When you think about nuclear radiation, you often think about power plants, but it turns out there’s radioactive stuff all over the place: smoke detectors, granite counters, bananas, and potatoes can all be mildly radioactive. Even some old tape dispensers . But those are all puny sources.

Radon is a colorless, odorless gas that is heavier than air. It is a byproduct of uranium decay, amonth other pathways, decays by emitting an alpha particle, and has a half-life of just about four days. As a gas, it escapes from rocks or soil. Usually, there isn’t much of it, so it vents harmlessly to the atmosphere. But it can easily get trapped in a house and accumulate. You might think your house is solid, but there are plenty of cracks, joints, and pipes where gas can weasel through, especially if your inside pressure is slightly lower than the pressure in the soil below, which is common.

As the radon decays, it can produce more radioactive material, including tiny amounts of polonium.…