Test your home for radon every two years, and again after any renovation that changes how air moves through the house. That is the Ohio Department of Health's recommendation for single-family homes; for multifamily, school, and commercial buildings, ODH recommends every five years.
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EPA gives the same two-year interval for homes with a mitigation system, and Ohio's Radon Mitigation Standards require mitigators to advise every client to retest at least every two years or after a significant alteration of the building. Kentucky homeowners can follow the same EPA guidance. Beyond the calendar, move the next test up when you start using a lower level such as a newly finished basement, since EPA says to retest on that level; when you add onto the house or replace the furnace or air handler; when you have waterproofing, sump, or drain tile work done; when you tighten the house with air sealing, insulation, or new windows; and when the radon system is altered or its manometer drifts from the startup mark. A sale is another trigger: EPA's Home Buyer's and Seller's Guide notes that a buyer may ask for a new test if the last one is not recent, for example within two years. For results you can compare over time, retest in the same room, ideally during the heating season, with windows and outside doors closed for 12 hours beforehand. Ohio homeowners can order free kits through ohio.radon.com; in Kentucky, a hardware-store kit or a measurement professional certified under state law does the same job. See radon testing for professional options, and our heating-season testing guide for placement.