Radon enters through cracks in the foundation, sump pits, gaps around pipes, and porous concrete. Even tightly built homes accumulate radon because indoor air pressure pulls it up from soil gas.
More detail
The mechanism is stack effect plus negative pressure. Warm air in the conditioned space rises and exits through upper-level penetrations (recessed lights, attic access, chimney chase, bath fans), creating slight negative pressure at the foundation level. That negative pressure pulls soil gas (which carries radon at concentrations 100-1000x indoor air) through every available path: visible slab cracks, sump-pit penetrations, plumbing rough-ins, expansion joints between slab and foundation wall, mortar joints in stone basement walls, and porous concrete itself. Cincinnati pre-1950 stone-basement homes (Hyde Park, Mariemont, Norwood) have the most paths and consistently show the highest pre-mitigation readings. Newer slab-on-grade construction with sealed penetrations leaks less, but never zero. Watch for: any slab penetration that was added or modified after original construction (post-1970 renovations sometimes cut the slab for a basement bathroom rough-in or sump installation). Those penetrations rarely get sealed to soil-gas-tight tolerance and become primary entry paths even in otherwise-tight homes. The on-site assessment maps every visible penetration before quoting scope, and we document each entry path with photographs in the post-install closeout package so the homeowner has a baseline if levels drift in the future.