Radon is the second-leading cause of lung cancer, and the only fix starts with a test

Radon is the second-leading cause of lung cancer after smoking and the leading cause in people who have never smoked, and the only way to know whether a home has a problem is to test for it [s1]. The gas is colourless and odourless, it seeps from uranium in soil and rock into buildings through cracks and gaps, and the health risk rises smoothly with how much is present — with no threshold below which exposure is demonstrably safe [s2].

The US Environmental Protection Agency attributes about 21,000 lung cancer deaths a year in the United States to radon, and recommends that homes be fixed when the measured level reaches or exceeds 4 picocuries per litre of air [s1]. The World Health Organization estimates that radon causes between 3% and 14% of lung cancers depending on the average level in a country and how common smoking is, and proposes a lower reference level of 100 becquerels per cubic metre, roughly 2.7 pCi/L [s3].

What the evidence actually shows

The strongest human evidence is a 2004 pooled analysis of individual data from 13 European case-control studies, covering 7,148 people with lung cancer and 14,208 without [s2]. It found that the risk of lung cancer increased by 16% (95% confidence interval 5% to 31%) for each 100 Bq/m³ increase in long-term average radon, after correcting for the measurement error that otherwise dilutes the association [s2]. The relationship was linear, with no sign of a safe threshold — the risk kept climbing across the whole range studied [s2].

The absolute numbers matter for interpreting that percentage. In the pooled analysis, the modelled risk of lung cancer by age 75, in the absence of other causes of death, was about 0.4% for a lifelong non-smoker living at 0 Bq/m³, 0.5% at 100 Bq/m³ and 0.7% at 400 Bq/m³ [s2]. For smokers, the corresponding risks were roughly 25 times higher at each level [s2]. That multiplier is the central public-health point: radon and tobacco smoke act together, so the same radon concentration is far more dangerous in a household where someone smokes [s2][s3]. Across those studies collectively, the authors concluded radon is responsible for about 2% of all deaths from cancer in Europe [s2].

Testing and remediation

Because radon cannot be sensed and varies enormously between neighbouring houses depending on soil and construction, measurement is the only way to know a home's level [s1][s3]. Short-term and long-term test kits are inexpensive and widely available; levels are typically highest in basements and ground-floor rooms and vary by season, which is why longer measurements give a more reliable annual average [s1].

Where levels are high, the established engineering fix is sub-slab depressurisation — a vent pipe and fan that draw radon from beneath the foundation and exhaust it above the roof before it enters living space [s1]. The EPA describes such systems as able to reduce indoor radon substantially, and sealing cracks alone is considered insufficient as a standalone measure [s1]. New homes can be built with passive radon-resistant features that are cheaper to install than to retrofit [s1].

The limits of the evidence

The case-control studies rest on reconstructing years of past exposure from measurements taken in current or former homes, which introduces error that the pooled analysis corrected for statistically rather than eliminated [s2]. The absolute lifetime risks are model estimates, not directly counted deaths [s2]. And most of the disease burden falls on smokers and recent ex-smokers, in whom radon multiplies an already large risk; for a lifelong non-smoker in an average home, the absolute risk remains low even though radon is, proportionally, their leading environmental lung-cancer cause [s2][s3].

None of that weakens the practical conclusion. The exposure is preventable, the test is cheap, the dose-response is real and has no safe floor, and the interaction with smoking means the highest-value action is to test — and, for anyone who smokes, to stop [s1][s2][s3]. This is an informational summary, not medical or home-safety advice.