What radon level is dangerous?
Short answer: The EPA action level is 4.0 pCi/L — at or above that level, mitigation is recommended. The EPA also recommends considering mitigation between 2.0 and 4.0 pCi/L. There is no safe level of radon; risk is cumulative with exposure.
The EPA's official action level is 4.0 pCi/L (picocuries per liter of air). At or above that level, the EPA recommends installing a mitigation system.
However, the EPA also explicitly recommends considering mitigation between 2.0 and 4.0 pCi/L, because there is no known safe threshold for radon — risk increases with both concentration and time of exposure. The World Health Organization recommends an even lower action level of 2.7 pCi/L.
For context:
- Outdoor air: ~0.4 pCi/L (the natural background level)
- National indoor average: ~1.3 pCi/L
- Iowa indoor average: ~8.5 pCi/L
- EPA action level: 4.0 pCi/L
- WHO action level: 2.7 pCi/L
In Iowa City, most of the homes we test come back between 5 and 25 pCi/L. We've seen single-family homes test as high as 80+ pCi/L — those families had no idea until a routine real estate test caught it.