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Understanding Your Radon Number: What 2, 4, and 10+ pCi/L Actually Mean in New England

By Chris Pagliccia, 603 Basement Solutions

Quick answer: Your radon result is measured in pCi/L, picocuries per liter, and the one line that decides anything is the EPA’s. Fix your home at 4.0 pCi/L or higher, weigh it between 2 and 4, and know the EPA says no level is truly safe [EPA]. But a number by itself doesn’t tell you much. What tells you something is where your reading lands next to the homes around you, in your own county, measured the same way. You got a result. Let’s read it.

Everything on this page is public agency data: the EPA, the CDC, state health departments, the American Lung Association. None of it is our own test numbers. For the full tri-state picture and the county-by-county table this page leans on, see our tri-state radon data guide.

What your number means, band by band

Your reading falls into one of four bands, and each comes with plain EPA guidance. Here’s what the EPA says at each one.

Your reading (pCi/L) What to make of it
Under 2 Low. No EPA action called for. Retest down the road [EPA].
2 to 4 The “consider it” band. The EPA says weigh fixing it [EPA].
4 to 10 At or above the EPA’s 4.0 action level. Time to act [EPA].
10 and up Well over the line. Act, and don’t sit on it [EPA].

Two things to keep in your head. The EPA doesn’t call any level safe, so 4.0 is a guideline, not a magic cliff where risk switches on [EPA]. And a single short-term test can bounce around. Your first number gets you started. A follow-up test settles it. Read on for what “normal” even looks like around here, because it’s not what most people think.

Is 4 pCi/L high? Compare it to your county, not a generic scale

Four is the EPA’s action line, and around here it’s common, not rare. Start with the baselines. Outdoor air runs about 0.4 pCi/L, and the average US home sits around 1.3 [EPA]. New Hampshire’s estimated average is about 5.9, roughly 4.5 times that US number [NH DHHS 2023]. So a reading that would raise eyebrows in a lot of the country is closer to ordinary here.

How common? In the CDC’s same-method county data, about 41.9% of tested homes in Rockingham County came back at or above 4.0, and 42.5% in Strafford [CDC EPHT]. Across the line in York County, Maine, it’s 48.4%, nearly half [CDC EPHT]. Down in Essex County, Massachusetts, right on the New Hampshire border, it’s 28.7% [CDC EPHT]. So if you’re holding a 4-something on the Seacoast, you’re not some unlucky exception. It’s common. We test basements around here all the time, and a 4-and-up reading is something we see constantly.

The full county table for all three states is in the tri-state guide.

The median trap: why 7.7 doesn’t mean what you think

Those scary New Hampshire county medians you’ll see quoted, Carroll 9.1, Rockingham 7.9, Strafford 7.7 pCi/L, are not the typical home [NH DHHS 2023]. Read fast, it sounds like half the homes in Strafford sit at 7.7 or higher. They don’t.

Those are the medians among homes that already tested elevated, 4.0 and up. Not the typical home. When you look at the median of all tested homes, it drops way down. Statewide it was about 2.3 pCi/L in the 1988 to 2011 data, Rockingham 2.7, Strafford 3.0 [NH EPHT 2016]. So “half of Strafford is at 7.7” is flat wrong. Half of the already-high homes are. That’s the single easiest radon stat to get backwards, and now you won’t.

Is 10 pCi/L radon bad? And why some homes read 100+

Yes, 10 is bad, in the sense that it’s 2.5 times the EPA’s action level, and around here even 100 shows up. Readings over 100 pCi/L, 25 times the action level, have been reported in every single New Hampshire county [NH DHHS 2023]. So a double-digit result isn’t a broken test to wave off. It’s real, and it happens.

Here’s what takes the panic out of it. The action level isn’t a danger threshold that flips at 4.0. It’s the line the EPA draws to say “get this fixed” [EPA]. Whether you’re at 4.2 or 42, what you do next is the same: confirm it, then bring it down with a mitigation system. A high number doesn’t change the plan. It just means don’t wait on it.

Your reading in New England context

Line your number up against the region and you’ll see New Hampshire and Maine run neck and neck, both well above Massachusetts. The one truly apples-to-apples comparison, every state measured the same way on CDC 2008 to 2017 data, looks like this [ALA/CDC 2022]:

State Estimated average radon (same method)
Maine 5.6 pCi/L
New Hampshire 5.5 pCi/L
Massachusetts 3.2 pCi/L

So a 5 in Rochester or Dover is right around your state’s average. That same 5 in the Boston suburbs would be on the high side for down there. Same number, different meaning, depending on the ground under the house.

One caution when you compare state headlines. You’ll see New Hampshire quoted around 37% of tested homes over 4.0 [NH DHHS 2023], Maine near that, and Massachusetts as “1 in 4 homes.” Those three were built three different ways. New Hampshire and Maine are counting actual test results from homes people chose to test. The Massachusetts “1 in 4” is a model of the whole housing stock, not a tally of tests. Don’t stack them side by side as if they’re the same measurement. The full breakdown of how each was built is in the tri-state guide.

What your number means for your health, in the EPA’s words

In the EPA’s telling, a higher radon number held over years raises your risk of lung cancer, and that’s the whole reason the reading matters. We’ll use their words, not ours. The EPA says radon is the second leading cause of lung cancer overall, and the number-one cause among people who never smoked [EPA]. It attributes about 21,000 lung-cancer deaths a year in the US to radon [EPA]. The risk climbs with both the level and how long you’re exposed, and it’s sharply higher for smokers [EPA].

You don’t need to panic over one reading. You do need to know your number. Radon is a gas you can’t see, smell, or taste, sitting in the lowest level of the house, and the whole point is that you can measure it and fix it.

So what do you do with your number?

It depends on your band, but step one is always the same. Confirm the reading with a test before you decide anything.

  • Under 2. Low. Nothing to do now. Retest in a couple years and after any big basement work. You don’t need a system, and we’ll tell you that straight.
  • 2 to 4. The consider band. Retest to confirm, since short-term numbers wander, and think about a system, especially if someone smokes or spends real hours down there.
  • 4 to 10. At or over the action level. Confirm with a follow-up test, then get a mitigation system in to bring it down.
  • 10 and up. Same plan, just don’t sit on it.

Where to go from here:

For the towns we’ve actually measured ourselves, our Seacoast radon report is in preparation, and we’ll link it here once it’s out.

One honest note on us. 603 is certified for radon measurement and testing (NRSB RMS-113966). This page is about reading your number. When it’s time to fix a high one, we’ll point you the right way.

Talk to a real person about your number

Got a result in hand and you’re not sure what it means? Call 603-610-1770 and talk to Chris or one of the crew. No pressure, no script. Or book your free inspection and we’ll come test the air and read the number with you. We cover New Hampshire, Maine, and Massachusetts, home base right here on the Seacoast.

Sources

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