The scale
| Result | What the EPA says | In practice |
|---|---|---|
| Under 2.0 pCi/L | No action needed | Retest every few years, or after any major foundation or HVAC work |
| 2.0 to 3.9 pCi/L | Consider mitigating | A judgment call. Weighs more if someone smokes, or if the basement is a bedroom or an office |
| 4.0 to 8.0 pCi/L | Mitigate | Standard system territory. Confirm with a second test first if the first was short term |
| Above 8.0 pCi/L | Mitigate promptly | Do not wait for a long term test to confirm. Mitigate, then verify with a post test |
pCi/L means picocuries per liter of air. It measures radioactive decay, not concentration by volume, which is why the numbers look small.
Why the same house gives different numbers
This is the single most confusing part for homeowners, and the answer is that radon is genuinely variable.
- Season. Winter readings run higher almost everywhere, and the effect is pronounced in Spokane. Cold outside plus warm inside creates a stack effect that actively pulls soil gas up into the house.
- Weather. A passing storm drops barometric pressure, and lower pressure outside means more gas gets drawn out of the ground.
- How the house is being run. Windows open for a week in September will give you a comfortable number that has nothing to do with January.
- Where the detector sat. Basement and second floor in the same house commonly differ by a factor of two or more.
This is why a long term test is the more honest measure of your actual exposure, and why a single short term result in mild weather should not be treated as the final word.
The one comparison worth knowing
The EPA estimates that living in a home at 4.0 pCi/L carries a lung cancer risk comparable to smoking about half a pack of cigarettes a day, and radon is the leading cause of lung cancer among people who never smoked. The risk multiplies sharply for smokers. That framing is what usually moves people from thinking about it to testing.