radon

Radon is a colourless, odourless, radioactive gas that seeps naturally out of rock and soil. You cannot see, smell or taste it, yet it is the second leading cause of lung cancer after smoking, and the leading cause among people who have never smoked. Outdoors it disperses harmlessly, but it can build up to risky levels inside buildings.

Radon forms as uranium in the ground slowly decays. The gas enters homes through cracks in foundations, gaps around pipes and from well water, accumulating especially in basements and poorly ventilated lower floors. When inhaled, radon and its short-lived radioactive decay products deposit in the airways and emit alpha particles — bursts of radiation that damage the DNA of lung cells and, over years, can lead to cancer.

The risk is dose-related, rising with both the concentration and the years of exposure, and there is no firm safe threshold. It is also strongly synergistic with smoking: a smoker exposed to radon faces a far greater risk than either factor alone. Crucially, radon is testable with inexpensive home kits and reducible: sealing entry points and installing sub-slab ventilation systems can lower indoor levels substantially.