The unit, the report, and who says so
Every radon test comes back as a number followed by “pCi/L” — picocuries per liter. A curie measures radioactive decay, a picocurie is a trillionth of one, and the “per liter” part means per liter of the air in your house. At 4 pCi/L, roughly nine atoms of radon are decaying every minute in every liter of air. You don’t need the physics — you need to know the number is a concentration, like parts per million, and higher means more.
A test report shows the average concentration over the test period, sometimes with hourly detail if a monitor ran the test. That average is what gets compared against published guidance.
The guidance comes from the EPA, and state radon programs, including South Dakota’s, carry the same figures. The big one is the action level: 4 pCi/L, at or above which the EPA says fix the home. Between 2 and 4, the EPA says consider fixing it. Mitigation is the fix in both cases; the difference is urgency.
One contract this page will keep: it explains the published scale, and that’s all. What a specific number means for your family’s health depends on exposure years, occupancy, and factors no webpage knows. This page tells you where the lines are, not which side of risk you personally sit on.
The published scale
About 0.4 pCi/L
- What it represents
- Average outdoor air
- What guidance says
- The practical floor
About 1.3 pCi/L
- What it represents
- Average U.S. indoor level
- What guidance says
- No action suggested
2 to 4 pCi/L
- What it represents
- Elevated but below the line
- What guidance says
- Consider fixing the home
4 pCi/L and above
- What it represents
- The EPA action level
- What guidance says
- Fix the home, then retest
| Level | What it represents | What guidance says |
|---|---|---|
| About 0.4 pCi/L | Average outdoor air | The practical floor |
| About 1.3 pCi/L | Average U.S. indoor level | No action suggested |
| 2 to 4 pCi/L | Elevated but below the line | Consider fixing the home |
| 4 pCi/L and above | The EPA action level | Fix the home, then retest |
Guidance figures come from EPA consumer publications. No level is called perfectly safe; 4 pCi/L is where "fix it" begins.
Why the number moves
Test the same basement twice and you’ll likely get two different numbers. That’s not a flaw in the kit. Radon levels genuinely rise and fall, for reasons worth knowing before you make decisions off a single decimal.
Season is the big one. In a shut-tight winter house, the furnace and rising warm air create gentle suction at the foundation, pulling soil gas in, so winter readings often run higher. Weather piles on: falling barometric pressure, soaking rain, and strong wind can each push a reading up or down for a few days.
How the house runs matters too. Windows open all week, bath fans running constantly, a busy back door: all of it dilutes the sample. That’s why short-term tests specify closed-house conditions: shut windows and doors for 12 hours before and during the test. The rules aren’t fussiness. They make the snapshot repeatable.
This is also why a single short-term reading near the line usually gets confirmed before money moves. A 4.6 in a January cold snap might be a 3-and-change annual average. Or the average might be higher. A second short test tells you if the first was a fluke. A long-term test, 90 days or more, absorbs the swings and gives the truest picture of what the household actually breathes. Short tests answer fast; long tests answer accurately. Each is good at its own job.
Have a number and want it lower? Start with an estimate
What to do with your number
Under 2 pCi/L. You’re below the average the guidance worries about. File the report, and retest in a few years or after big changes like a finished basement, new mechanicals, or foundation work. Levels drift as houses age.
Between 2 and 4. The consider zone. Nothing obligates you, but mitigation works just as well on a 3 as on a 12, and homes in this range often drop near outdoor levels with a system. Worth weighing if the basement holds bedrooms or a home office, since time-in-space is what exposure is made of.
At 4 or above. Published guidance stops hedging here: fix the home. The good news is the fix is routine and provable. Radon mitigation pulls the gas from under the house, and a follow-up test verifies the drop. Start with the high-result walkthrough; it covers confirming borderline numbers and what installation actually involves.
No number yet? Then the scale is theory. The testing guide matches the test to the question, whether that’s a quick screen, a transaction deadline, or a true annual average. Any of them beats wondering.
Keep reading
-
Your test came back high
What to do when the number lands at 4 or above — without the panic.
Read more -
The testing guide
Kits, monitors, and 90-day tests, matched to the question you're asking.
Read more -
Radon levels in your area
What the published data says about this corner of South Dakota.
Read more