Acceptable Radon Levels: How to Read Your Test Number

You ran a radon test and got a number. Now the real question: what counts as acceptable? The honest answer from the EPA is that no level of radon is truly “safe” — risk scales with exposure all the way down. But there are well-defined thresholds for when to act, and they're lower than many buyers assume.

Here’s the full scale in plain English, where the EPA and WHO draw their lines, and what to do at each level.

The radon scale, in pCi/L

Context for the 4.0 line: living long-term at 4 pCi/L carries roughly the lung-cancer risk of smoking about half a pack a day, per EPA risk comparisons — and the agency attributes about 21,000 lung cancer deaths a year to residential radon. The action level is a cost-benefit threshold, not a safety certificate: the EPA itself recommends considering mitigation anywhere above 2.

Why one test isn't the whole story

• Radon fluctuates. Levels swing with seasons (higher in winter, when houses are sealed), weather, and even barometric pressure. A short-term test is a snapshot.
• Short-term vs long-term: 2–7 day charcoal or electret tests are what real-estate transactions use; 90+ day alpha-track tests measure the true annual average. If a short-term result lands between 2 and 8, a long-term follow-up is worth the $30.
• Test the right floor. Measure the lowest level you actually use — a finished basement bedroom matters; an unused crawlspace doesn't (though it tells you what's coming through).
• In a transaction, use a professional. Tamper-resistant continuous monitors ($150–$300) produce hour-by-hour data both sides can trust.

If the number is high: mitigation works

Radon is among the most fixable environmental problems a house can have. A standard active sub-slab depressurization system — a pipe and fan that vents soil gas above the roofline — costs $800–$2,500 installed and typically lands homes below 2 pCi/L, often near outdoor ambient. Post-mitigation, retest to confirm, then check the system's manometer occasionally and re-test every couple of years.

Buying in a high-radon area? See our Zone 1 guide for how to handle testing and mitigation inside a purchase negotiation, and the state-by-state map for your region's baseline odds.