Air
Radon
A radioactive gas produced by the decay of uranium in soil and rock, which enters buildings through the foundation. EPA identifies it as the second leading cause of lung cancer in the United States.
In more detail
Radon is part of the uranium decay chain and is itself unstable. The exposure of concern is less the gas than its short-lived decay products, which are solid, attach to airborne particles, and can deposit in the lungs, where their alpha emissions irradiate lung tissue at close range.
Entry is driven by pressure. A house is usually slightly negative relative to the soil beneath it because of stack effect and exhaust equipment, so soil gas is drawn in through slab cracks, control joints, service penetrations, sump openings and crawlspace floors. Concentration therefore depends on the geology under the specific house and on how that house handles pressure — which is why a neighbor's result predicts very little, and why testing is the only way to know.
EPA sets an action level of 4 picocuries per litre and recommends fixing homes at or above it, while also stating that there is no known safe level of radon and that homeowners may wish to consider action between 2 and 4 pCi/L. Testing comes in short-term and long-term forms; because levels fluctuate with weather, season and house operation, a long-term test or a confirmed pair of short-term tests gives a better estimate of actual exposure than a single two-day result.
Mitigation is a pressure solution rather than an air-cleaning one. The standard approach is active sub-slab depressurization: a pipe through the slab and a continuously running fan that keeps the region under the house at lower pressure than the interior, so soil gas is vented above the roof instead of entering.
Why it matters
Radon is the rare indoor air problem with a cheap, definitive test and a well-established fix, and it is entirely invisible to every other signal of a good house. A new, tight, beautifully built home can test high, and an old leaky one can test low; the geology does not care about the finishes. It is also the strongest argument for running the test before finishing a basement, because the mitigation route is trivial through an unfinished slab and disruptive through a completed floor.
The common misunderstanding
That an air purifier addresses it. Particle filters do not capture a noble gas, and sorbent media are not a residential radon control. The exposure comes largely from decay products that form continuously from whatever gas is present, so the effective interventions are preventing entry and venting the soil gas outdoors — not cleaning the air after it arrives.
Related terms
On Compound
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Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.