EPA recommends testing every 2 years and after any foundation work, HVAC changes, or basement renovation. Real-estate transactions also require fresh testing within the past 6 months.
More detail
EPA testing cadence guidance comes out of the Citizen's Guide to Radon and assumes that radon levels in a given home shift over time as the foundation settles, sealing degrades, HVAC changes, and adjacent excavation alters soil-gas pressure. The two-year cadence is conservative; many Cincinnati homeowners with documented post-mitigation readings under 2.0 pCi/L test annually because the cost is low ($15-$30 for a hardware-store kit) and the upside is catching fan failure or seal degradation early. Triggers that warrant immediate retest regardless of cadence: any foundation work, new sump pit or modification, basement renovation, HVAC equipment swap, attached-garage retrofit, or significant exterior excavation within 10 feet of the foundation. Real-estate transactions universally require fresh testing inside a 6-month window per FHA, VA, and most conventional lender contingency clauses. Cincinnati specific: Hamilton County and parts of Mariemont, Madeira, and Hyde Park have aggressive freeze-thaw cycles that crack slabs on a 5-10 year timeline, opening fresh radon entry paths even when the original mitigation system is still working. Testing every 2 years catches these geological shifts before levels drift back above the action level.