What a picocurie actually is
Your test report says something like “4.7 pCi/L,” and nobody ever explains the unit. Here’s the plain version.
A picocurie is a tiny measure of radioactivity, a trillionth of a curie. “Picocuries per liter” counts how much radon radioactivity sits in each liter of your indoor air. At 1 pCi/L, roughly two radon atoms decay every minute in every liter of air in the room. At 10 pCi/L, about twenty. The report is literally counting atomic events in your basement air. That’s all the number is.
The guidance for reading that number comes from the EPA, whose consumer publications set the reference points everyone in this trade uses: an action level at 4 pCi/L, a consider-action range from 2 to 4, and the blunt note that no level is officially “safe”. Outdoor air itself carries a background of around 0.4 pCi/L.
One contract this page will keep: it explains the scale, and that’s all. What your specific number means for your specific family over your specific years in the house is not something a webpage can calculate, and you should be suspicious of anyone who claims otherwise in either direction, whether they’re selling panic or selling reassurance.
The published scale
Under 2 pCi/L
- What EPA guidance says
- Below the consider-fixing range
- Typical next step
- Retest every few years
2–4 pCi/L
- What EPA guidance says
- Consider fixing the home
- Typical next step
- Confirm, then weigh mitigation
4 pCi/L and above
- What EPA guidance says
- Fix the home
- Typical next step
- Mitigation with a verification retest
| Result range | What EPA guidance says | Typical next step |
|---|---|---|
| Under 2 pCi/L | Below the consider-fixing range | Retest every few years |
| 2–4 pCi/L | Consider fixing the home | Confirm, then weigh mitigation |
| 4 pCi/L and above | Fix the home | Mitigation with a verification retest |
The scale is from EPA consumer guidance on radon.
Why the number moves
Test the same house twice and you’ll likely get two different numbers. That’s not sloppy testing. It’s the nature of the thing being measured.
Radon enters a house on pressure differences, and pressure differences change constantly. Winter is the big one: a closed-up, heated home pulls harder on the soil beneath it, so cold-month readings typically run higher than summer ones. Weather matters day to day. Falling barometric pressure ahead of a storm can push more gas out of the ground, and saturated soil after heavy rain redirects gas toward the dry path under your foundation. Even daily living moves the number: furnaces, fireplaces, bathroom fans, and clothes dryers all throw air out of the house, and that air gets replaced partly from the soil.
So a single reading is a photograph of a moving subject. That’s why guidance treats results near the line carefully. A 4.4 in February might be a 3.2 annual average, or it might not, and that’s why near-the-line results usually earn a confirming test before money gets spent.
The two test styles handle this differently. Short-term tests, two to seven days, take the quick photo: right for transactions and quick answers, but exposed to whatever week they capture. Long-term tests, ninety days or more, average across seasons and weather, and get closest to the number that actually matters: what your home does over time. Quick answer, short test; true average, long test.
Have a number and want it lower? Start with an estimate.
What to do with your number
The scale above sorts the next move into three lanes.
If you’re under 2 pCi/L, file the report and put a retest on the calendar a few years out, or after any renovation that touches the foundation, which can change how a house breathes.
Between 2 and 4 is the judgment zone. EPA guidance says consider fixing. A long-term test is a sensible next step here. It converts a borderline snapshot into a real average, and the average is what should drive the decision. The testing guide covers the options and how each one runs.
At 4 or above, the guidance stops hedging: fix the home. Start with your test came back high if you want the full walkthrough of confirming and deciding, or go straight to radon mitigation to see what the fix involves: a system designed to your foundation, with a retest at the end that proves the level came down.
Whatever the lane, the number on your report is information, not a grade. Houses aren’t good or bad at radon. They’re just tested or untested, and yours is tested. That’s the harder half done.