Radon Entry, Transport, and Building Dynamics
This domain tests how soil gas moves into a building and why the same house can test differently from season to season or floor to floor.
Driving forces
Radon entry is mainly pressure-driven, not diffusion-driven. Warm indoor air rises and leaks out high in the building-the stack effect-while lower levels draw soil gas through openings. Exhaust fans, clothes dryers, combustion appliances, and unbalanced HVAC can depressurize a basement. Wind and falling barometric pressure also change the soil-to-indoor pressure difference. Diffusion through concrete is real but usually small compared with advective flow through cracks and penetrations.
Entry routes
Common paths are slab cracks and control joints, cove joints, sump pits, drain tile, plumbing and utility penetrations, hollow block walls, exposed soil in crawlspaces, and gaps around posts or drains. Well water can release radon when used, but water is usually a minor source compared with soil gas unless the water concentration is very high.
Building dynamics
Lower levels generally show higher concentrations because they are closer to the soil and more often depressurized. HVAC operation can pressurize some zones and depressurize others. A closed house reduces dilution by outdoor air, which is why test protocols require it. Radon is denser than air, but indoor mixing and pressure-driven flow dominate; it does not simply settle into the basement like water.
Worked example
A tester finds 9 pCi/L in a basement bedroom in winter with a bath exhaust fan running, and 3 pCi/L upstairs. The likely explanation is not that radon "rose" upstairs, but that the basement is the entry zone and the exhaust fan increased depressurization. A valid test would control the fan per protocol and place the device in the lowest livable level.
Traps
Claiming radon sinks because it is heavy; assuming only basements can have a problem; treating diffusion as the main entry mechanism; and ignoring exhaust fans, season, or pressure effects when comparing results.