There is no point at which it suddenly makes you sick. Radon causes no symptoms; its risk is lung cancer that grows with the level and the years of exposure. That is why a high result calls for a fix this season, not an evacuation.
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The EPA's risk tables are written in terms of a lifetime of exposure, which is the honest frame. A family that finds 8 pCi/L in March and has a system running by April has changed very little about its lifetime risk compared with fixing it the same day; what matters is the years that follow. Two groups gain the most from moving quickly. Smokers face much higher risk at every level, about 120 per 1,000 at 8 pCi/L on the EPA table against about 15 for never smokers. And anyone who sleeps or works on the lowest level spends the most hours where readings run highest. In the meantime, use the house normally, and consider moving a basement bedroom upstairs until the fix is in. See radon and lung cancer.