Only homes that test above 4.0 pCi/L (the EPA action level) require mitigation. About 22 percent of Hamilton County tests and 36 to 37 percent in Butler, Warren, and Clermont exceed it (ODH data). Test first.
More detail
EPA action level is 4.0 pCi/L; homes testing above that should mitigate. EPA recommends "considering" mitigation for homes testing between 2.0 and 4.0 pCi/L, especially if children or chronic-respiratory residents live in the home, because the lung-cancer-risk reduction from mitigation continues down to roughly 1.0 pCi/L. Below 2.0 pCi/L, the marginal risk reduction from further mitigation is small and the EPA does not recommend mitigation. Ohio Department of Health data from licensed testers since 2010 puts about 22 percent of Hamilton County tests above 4.0 pCi/L and roughly 36 to 37 percent in Butler, Warren, and Clermont counties. The cost of testing ($15-$30 hardware-store kit, $150-$300 professional) is so low relative to the cost of unnecessary mitigation that test-first-mitigate-if-needed is the universal recommendation. Edge case worth knowing: even Cincinnati homes that test below 2.0 pCi/L can drift higher after foundation work, attached-garage retrofits, or significant air-sealing projects (spray foam, new windows). Re-test 30-90 days after any major project. The cost-benefit math always re-resolves in favor of testing because the test itself is so cheap relative to the eventual mitigation.