Radon and cancer

Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock. It is colourless and odourless, and can accumulate to elevated concentrations inside buildings, particularly those with limited ventilation or in areas with uranium-rich geology. Radon is recognised as the leading cause of lung cancer in people who have never smoked, and the second leading cause of lung cancer overall after tobacco smoking. Lung cancer is the primary cancer type linked to radon, though emerging research examines associations with other malignancies including multiple myeloma and brain tumours.

Radon gas itself is inert, but its short-lived radioactive decay products, including polonium-218, lead-214, bismuth-214, and polonium-214, are reactive and can attach to airborne particles. When inhaled, these particles deposit in the bronchial airways and lung tissue. During radioactive decay, they emit alpha particles, which are densely ionising and deposit a concentrated dose of radiation in nearby cells. This alpha radiation causes DNA double-strand breaks, chromosomal damage, and mutations in critical genes such as TP53, initiating the sequence of events that can lead to lung cancer. Research on combined radon and cigarette smoke exposure suggests the two act synergistically, with smoke-induced changes to airway epithelium potentially increasing retention of radon decay products. Dose modelling studies estimate that absorbed radiation doses in children may exceed those in adults due to smaller lung capacity and higher breathing rates.

A population-based study using SEER (Surveillance, Epidemiology, and End Results) cancer registry data from 1975 to 2022 compared lung cancer incidence in counties classified by the Environmental Protection Agency (EPA) as low-risk or high-risk (above 4.0 picocuries per litre) for indoor radon, finding higher incidence rates in high-radon counties. An ecological study in Denmark found a linear association between age- and sex-standardised multiple myeloma (MM) incidence rates and indoor radon concentrations across municipalities, suggesting a possible link beyond lung cancer, though ecological designs limit causal inference. Machine learning models applied to ZIP code-level data in Pennsylvania demonstrated that county-level average estimates used in most studies substantially underestimate regional variability in indoor radon exposure, with implications for risk assessment accuracy. A systematic review confirmed that children are more vulnerable to radon health effects due to physiological differences, though the 10 to 30 year latency period for mutations to manifest as cancer means observed childhood cancer incidence does not reflect the full lifetime risk of early exposure. A qualitative study of home radon testing behaviours found that low risk perception and lack of awareness were primary barriers to testing, despite the availability of low-cost detection methods.

Non-smokers represent the population in whom radon-attributable lung cancer risk is most prominent on a relative basis, since tobacco smoking is an overwhelming risk factor in its own right. However, smokers also exposed to high indoor radon levels face a combined risk substantially greater than either factor alone, as the two exposures appear to interact more than additively. Geographic variability is large, with high-risk areas concentrated above geological formations containing elevated uranium content. People living or sleeping in basement or ground-floor rooms in high-radon regions face the greatest indoor exposure. Children are particularly vulnerable because their higher breathing rates and longer remaining lifetimes mean greater cumulative radiation dose from chronic exposure.

The causal link between residential radon exposure and lung cancer is well established, recognised by the International Agency for Research on Cancer (IARC) and major public health agencies, based on miner cohort studies and pooled analyses of residential studies in Europe and North America. The association with multiple myeloma and other non-lung cancers is biologically plausible but supported mainly by ecological studies that cannot establish individual-level exposure-response relationships or control adequately for confounders. Dose modelling for low-level chronic residential exposure remains an active area of research, with machine learning tools improving the granularity of exposure estimates beyond what traditional county-level surveys provide.

Radon is a well-documented environmental cause of lung cancer that is invisible to the senses but detectable with low-cost testing. Evidence supports that areas with elevated geological radon potential have higher lung cancer rates, and that indoor exposure can be meaningfully reduced through building ventilation improvements and radon mitigation systems. The interaction between radon and tobacco smoking appears to be more than additive, meaning both risk factors together confer considerably greater risk than each alone.