Your Radon Test Said 4.2. Now What Do the Numbers Actually Mean?
You ran a radon test, waited a few days, and got back a single number: something like 4.2 pCi/L. Great. Except nobody handed you the scale that number sits on, so you have no idea whether 4.2 is a shrug or an emergency. That's the gap we're closing today — what the picocuries-per-liter scale actually means, where the action line is and why, and what a sensible person does at each level.
None of this is fear-mongering. Radon is the second-leading cause of lung cancer in the United States and the number one cause among people who never smoked — the EPA estimates about 21,000 radon-related lung cancer deaths a year. But it's also one of the most fixable hazards in your whole house. You just have to read your number correctly first. And then there's Joe, who read his number and drew exactly the wrong conclusion.
The scale, in plain numbers
Radon is measured in picocuries per liter of air (pCi/L). Here's the scale the EPA uses, with the risk anchored to real comparisons:
| Your test reads… | What it means |
|---|---|
| ~0.4 pCi/L | The average outdoor level. This is the floor; you can't realistically get a house below the low 1s. |
| ~1.3 pCi/L | The average indoor level in American homes. |
| 2 to 4 pCi/L | The EPA says consider fixing in this band. Not an emergency, but not nothing. |
| 4.0 pCi/L and up | The EPA action level. Fix your home. |
| 10, 20 pCi/L+ | High. At 20 pCi/L, a never-smoker's lifetime lung-cancer risk is roughly 36 in 1,000 — about 35 times the risk of drowning. |
So your 4.2? That's just over the action line. Not a reason to panic and move out tonight, but a clear "yes, fix this" — and confirm it with a second test first, because radon bounces around day to day.
Why 4.0 and not zero?
People assume the goal is zero. It isn't, because it can't be — outdoor air already carries radon, so no house sits at zero. The EPA set 4.0 pCi/L as the action level because that's where reducing the level reliably makes sense against what mitigation costs, and because pushing a home below about 2 pCi/L gets technically difficult. If you land in the 2-to-4 gray zone, it's a judgment call: how much time do people spend on the lowest level, is anyone a smoker (radon and smoking multiply each other's risk), and is a young family involved.
What to do at your number
- Under 2 pCi/L: You're near the practical floor. Re-test every few years or after major foundation/HVAC work.
- 2 to 4 pCi/L: Consider fixing, especially if you have a finished basement you live in or a smoker in the house.
- 4 pCi/L and up: Confirm with a second test, then get a mitigation quote. This is routine work, not a demolition.
- Anything over ~8: Confirm and fix promptly. A short-term follow-up test is fine here since you already know you're acting.
If you want to double-check your number against the official scale — including the side-by-side risk tables for smokers versus never-smokers — the EPA's Health Risk of Radon page is the authoritative source, and it's where the figures in this post come from.
He got a 3.6 and threw a party.
Joe — the basement-dwelling legend himself — finally tested and got a 3.6 pCi/L. Under 4! He interpreted "under the action level" as "safe," cracked a beer in his basement man-cave, and never thought about it again.
Trouble is, Joe smokes, and Joe spends four hours a night down in that basement. For a smoker, 3.6 is squarely in the "seriously consider fixing" zone — radon and cigarette smoke amplify each other. Joe treated a soft judgment-call number like a hard all-clear. Don't be Joe. "Under 4" isn't "zero risk" — read your number in the context of who you are and where you spend your time.
The bottom line
Your radon number only means something once you can place it on the scale. Outdoor air is around 0.4, the average home is around 1.3, and 4.0 is the line where the EPA says fix it. If you're over 4, confirm with a second test and get a quote — mitigation is routine and effective. If you're in the 2-to-4 band, weigh how you actually live in the space. And whatever you do, don't read "under 4" as "done thinking about it."