Heating season (October-March) typically shows the highest radon levels because homes are sealed up. Year-round long-term testing gives the most accurate average. EPA action level applies regardless of season.
More detail
Stack effect peaks in winter when warm interior air rising creates the strongest negative pressure at the foundation, pulling more soil gas (and thus more radon) into the home. Cincinnati winter testing typically reads 30-60% higher than summer testing in the same home. EPA recommends mitigation based on either short-term winter testing (which captures the worst-case) or long-term year-round testing (which gives the true annual average). Either approach is defensible. Borderline 2.0-4.0 pCi/L homes are the trickiest: a winter-only short-term test might read above 4.0 and trigger mitigation, while a year-round long-term test might read 2.5 and not trigger. Many Cincinnati homeowners run both: a winter short-term as the screening test, then a year-round long-term to confirm the reading before deciding. Ohio Department of Health's January Radon Action Month is an intentional alignment with the highest-reading season. Cincinnati seasonal dynamics: typical year-over-year heating-season radon readings run 30-60% higher than summer readings in the same home. Late January through early March in Cincinnati typically captures the highest stack-effect pull. Homeowners testing in summer should be aware their reading is on the low end of the annual range; a summer 3.5 pCi/L reading often becomes a winter 5.5-6.0 pCi/L reading in the same home.