Radon is a radioactive gas produced by the decay of uranium in soil and rock. It seeps through floors and foundations into buildings, where it can accumulate to levels far above outdoors. Breathing radon and its decay products is the second leading cause of lung cancer after smoking. Indoor levels swing by the hour with weather and ventilation and by a factor of two or more between summer and winter, so a single short test can be badly wrong. Long-term monitoring over months, or a continuous monitor, gives the number that actually describes exposure.
Where it comes from
Uranium is present in trace amounts in almost all rock and soil, and more in granite, shale and some limestones. It decays through radium to radon-222, a gas with a half-life of 3.8 days, long enough to migrate through soil pores and cracks into a building. Indoors, radon decays into short-lived solid isotopes that attach to dust and are inhaled; their alpha radiation damages lung tissue. Water from wells and some building materials contribute smaller amounts.
Units and action levels
| Body | Level | Meaning |
|---|---|---|
| WHO reference level | 100 Bq/m³ (2.7 pCi/L) | Recommended maximum for homes |
| US EPA action level | 4 pCi/L (148 Bq/m³) | Fix the home above this; consider fixing above 2 pCi/L |
| UK action level | 200 Bq/m³ | Target level 100 Bq/m³ |
| Outdoor air | 5 to 15 Bq/m³ | Background |
1 pCi/L = 37 Bq/m³. Risk rises roughly in proportion to concentration and duration; there is no safe threshold, only lower risk.
Why it varies so much
- Stack effect. Warm indoor air rises and draws soil gas in through the foundation. Winter, with heating on and windows closed, is usually the high season.
- Weather. Falling barometric pressure pulls radon out of the ground; rain and frozen soil cap it and push it toward the house.
- Ventilation. Opening windows dilutes it within hours; closing up lets it rebuild overnight.
- Occupancy patterns. Basements used at night read differently from those used by day.
A two-day test in summer with windows open can read 30 Bq/m³ in a home that averages 250 Bq/m³ over the year.
How monitors measure it
- Passive detectors (activated charcoal or alpha-track) sit for days to months and are mailed to a laboratory. Cheap and accurate for an average; no live reading.
- Consumer electronic monitors (Airthings, Aranet, Ecosense) use a passive diffusion chamber with an alpha detector. They show a rolling average, need a week or so to settle, and improve in accuracy as the averaging period lengthens. Their short-term readings are noisy by design.
- Continuous professional monitors with ion chambers or pulsed detectors give hourly resolution for diagnostics and mitigation checks.
The distinction between short-term and long-term readings on a monitor matters; see short-term vs long-term radon readings.
Reducing it
- Seal obvious floor cracks and sump openings; this alone rarely suffices.
- Improve ventilation of the lowest level.
- Install sub-slab depressurization: a fan drawing soil gas from beneath the floor and venting it above the roof. This is the standard fix and typically cuts levels by 80 to 99 percent.
- Keep monitoring after mitigation to confirm the fan is working.
Combination monitors that read radon alongside CO₂ and particles are compared in the office air quality guide; see also why indoor air can be worse than outdoor air.
Recommended Tools
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Frequently Asked Questions
What radon level is dangerous?
The WHO reference level is 100 Bq/m³ (2.7 pCi/L); the US EPA action level is 4 pCi/L (148 Bq/m³). Risk is proportional to exposure with no threshold, so lower is always better.
How long should I monitor radon?
At least 90 days, ideally a full year covering a heating season. Short tests of 2 to 7 days screen for very high levels but can miss a home that is high only in winter.
Which rooms should be tested?
The lowest lived-in level: basement rooms, ground-floor bedrooms and living rooms. Radon is lowest on upper floors.
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