Radon in US homes is measured in picocuries per liter of air (pCi/L). EPA recommends fixing a home at 4 pCi/L or more and considering a fix between 2 and 4 pCi/L. There is no level that is completely safe: the lower the number, the lower the risk (EPA, A Citizen’s Guide to Radon; CDC).
Radon levels chart
| Radon level | What it means | What EPA recommends |
|---|---|---|
| 0.4 pCi/L | Typical outdoor air | Nothing; this is the background level |
| 1.3 pCi/L | Average indoor level in US homes | Nothing required |
| Below 2 pCi/L | Below EPA’s levels for action | Most homes that are fixed end up here or lower |
| 2 to 4 pCi/L | Moderate | Consider fixing |
| 4 pCi/L or more | Elevated (EPA’s action level) | Fix your home, after a confirming test |
| Above 8 pCi/L | More than twice the action level | Take a second short-term test straight away; fix if it confirms the level |
Sources: EPA, Citizen’s Guide and Home Buyer’s and Seller’s Guide. Nearly 1 in 15 US homes is estimated to be at or above 4 pCi/L.
Why 4 pCi/L?
The 4 pCi/L action level is the point at which EPA says you should fix your home. It is not a line between safe and unsafe. EPA says any radon exposure carries some risk, and Congress set a long-term goal that indoor air should be no more radioactive than outdoor air (about 0.4 pCi/L). With today’s methods, most homes can be brought to 2 pCi/L or below (EPA, Citizen’s Guide; EPA action level page).
Lifetime risk at each level
EPA estimates how many people out of 1,000 would get lung cancer if they were exposed to a given level over a lifetime. Smoking multiplies the risk (Citizen’s Guide).
| Radon level | Never smoked (per 1,000) | Smokers (per 1,000) |
|---|---|---|
| 20 pCi/L | About 36 | About 260 |
| 10 pCi/L | About 18 | About 150 |
| 8 pCi/L | About 15 | About 120 |
| 4 pCi/L | About 7 | About 62 |
| 2 pCi/L | About 4 | About 32 |
| 1.3 pCi/L (average indoor) | About 2 | About 20 |
For people who have never smoked, the risk at 4 pCi/L is about the same as the risk of dying in a car crash (EPA).
pCi/L and Bq/m³
Most of the world, and many radon monitors, report radon in becquerels per cubic meter (Bq/m³). One pCi/L equals 37 Bq/m³.
| pCi/L | Bq/m³ |
|---|---|
| 1 | 37 |
| 2 | 74 |
| 2.7 | 100 (WHO reference level) |
| 4 | 148 |
| 8.1 | 300 (WHO upper limit for a national reference level) |
The World Health Organization recommends that countries set a reference level of 100 Bq/m³ (about 2.7 pCi/L), and no higher than 300 Bq/m³ where that can’t be reached. WHO estimates lung cancer risk rises by about 16% for every 100 Bq/m³ increase in long-term average radon (WHO).
One reading isn’t the whole story
Radon levels rise and fall from day to day and season to season. A short-term test is a snapshot; a test longer than 90 days is closer to your year-round average. Close to 4 pCi/L, short-term results are uncertain: when two short-term tests average 4.1 pCi/L, there is about a 50% chance the year-round average is somewhat below 4 (EPA, Citizen’s Guide). Levels are often higher in basements and lower floors (CDC), which is why EPA asks you to test the lowest level you use.
Your neighbours’ results don’t predict yours. Homes next to each other can have very different levels (EPA, Home Buyer’s and Seller’s Guide). County figures show how often tests in your area come back high, which tells you how seriously to take testing, not what your own home holds. See how to test for radon.