Sources, Pathways, and Movement

Three factors are necessary for radon to be present in indoor air:

  • A source of radon
  • A pathway (opening into the building)
  • A driving force (transport mechanism)

First and foremost there must be a source of radon. Thinking back to the uranium decay chain, remember that radon is the by-product of radium, which can be found in soil or bedrock, well water, and building materials.  If a source is present, an opening or pathway into the home must also exist and work collectively with a driving force transport mechanism to bring radon gas into the home or building. 

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. 

Factors of Accumulation:

Radon is constantly entering buildings and can accumulate to elevated levels due to:

  • Strength of the Radium Source
  • Building Materials & Construction design
  • Ventilation System Adequacy
  • Indoor Air Quality, Temperature, & Humidity
  • Weather
  • Pathways Into the Home


There are three primary sources of indoor radon:

  1. Soil or Bedrock – The Typical Culprit, 80-90%
  2. Building Materials – Sometimes but not Typical
  3. Water – Rarely