Short-term (48-hour) tests work for real-estate timelines and initial screening. Long-term (90-day) tests give the most accurate average exposure. Both have legitimate uses; pick based on the question being asked.
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Short-term tests use a continuous radon monitor (CRM) or a charcoal canister and give a 48-hour to 7-day snapshot. Required for real-estate transactions because the closing window does not accommodate longer testing. Costs $150-$300 for professional CRM testing, $15-$30 for a charcoal kit. Long-term tests use alpha-track detectors that integrate radon decay over 90 days to 1 year, producing a true annual-average exposure figure. Costs $30-$50. The accuracy difference matters most for homes near the action-level borderline (between 2.0 and 4.0 pCi/L) where a single short-term snapshot might fall on either side of the threshold depending on weather and home occupancy that week. Heating season (October-March) typically shows the highest readings because homes are sealed and the stack effect is strongest. Year-round long-term testing averages out that seasonality. Cincinnati borderline-reading reality: about 30% of Hamilton County homes that test fall in the 2.0-4.0 pCi/L range. For these, the choice between short-term and long-term testing affects the mitigation decision. EPA risk tables show measurable lifetime cancer-risk reduction at every pCi/L level above 1.0; below the action level, the cost-benefit decision shifts toward household-specific factors (children, smoking history, basement occupancy hours).