or exit (i.e., natural ventilation from outdoors) in such energy-efficient homes.

Radon releases from groundwater also contribute to exposure. Radon can be released from water into the air, resulting in inhalation exposure when

  • Clothes are washed,
  • Dishes are washed,
  • Toilets are flushed, and
  • Water splashes during showering.

Radon is released into the air when natural gas or propane is burned in a stove or furnace.

Because tobacco is naturally sticky, many radon decay products actually stick to tobacco products. When smoked or otherwise used, these radon progeny may enter your body.


Exposure to radon by the oral route can occur as a result of radon gas dissolving in water. Radon and its progeny are present in rocks and soil; the water that contacts the rocks and soil will dissolve out some radon. As such, in most drinking water radon and its progeny are naturally present.

Some radon and its progeny swallowed in drinking water pass through the stomach walls and intestine(Ishikawa et al. 2003; NAS 1999). Yet radon is biologically inert; after it reaches the lungs, it is readily breathed out through pulmonary circulation.

Dermal Exposure

Data are very limited regarding the absorption of radon following dermal exposure (ATSDR 2008). Because radon is a noble gas, transfer across the dermis should be by diffusion only and should involve no active transport. The layers of dead skin protect the body from exposure to alpha radiation from radon and its progeny. Dermal exposure to radon is not considered a significant exposure route.

Key Points
  • For the U.S. general public, radon is second only to medical radiation as the principal ionizing-radiation exposure source.
  • Inhalation is the most important radon exposure route.
  • Data are limited regarding the absorption of radon following dermal exposure. But dermal is not considered a significant radon exposure route.