Radon Entry, Transport, and Building Dynamics
What you must understand
Soil gas is the dominant source in most homes. Radon moves by two mechanisms: diffusion (slow, short distances) and advection (pressure-driven bulk flow, the major pathway). Pressure differentials arise from stack effect (warm indoor air rising), exhaust fans and HVAC imbalance, and wind. Entry points include cracks in slabs and foundations, sumps, floor drains, block-wall cavities, utility penetrations, and exposed soil in crawlspaces. Water can contribute, especially from private wells, but usually a small fraction of airborne concentration unless concentrations are very high. Building dynamics explain why concentrations vary seasonally (winter heating increases stack effect), diurnally, and with weather (falling barometric pressure, rain-saturated soil). Newer, tighter homes often show higher concentrations because of reduced dilution by outdoor air.
Traps
Questions imply diffusion is the main driver, or that opening windows is a valid long-term mitigation. Don't assume a home with no basement is safe - slab-on-grade and crawlspace homes also draw soil gas. Distinguish the pressure field extension from the entry point itself.
Worked example
A home with a basement and a running bath fan develops a −5 Pa basement-to-soil pressure difference; soil gas with 2,000 pCi/L flows along the pressure gradient into the basement. Rule of thumb: reduce the pressure difference (seal, balance HVAC, sub-slab depressurization) and entry drops dramatically - dilution alone is not control.