Radon (Rn-222) is a colorless, odorless, naturally occurring radioactive gas produced by the breakdown of uranium in soil. It seeps into homes through cracks and gaps and accumulates indoors, where it becomes the second leading cause of lung cancer in the US.
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Radon-222 is part of the uranium-238 decay chain. Half-life is 3.8 days, which is long enough for the gas to migrate from soil into homes but short enough that it decays into solid radioactive daughter products (polonium-218, lead-214, etc.) once inhaled. The daughter products attach to lung tissue and continue decaying, releasing alpha radiation that damages cellular DNA. Cumulative exposure over years is what drives lung-cancer risk; brief high-dose exposure does not produce the same harm because the daughter products clear from the lungs within hours. EPA estimates 21,000 US lung-cancer deaths annually from radon exposure, second only to smoking and roughly 7-8x the death rate from drunk driving. Cincinnati-specific perspective on the death toll: 21,000 annual US deaths from radon translate to roughly 600-800 Ohio deaths per year given Ohio's population share, with Hamilton County and Greater Cincinnati accounting for a disproportionate fraction because of Zone 1 geology. Mitigation at the Cincinnati Zone 1 level moves a household from "elevated risk" to "general population baseline" risk, the largest single cancer-risk reduction available from a one-time home modification.