There is no threshold moment and no acute illness at residential levels. Radon risk accumulates over years and decades of exposure, which is why nobody needs to leave a house over a test result and why fixing it is measured in weeks.
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This is the most common question people ask after a failed test, usually at night, and it deserves a calm and specific answer. There is no exposure duration at which something happens. Radon does not cause an acute illness, there are no symptoms of radon exposure to look for, and no residential concentration produces an effect anyone would feel. If you go searching for a symptom list, you will not find a useful one, and that absence is the honest answer rather than a gap in the research. What the risk actually is. Radon decay products lodge in lung tissue and emit alpha radiation that damages the cells lining the airway. Damage of that kind is probabilistic and cumulative: each year of exposure adds to lifetime risk, and the models the EPA uses are built on decades of occupancy, not months. That is why the recommendation for a result above 4.0 pCi/L is to fix the home, not to evacuate it. So the practical timeline. A household that discovers an elevated result and installs a system within a few weeks has changed essentially nothing about their lifetime risk compared to fixing it tomorrow. What matters is the years after, not the days between. The genuinely encouraging part, and it is the reason this question has a good answer. Exposure that already happened cannot be undone, and exposure from here forward is entirely within your control. A correctly designed system typically takes a reading in the 6 to 8 pCi/L range down under 2.0, and it keeps doing that every day the fan runs, for the life of the house. Very few health risks can be removed permanently for the cost of a system in this market. One group deserves a specific note: risk multiplies for smokers, so a household that includes one has more to gain from mitigation than the numbers alone suggest. 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. That geology is why much of Greater Columbus and Central Ohio sits in EPA Zone 1, and why this comes up here more than in most of the country.