Radon Pathways

Openings That Allow Radon into Homes

Characteristics that enhance the radon pathway:

  • Ease of air movement
  • Proximity to radium
  • Interconnection to other airflow passages

There are a number of ways that air passes through a source of radium in the bedrock or soil on its way from a higher pressure to a lower one. For example, many water drainage methods, used to prevent surface or groundwater from entering a building, also connect large surface areas of soil and bedrock with passages that have little resistance to airflow. When digging to place a foundation or utility entrance, the resistance to airflow in the surrounding soil can be changed.

Among the pathways allowing easy airflow are:

  • Cracks or openings in foundations
  • Sand, gravel & uneven pebbles
  • Cracks in soil, fissures, and caverns in underground bedrock
  • Perforated pipe (as used in drainage systems)
  • Plumbing & electrical chases
  • Floor drains
  • Unsealed sill plates & window frames in lower levels
  • Void spaces in hollow block walls

There are many airflow routes through seemingly impermeable soils:

  • A layer of shattered shale below 4 feet of clay
  • An area of sand found in only one corner of a basement
  • A water service or sewer line from the street, backfilled with sand or loose fill

Many houses have hidden airflow routes, which interact with each other. When suction is put on the basement, the pressure differential draws air from the outside through soil pathways. Air movement through these pathways is complex and changes direction and magnitude with a variety of factors including wind, temperature, and the operation of home appliances such as ventilation fans or furnaces.   For example, when both the radium and the house are on a highly permeable soil made of sand or gravel, radon attached to soil gas is easily drawn through cracks and holes in the foundation by stack effect or mechanical equipment.