Only homes that test above 4.0 pCi/L (the EPA action level) require mitigation. About 1 in 3 tested Cincinnati-area homes exceed this threshold. Test first, mitigate only if needed.
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. About 1 in 3 Cincinnati-area homes tests above 4.0 pCi/L; another 1 in 3 falls in the 2.0-4.0 borderline. The remaining third tests below 2.0. 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.