Because radon is produced in the ground, it is present nearly everywhere. Soil is porous, so radon and other soil gases are able to move up through the dirt and rocks and into the air we breathe.
Without proper ventilation, radon can accumulate in indoor spaces, including your home, office, school, etc.
Two factors that determine how much radon will accumulate are pathways and air pressure. Because these factors differ from home to home, indoor radon levels vary from home to home.
Pathways are routes that gasses use to enter your home (openings between the soil and the home).
Radon can enter your home through the following pathways:
- Cracks in concrete slabs
- Uncapped hollow-block foundations
- Floor-wall joints
- Mortar joints
- Pores and cracks in concrete blocks
- Weeping drain tile (if drained to an open sump)
- Exposed soil (in a sump or crawl space)
- Loose-fitting pipe penetrations
- Building materials (brick, concrete, rock, etc.)
- Well water
Homes generally operate under a negative air pressure, especially during the heating season. The air pressure inside your home is typically lower than the surrounding air and soil, creating a vacuum that pulls soil gases into the home through the pathways. While the ground around the home may be frozen or soaked by rain, the gravel and soil underneath the home remain warm and permeable, attracting radon gas from the surrounding soil.
Factors that contribute to a home's air pressure changes include:
- Stack effect - Warm air rises to upper portions of the home and is replaced by cooler, denser outside air (some of which comes from the soil).
- Down wind draft effect - Strong winds create a vacuum on the downwind as they blow past a home.
- Vacuum effect - Combustion appliances (furnaces, hot water heaters, fireplaces, etc.), exhaust fans, and vents can remove a significant amount of air from a home. As indoor air is exhausted, outdoor air enters the home to replace it. This replacement air often comes from the soil underneath the home.
In general, any time air enters a home from the underlying soil, radon can come along with it.