St. Louis Metro · Missouri & Illinois
Radon Levels and pCi/L
Picocuries are the unit radon is measured in. Picocuries per liter, written pCi/L, counts radioactive decays in air rather than the mass or volume of gas present, and every radon reading you will ever see is expressed in picocuries per liter.
One picocurie per liter equals about 0.037 radioactive decays per second in one liter of air. In picocuries per liter, typical outdoor air sits near 0.4, the average US home sits near 1.3, and the EPA action level is 4.0 picocuries per liter.
That action level is not a safety threshold. It is the point at which the EPA recommends fixing the home, and the EPA is explicit that there is no level of radon considered entirely risk-free.
Reading the Scale
Systems are engineered to bring your home below the EPA action level of 4.0 pCi/L.
Measured in pCi/LThe Unit
What are picocuries and what does pCi/L mean?
A picocurie is one trillionth of a curie, the standard unit of radioactivity, and pCi/L expresses how many of those are present in each liter of air. Because radon is measured by its radioactivity rather than its concentration by volume, the unit counts decay events per unit time.
In practical terms, 1 pCi/L means roughly 0.037 radioactive disintegrations per second in every liter of air in the room, or about 2.2 decays per minute.
Most of the world reports radon in becquerels per cubic metre instead. The conversion is straightforward: 1 pCi/L is approximately 37 Bq/m3, so the EPA action level of 4.0 pCi/L is about 148 Bq/m3.
The Scale
What do different radon levels mean?
Different radon levels mean different recommended actions, and radon levels are interpreted against the EPA action level of 4.0 pCi/L, above which the EPA recommends installing a mitigation system. Radon levels below that line are not risk-free, they are simply below the threshold at which the EPA advises fixing the home.
The table below gives the radon levels that come up most often and what each one means.
| Reading | What it represents | Typical recommendation |
|---|---|---|
| 0.4 pCi/L | Typical outdoor air | None |
| 1.3 pCi/L | Average US indoor level | Retest every few years |
| 2.0 pCi/L | EPA suggests considering action | Mitigation optional |
| 4.0 pCi/L | EPA action level | Fix the home |
| 8.0 pCi/L | Twice the action level | Fix the home |
| 20.0 pCi/L | Five times the action level | Fix promptly |
A reading between 2.0 and 4.0 pCi/L sits in a genuine grey zone. The EPA notes that homeowners may want to consider fixing at those levels, because the risk does not drop to zero below the action line.
Common Query
Is a radon level of 8 pCi/L dangerous?
A radon level of 8 pCi/L is twice the EPA action level of 4.0 pCi/L, and the EPA recommendation at that reading is unambiguous: fix the home.
Some context on what that number does and does not mean. It is not an emergency requiring anyone to leave the house, because radon risk accumulates over years of exposure rather than acutely. It is also well within what a standard mitigation system handles, since active sub-slab depressurization reduces indoor radon by as much as 99 percent.
What an 8 pCi/L reading does mean is that the house should not be left as it is. Left alone, that level persists indefinitely, and it will typically be higher still in winter.
Comparison
How do radon levels compare to smoking cigarettes?
The EPA compares living in a home at 4.0 pCi/L to the lung cancer risk of smoking roughly eight cigarettes a day, and uses that comparison in its own consumer materials to make an abstract number concrete.
Some important qualifications on that comparison:
- It is statistical, not individual. It describes population-level risk, not what will happen to any one person.
- Smoking and radon compound each other. The EPA notes the combined risk for smokers exposed to radon is substantially higher than either factor alone.
- Radon is the leading cause of lung cancer among people who have never smoked, according to the EPA, which estimates roughly 21,000 radon-related lung cancer deaths a year in the United States.
The useful takeaway is comparative rather than precise. It places an invisible household exposure on a scale people already understand.
Variability
Why do radon levels change between tests?
Radon levels in the same house vary substantially from test to test, and a single short-term reading is a snapshot rather than an annual average.
- Season. Levels are highest from December through February, and elevated from roughly October through April.
- Weather. Barometric pressure, wind, rain, and frozen ground all change how much soil gas enters.
- House operation. Open windows, exhaust fans, and HVAC settings shift indoor pressure.
- Test placement. A reading from the lowest lived-in level is not comparable to one from an upper floor.
This is why a summer test can understate a home’s true exposure, and why closed-house conditions are required during short-term testing. Our page on radon levels in winter versus summer covers the seasonal swing in more detail.
Local Conditions
Typical radon levels in St. Louis homes
Typical radon levels in St. Louis homes span the full range, and county averages conceal how much variation in radon levels exists street to street. Homes in the same subdivision, built by the same builder in the same year, routinely test differently because of what is under the individual lot.
The geology helps explain the spread. Much of St. Louis County and St. Charles County sits on limestone and shale with meaningful uranium content, Jefferson and Franklin counties include historically mineralized areas, and Madison and St. Clair counties on the Illinois side carry their own elevated zones.
None of that substitutes for a measurement at the address in question. See radon testing for how a test is run, or radon mitigation for what follows an elevated result.
Common Questions
Frequently Asked Questions About Radon Levels and pCi/L
What is a safe radon level in pCi/L?
There is no radon level the EPA describes as entirely safe. The EPA action level is 4.0 pCi/L, meaning homes at or above that should be fixed, and the EPA suggests homeowners may want to consider action between 2.0 and 4.0 pCi/L. Typical outdoor air is around 0.4 pCi/L, which is the practical floor for indoor air.
How many picocuries is dangerous?
The EPA recommends fixing any home testing at or above 4.0 pCi/L. Risk rises with both concentration and years of exposure rather than crossing a single threshold, so 8 pCi/L is roughly twice the concern of 4 pCi/L and 20 pCi/L roughly five times. No reading requires leaving the house, because radon risk is cumulative rather than acute.
What is the average radon level in a home?
The average indoor radon level in US homes is about 1.3 pCi/L, compared with roughly 0.4 pCi/L in outdoor air. The EPA estimates about 1 in 15 homes nationally, roughly 6.6 percent, test at or above the action level of 4.0 pCi/L. Regional averages vary widely and say little about any specific address.
How do you convert pCi/L to Bq/m3?
Multiply by 37. One picocurie per liter equals approximately 37 becquerels per cubic metre, so the EPA action level of 4.0 pCi/L is about 148 Bq/m3. Becquerels per cubic metre is the unit used in most countries outside the United States, and the World Health Organization references a lower reference level than the EPA.
Can radon levels go down on their own?
Radon levels fluctuate but do not permanently decline on their own, because soil gas enters the house continuously. A retest can read lower simply because it was run in a different season or under different weather, which is why the EPA specifies closed-house conditions for short-term tests. Only mitigation produces a durable reduction.
References: EPA radon action-level guidance and A Citizen’s Guide to Radon.
Turn the Number Into a Plan
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A test in the lowest lived-in level under closed-house conditions gives you a number that means something. We handle both the test and the fix.