The numbers
What the risk from radon actually is
Radon decays into short-lived radioactive particles that attach to dust and are inhaled. Once in the lung they emit alpha radiation directly into the tissue lining the airway. Over years, that damage is what causes cancer.
The EPA publishes its risk estimates as plain frequencies rather than percentages, and they are reproduced below exactly as issued. They are the most useful thing anyone has written about radon, and they are the reason this page exists.
EPA risk tables
Lifetime lung cancer deaths per 1,000 people exposed
Read a row as: if 1,000 people spent a lifetime in a home at this radon level, this many would be expected to die of lung cancer caused by it. The two columns are the whole argument of this page.
| Radon level | If you smoke | If you have never smoked |
|---|---|---|
| 20 pCi/L | 260 out of 1,000 | 36 out of 1,000 |
| 10 pCi/L | 150 out of 1,000 | 18 out of 1,000 |
| 8 pCi/L | 120 out of 1,000 | 15 out of 1,000 |
| 4 pCi/Laction level | 62 out of 1,000 | 7 out of 1,000 |
| 2 pCi/L | 32 out of 1,000 | 4 out of 1,000 |
| 1.3 pCi/L | 20 out of 1,000 | 2 out of 1,000 |
| 0.4 pCi/L | 3 out of 1,000 | Less than 1 in 1,000 |
EPA, A Citizen’s Guide to Radon. Figures are the EPA’s published estimates of lifetime lung cancer deaths per 1,000 people exposed at that level, and are reproduced here unmodified. They describe populations over a lifetime of exposure. Lung.com publishes environmental and public-health data. It is not medical advice, diagnosis or treatment, and it is not a substitute for a clinician.
The interaction
Smoking multiplies radon risk, it does not add to it
At 4 pCi/L the EPA puts the lifetime risk at 62 in 1,000 for a smoker and 7 in 1,000 for someone who has never smoked. That is close to a ninefold difference at the same radon level in the same house.
The mechanism is not mysterious. Tobacco smoke damages the airway lining and impairs the clearance that would otherwise remove inhaled particles; radon decay products then deliver radiation to tissue that is already compromised and staying put longer. Each makes the other worse, which is why epidemiologists describe the effect as multiplicative rather than additive.
The practical consequence: a household where someone smokes has far more to gain from fixing a radon problem than an identical household where nobody does. It is also why the county pages on this site put the local smoking rate next to the radon zone rather than on a separate page.
Why 4
What the action level represents
4 pCi/L is not a threshold below which radon stops causing cancer. The dose-response relationship runs down through the lowest levels ever studied, and the EPA states plainly that no level is risk-free.
The number is a practical judgement about where reduction becomes clearly worth doing: it is achievable in almost any house with existing technology, and the risk reduction at that point justifies the cost. The EPA also suggests considering a fix between 2 and 4 pCi/L, and the World Health Organization recommends a lower reference level of 2.7 pCi/L (100 Bq/m³).
A well-designed mitigation system commonly takes a house well below 2 pCi/L, so for many homes the choice is not between 4 and 3.9 but between 8 and 1.
Questions
Common questions about radon and health
How many people does radon kill in the US?
The EPA estimates about 21,000 lung cancer deaths a year in the United States are attributable to radon. That makes it the second leading cause of lung cancer after smoking, and the leading cause among people who have never smoked.
Is there a safe level of radon?
No. The EPA is explicit that there is no known level of radon exposure considered risk-free. Outdoor air averages about 0.4 pCi/L, so no home tests at zero. The 4 pCi/L action level is the point at which the EPA judges reduction to be clearly worthwhile, not a boundary between safe and unsafe.
How long does exposure have to last to matter?
Radon risk is driven by cumulative exposure over years, which is why long-term tests are a better guide than short-term ones and why the risk tables are expressed over a lifetime. A few days in a high-radon building is not the concern; a few decades of sleeping above one is.
Does radon cause anything other than lung cancer?
Lung cancer is the only health effect the EPA and the World Health Organization consider established for residential radon. Studies have looked at leukaemia and other cancers without producing consistent evidence. Radon is inhaled and delivers its radiation dose to lung tissue, which is consistent with lung cancer being the effect that shows up.
Next
None of this tells you about your house
Every number on this page is about populations at a given exposure. What your building is actually at is a separate question with a cheap, definite answer.