Radon, the soil gas under your house

Where it comes from

Almost all rocks contain trace amounts of uranium-238, a naturally-occurring radioactive element that decays in a long chain ending at stable lead. One step in that chain is radium-226; one step after that is radon-222 - a noble gas, chemically inert, with a half-life of 3.8 days. Because radon is a gas, it migrates out of the soil where the uranium and radium sit, percolates through cracks and pore spaces in rock, and reaches the surface. Outdoors, it disperses in the atmosphere to harmless levels almost immediately. Indoors, in a building sitting on uranium-bearing geology, it can concentrate to many times the outdoor level - especially in basements, in well-sealed modern construction, and in winter when buildings are closed up against the cold^[1].

Radon itself decays into a series of short-lived radioactive solid “daughter” products (polonium-218, lead-214, bismuth-214, polonium-214). Those daughters are charged particles. When you breathe air containing radon, the daughters stick to lung tissue, where they continue to decay - emitting alpha particles that damage cell DNA in the bronchial epithelium. The dose-response evidence for lung-cancer risk from this mechanism is one of the best-characterised in all of cancer epidemiology^[2].

How much risk, exactly

The most-cited pooled analysis of residential radon and lung cancer is Darby et al. (2005), which combined 13 European case-control studies covering more than 7,000 lung-cancer cases and more than 14,000 controls^[3]. The result: the relative risk of lung cancer increases by about 16 percent for every 100 Bq/m3 increase in long-term residential indoor radon concentration. North American and Chinese pooled analyses arrived at similar slopes within their confidence intervals.

That increase looks small written out. It is not. The lung cancer death rate among lifelong non-smokers is low in absolute terms, but the multiplier is the part that does the work. The WHO summarises the joint behaviour explicitly: in a person who has never smoked, indoor radon at the WHO reference level (100 Bq/m3) contributes a small but measurable share of lifetime lung-cancer risk. In a person who has smoked, the same radon exposure contributes roughly an order of magnitude more, because the damage pathways from cigarette smoke and from radon daughters multiply rather than add at the cellular level^[4].

This is the reason the highest-leverage public-health framing of indoor radon is paired - test for radon, and pair the result with a cessation conversation if applicable - not radon alone.

Where it is worst

Indoor radon concentrations vary by orders of magnitude across countries, across regions inside countries, and across individual homes on the same street. The single best predictor at the country scale is geology: granitic basement rocks, alum shale, karst limestones over uranium-bearing strata, and certain volcanic substrates all produce elevated indoor radon. The country scale hides enormous local variability: an “average” country can contain individual postcodes where a significant fraction of homes test above the WHO reference.

The European Atlas of Natural Radiation, published by the European Commission Joint Research Centre, is the most-cited cross-border survey^[5]. The WHO Handbook on Indoor Radon (2009) is the canonical reference for national programs^[6]. National programs are how individual readers actually get to numbers: the US EPA, UKHSA, BfS in Germany, SURO in Czechia, SSM in Sweden, STUK in Finland, Health Canada, IRSN in France, and equivalents in dozens of other countries publish regional risk maps, subsidise testing in high-potential areas, and certify mitigation contractors.

What you actually do about it

The action is testing your own home. National survey averages tell you the prior probability; the only way to know whether your house is affected is to measure it. Short-term home radon test kits cost $15-30 and run for 2 to 7 days on the lowest lived-in level of the building. You return the device to the lab in the post; the lab posts back a value in Bq/m3 or pCi/L. There is no calibration to do, no software to install, no follow-on subscription.

If the result is below the WHO reference level (100 Bq/m3), you are done; many homes test very low and never need to test again. If the result is between 100 and 300 Bq/m3, the WHO advises action and most national programs recommend a follow-up long-term test (because short-term tests can over- or under-shoot seasonally). If the result is above the national action level (200 or 300 Bq/m3 in most European programs, 4 pCi/L = 148 Bq/m3 in the US), the action is professional mitigation - usually a sub-slab depressurisation system, installed by a certified contractor for roughly $500-1,500 in most affected countries. The system runs a small fan continuously and reliably reduces indoor radon to a fraction of its original value.

None of this is medical advice. The dose-response slope is published; the action thresholds are published; the certified-contractor lists are published. Indoor radon is one of the few public-health risks where a normal household can walk through the entire decision tree from awareness to mitigation using only public-agency materials and a $20 test kit.

Why this story is not better-known

Radon does not photograph. It does not have a corporate villain or a political tribe. The action is small, individual, and only personally helpful - testing your own house does not protect your neighbours. The risk it carries is statistical, not acute. None of those properties make for good engagement on social platforms, and so radon barely registers in popular radiation coverage even in countries where national programs have run for forty years.

The countervailing observation is that more than half of the WHO European Region member states have national radon action plans under the EU Basic Safety Standards Directive, and most affected English-speaking countries (US, UK, Canada, Ireland, Australia) have decades of continuously-funded public programs. The infrastructure exists. The bottleneck is that individual homeowners do not yet associate the word “radiation” with the action “order a $20 kit.”

This site exists in part to help close that gap.

What to read next

If you live in a country with a national survey, the map shows you what is published for your country, with the source linked on the country card. If you want the underlying physics and epidemiology, the WHO Handbook on Indoor Radon is the right reference. If you want the personal-dose context - what proportion of your annual exposure is radon versus everything else - the next primer is Your personal radiation dose budget.

Sources

1. World Health Organization. (2021). Radon and health (fact sheet). https://www.who.int/news-room/fact-sheets/detail/radon-and-health
2. International Agency for Research on Cancer. (2012). IARC Monographs Volume 100D - Radiation. Residential radon is classified Group 1 (carcinogenic to humans). https://publications.iarc.fr/
3. Darby, S., Hill, D., Auvinen, A., et al. (2005). Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case-control studies. BMJ, 330(7485), 223. https://www.bmj.com/content/330/7485/223
4. US Surgeon General. The Health Consequences of Smoking (lung cancer chapters). https://www.cdc.gov/tobacco-surgeon-general-reports/about/index.html
5. European Commission Joint Research Centre. European Atlas of Natural Radiation. https://remon.jrc.ec.europa.eu/About/Atlas-of-Natural-Radiation
6. World Health Organization. (2009). WHO Handbook on Indoor Radon: A Public Health Perspective. https://www.who.int/publications/i/item/9789241547673

This primer is published under CC BY-SA 4.0. Copy, translate, and republish freely; please keep the citations intact.