How to Read Your Radon Test Results

By Steve Crews · Last updated June 19, 2026

What pCi/L actually means

Radon in the US is measured in picocuries per liter, abbreviated pCi/L. One picocurie is a unit of radioactivity, and “per liter” just refers to a liter of air. In most countries outside North America, the equivalent unit is becquerels per cubic meter (Bq/m3). The two measure the same thing on different scales: 1 pCi/L equals about 37 Bq/m3.

If your results come back as a single number with no unit printed, it is almost certainly pCi/L. Labs in the US report it that way by default.

What the full range of results tells you

The EPA does not publish a single cutoff with a clean “safe” and “unsafe” on either side. Risk is a gradient, and the guidance reflects that.

Below 2 pCi/L: The EPA does not recommend taking action at this level. That said, even a result of 1.5 pCi/L is above the typical outdoor air concentration of about 0.4 pCi/L. You cannot reduce indoor radon to zero, and the EPA acknowledges that. Below 2 is considered lower priority, not zero risk.

2 to 4 pCi/L: The EPA specifically says you “should consider” fixing your home in this range. The average level in US homes is around 1.3 pCi/L, so a result between 2 and 4 is meaningfully elevated compared to the national average, even if it sits below the action threshold. How seriously to take it depends on your circumstances: how much time your household spends in the lowest level of the home, and whether anyone smokes. Radon and tobacco are each lung-cancer risks; together, their effect is far greater than either alone.

4 pCi/L and above: This is the EPA action level. The agency recommends mitigation as soon as practical. If you are sitting at 5 or 6 pCi/L, you do not need to treat it as an emergency, but you should move forward rather than delay. Most mitigation systems can bring even a moderately high reading well below the action level within a short time.

Above 10 pCi/L: Get a certified mitigator involved promptly. Very high results sometimes reflect testing error (a test placed too close to the sump or in an unfinished utility room), so your mitigator will confirm the reading and locate the entry points, but numbers in this range are taken seriously. See how much radon mitigation costs if you are dealing with a result in this territory.

For international context: the World Health Organization recommends a reference level of 100 Bq/m3 (about 2.7 pCi/L) where achievable. Health Canada’s guideline is 200 Bq/m3, roughly 5.4 pCi/L. These figures differ from the EPA’s not because the underlying health science disagrees, but because each agency weighs local housing conditions, costs, and what is realistic to achieve.

Short-term vs. long-term: the test type changes how you interpret the number

This distinction matters more than most people realize.

A short-term test, typically a charcoal canister left out for 2 to 7 days, captures radon levels during those specific days. Weather patterns, seasonal variation, whether the house was closed or aired out, and even wind direction can all move the number. A charcoal canister placed in November during a cold snap may return a noticeably different result than one placed in June. Neither is wrong; they are both accurate snapshots of what the air held during that window.

A long-term alpha track detector, placed for 90 days to a full year, averages across all of those fluctuations and gives you something closer to your actual year-round exposure. The EPA and most state radon programs consider long-term testing the most reliable method for assessing your true risk.

Practical guidance from AARST-NRPP and most state programs:

• Short-term result at 4 pCi/L or above: do not wait for a long-term test. Confirm with a second short-term test if you want a second data point, or contact a certified mitigator directly.
• Short-term result between 2 and 4 pCi/L: a follow-up long-term test is reasonable before deciding whether to act.
• Short-term result below 2 pCi/L: retest every few years, especially after any significant renovation or addition.

Real estate test results: useful but not the whole story

If your results came from a home inspection, they were almost certainly run under “closed-house conditions”: windows closed for at least 12 hours before and during the test. This is intentional. Closed-house conditions push levels higher than normal everyday ventilation would, giving the test a more conservative read. That is considered a feature in real estate, not a flaw, because it surfaces potential problems that might otherwise look marginal.

What that means in practice: a result of 3.8 pCi/L under closed-house conditions could end up averaging closer to 3 in normal daily use, or it might run higher depending on season and habits. Neither you nor the seller can know for certain. What is clear is that 3.8 is close enough to 4 that treating it as a non-issue would be unwise.

If the home you are buying already has a mitigation system installed, the relevant question is whether it is working, not whether it exists. Check out our guide on buying a house with a radon mitigation system before you close.

If you have not tested yet

Your state radon program is the cheapest starting point. Most state health or environmental agencies offer free or low-cost test kits by mail, and some run periodic discount periods. A quick search for your state name plus “radon test kit” will find the program.

If you prefer to buy directly, look for a kit certified through NRPP or NRSB, the two national proficiency programs for radon measurement. Short-term charcoal canisters are widely available and work well for initial screening. Check price on Amazon

Our guide on how to test your home for radon covers placement, timing, and how to read the lab report in more detail.

What comes next once you have a number

Below 2 pCi/L: no action required. Plan to retest every two years or after any major renovation. Radon entry points can shift as a home settles or gets modified.

Between 2 and 4 pCi/L: run a long-term test if you used a short-term canister. Factor in how much time the household spends in the lowest level and whether anyone smokes. The EPA has specifically said this range warrants consideration, so the call is yours, but it is not a call you should skip.

At 4 pCi/L or above: contact a mitigator certified through AARST-NRPP (or C-NRPP if you are in Canada). The standard fix is a sub-slab depressurization system: a fan and pipe that draws radon from under the floor and vents it above the roofline before it reaches living space. Done correctly, these systems bring levels down substantially in most homes. After installation, retest. A post-mitigation test is the only way to confirm the system actually worked.

FAQ

What if my result includes a margin of error, such as “3.2 +/- 0.4 pCi/L”? Use the central number to make decisions. The margin of error reflects normal measurement precision at the lab level, not a range of possible true values in your home. Do not use the low end of the range to argue your way below the action level. Treat 3.2 as your result.

Does a result below 4 mean my home is safe? It means your reading is below the EPA action level, not that exposure is risk-free. The EPA explicitly recommends considering mitigation between 2 and 4 pCi/L, and states clearly that there is no level of radon considered completely without risk. Below 4 is a lower-priority range, not a green light to stop thinking about it.

Can radon levels change from year to year without me doing anything? Yes. Seasonal variation, weather patterns, changes in how you heat and ventilate the home, and foundation settling over time can all shift your reading. A single test is a snapshot. The EPA recommends retesting every couple of years and after any significant renovation, because what the home showed three years ago may not reflect current conditions.

My neighbor tested and came back under 2 pCi/L. Does that tell me anything about my home? Not really. Radon concentrations vary sharply between adjacent houses because of differences in soil type, foundation construction, and how tightly each home is sealed. Two houses on the same street can differ by a factor of four or more. Your neighbor’s result is their result. Test your own home to know where you actually stand.