Radon levels explained: what your pCi/L actually says
One unit, two thresholds, and a lot of misreading. Here’s what picocuries per liter measure, where EPA’s lines come from, and the honest action attached to every range your report could show.
The unit
A picocurie per liter (pCi/L) measures radioactive decays happening in a liter of air. Radon — an invisible, odorless gas produced by uranium decaying in soil and rock — exists outdoors everywhere at low background levels; the question a test answers is how much your house concentrates it. There is no absolute zero to chase, which is exactly why guidance works in thresholds rather than absolutes.
The two lines that matter
EPA recommends action at 4 pCi/L and suggests considering action between 2 and 4. The 4 pCi/L figure is a practical policy line — the level at which EPA judges reduction both warranted and reliably achievable — not a cliff where air becomes dangerous at 4.0 and safe at 3.9. Risk from radon is cumulative and roughly proportional to concentration and time; EPA identifies long-term exposure as the leading cause of lung cancer among people who never smoked. Lower is simply better, and 4 is where “fix it” becomes the standing advice.
Every range, with its honest next step
| Your result | What it means | Honest next step |
|---|---|---|
| Under 2 pCi/L | At or near typical background; no guidance calls for action. | File it. Retest in a couple of years or after major renovations. |
| 2 – 3.9 pCi/L | EPA’s consideration band — a judgment zone, weighed by basement use and household factors. | Long-term test for a truer average; act if the lowest level is heavily lived in. |
| 4 – 7.9 pCi/L | At or above the action level; the standing recommendation is to reduce it. | Confirm if unhurried, then quotes; skip confirmation inside a deal window. |
| 8 pCi/L and up | Well above the action level — same fix, more reason to schedule it. | Straight to licensed quotes; retesting rarely changes this decision. |
Why your number moves between tests
Radon entry rides on weather, soil moisture, frost, wind, and how the house was operated during the test — closed-house winter readings run higher than open-window June ones because the house holds more of what enters. Two honest tests can differ meaningfully. That’s not measurement failure; it’s why averages and long-term tests exist, and why serial retesting in pursuit of a number you like tells you about your patience, not your house. More on sequencing: testing vs. mitigation.
What mitigated homes typically reach
EPA’s consumer guidance is that most homes can be reduced below 4 pCi/L, and well-designed systems commonly verify below 2. The proof is always the post-mitigation test — insist on it in any scope of work. See how systems get there.
Common questions
Is 3.9 “safe” and 4.1 “dangerous”?
Neither word fits. Radon risk is a continuous curve — 4 pCi/L is where EPA plants its action flag for policy and practicality, not a boundary in biology. A 3.9 and a 4.1 describe essentially the same house. Read results in bands, decide with the band’s guidance, and spend exactly zero energy on the second decimal.
What does WHO say versus EPA?
The World Health Organization discusses a lower reference level (commonly cited at 2.7 pCi/L) while EPA uses 4 with a 2–4 consideration band. The two agree on the substance: risk rises with concentration, there is no perfectly safe level, and reduction is worthwhile as numbers climb. For U.S. homes and transactions, EPA’s framework is the operating standard — and this site cites it exclusively.
My result came back in becquerels (Bq/m³). How do I convert?
Divide by 37: 148 Bq/m³ equals 4 pCi/L, so a 200 Bq/m³ report is about 5.4 pCi/L — above EPA’s action level. Some labs and international devices default to becquerels; the guidance bands on this page apply identically once converted.
Does an elevated result mean we’ve been harmed already?
It means exposure, not injury — radon risk accumulates over years, which is why discovering a high number after living with it for a while calls for fixing it now rather than panic about the past. If you smoke, the combined risk is substantially higher (EPA is direct about this), making mitigation and quitting resources a powerful pair.
Sources
- U.S. EPA — Radon
- U.S. EPA — A Citizen’s Guide to Radon
- U.S. EPA — Health Risk of Radon
- U.S. EPA — Consumer’s Guide to Radon Reduction
- IEMA-OHS — Illinois Emergency Management Agency and Office of Homeland Security — Radon Program
Your number is in the action band?
Turn it into a priced, scheduled fix — free quotes from independent IEMA-licensed contractors.