It is above the EPA action level of 4.0 pCi/L, so mitigation is recommended. It is not an emergency and nobody needs to leave the house. A system typically brings a 6.5 reading below 2.0.
More detail
Radon is measured in picocuries per liter of air. The EPA sets 4.0 pCi/L as the action level, the point at which it recommends fixing the home, and suggests considering mitigation between 2.0 and 4.0. A 6.5 result sits in the range most Columbus-area homes that fail come back at, and a correctly designed sub-slab depressurization system routinely takes readings in that band down under 2.0 pCi/L. What the number does not mean: it is not a measure of immediate danger, and no exposure at 6.5 requires anyone to leave the house while the work is scheduled. Radon risk is cumulative over years of exposure, which is why the fix is measured in weeks rather than hours. What matters more than the single number is the test type behind it. A 2-to-7-day short-term charcoal or continuous-monitor test under closed-house conditions is a snapshot, and readings swing with weather, season, and how the house was operated during the test. Central Ohio sits on flat-lying glacial till with dense clay subsoils that drain slowly and perch water above the till rather than letting it through. A long-term test of 90 days or more produces the annual average the EPA guidance is actually written against. If the 6.5 came from a short-term test in a real estate transaction, that is the correct test for that purpose, and the mitigation decision is normally made on it. One practical note for anyone under contract: post-mitigation verification testing should follow the install, and the target is a documented reading under 4.0 pCi/L with the system running, not an installer's assurance.