How Common Is High Radon in New Hampshire, Maine, and Massachusetts? A County-by-County Data Guide

By Chris Pagliccia, 603 Basement Solutions

Quick answer: In New Hampshire, about 37% of tested homes came back at or above the EPA action level of 4.0 pCi/L [NH DHHS 2023]. In Maine, about 36.5% of radon tests came back that high [ALA 2025]. In Massachusetts, the state estimates roughly 1 in 4 homes sit above 4.0 [MA DPH]. Those three numbers look like a tidy race. They aren’t. New Hampshire and Maine are counting actual test results from homes people chose to test. Massachusetts’ “1 in 4” is a modeled estimate of the whole housing stock, not a tally of tests. Different years, different math, different meaning.

Everything on this page comes from public agency data. State health departments, the EPA, the USGS, the American Lung Association. None of it is our own test numbers. When you want to see what we actually measured in Seacoast basements, that lives in a separate report we’re finishing now, and we point you to it at the bottom.

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The short version, by state

• New Hampshire runs high, statewide. Estimated average around 5.9 pCi/L, roughly 4.5 times the U.S. average of 1.3 [NH DHHS 2023]. Readings over 100 pCi/L (25 times the action level) have turned up in every NH county.
• Maine is right there with it. Roughly a third of Maine homes are estimated to have elevated radon, closer to two-thirds in some pockets, and the statewide average concentration sits above the 4.0 guideline [Maine DHHS].
• Massachusetts is lower on average but far from clear. The state’s own estimate is about 650,000 homes above 4.0 [MA DPH]. Its highest-risk counties are the three EPA Zone 1 counties in the northeastern and central part of the state: Essex, Middlesex, and Worcester.

If you’re on the Seacoast, that matters more than the state averages. Your two home counties, Rockingham and Strafford, are among New Hampshire’s worst. More on that below.

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How to read these numbers

Radon data is messy, and any single “average” for a whole state leaves out how it was built. If you don’t know how a number was made, you can’t tell what it’s telling you. Four things to keep straight.

1. Two of the three are exceedance rates from self-selected tests. New Hampshire’s 37% and Maine’s ~36.5% come from homes that got tested. Homeowners who chose to test, plus real-estate and (in Maine) rental testing. That’s not a random survey of all homes. People who suspect a problem, or who are buying and selling, test more, so the sample leans toward homes with a reason to check. New Hampshire’s figure covers tests from 2007 to 2021 (n=32,747) [NH DHHS 2023]. Maine’s comes from 57,000+ air tests, mostly 2013 to mid-2019, reported by labs, contractors and landlords, with already-mitigated homes left out [Maine Tracking; ALA 2025].

2. The famous Massachusetts number is a model, not a test count. Massachusetts’ “1 in 4 homes” / 650,000 homes / 628 lung-cancer deaths a year are EPA modeled estimates across the housing stock [MA DPH]. That “1 in 4” traces back to a 1988 MA DPH/EPA study. Massachusetts does keep a separate database of real certified-lab test results by town and county, and that would be the true match for New Hampshire and Maine. But those exact figures live inside the MA EPHT dashboard, so we point you there for your county instead of freezing a copy here [MA EPHT].

3. EPA “Radon Zones” are a 1993 geologic prediction, not a measurement. You’ll see counties colored Zone 1, 2, or 3 on EPA maps. That’s a forecast based on rock and soil from 1993, not what testing actually found [EPA]. It’s useful, but around here it undershoots the real picture. Plenty of New Hampshire “Zone 2” counties test at 32 to 41% over the action level. Don’t read your zone color and relax.

4. Different years, different units, different math. NH’s 2007-21 against Maine’s 2013-19 against an undated MA model. Percent-of-tests against a modeled home count against “1 in 4.” Never line them up as if they’re the same measurement without saying how each one was built.

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The one apples-to-apples comparison

There is a single dataset that measured all three states the same way: the American Lung Association’s 2022 “Radon Testing Disparities” reports, built on CDC national tracking data from 2008 to 2017, Bayesian-smoothed, the same method in every state [ALA/CDC 2022].

Two takeaways. First, Maine and New Hampshire are effectively tied and both run well above Massachusetts. Second, look at that testing-volume column. Massachusetts tests five times more per home than Maine does. That’s a testing habit, not a radon level. So don’t read “Massachusetts has way more tests” as “Massachusetts has way more radon.” The opposite is closer to the truth.

(Note the two NH averages you’ll see on this page: 5.9 pCi/L is NH DHHS’s 2007-21 figure, 5.5 is the ALA/CDC 2008-17 model. Different samples and years, so we don’t merge them.)

What each state does about it

The same picture shows up in the Indoor Environments Association / AARST 2026 Radon Report Cards, which run off that same CDC 2008-2017 test data [AARST 2026]:

Maine is the only one of the three that makes every new home go in with a radon system, in all counties, not just the high-zone ones [AARST 2026]. And Massachusetts shows more than ten times the tests Maine does, which is again about population and testing habit, not a higher radon level.

Every county, measured the same way (CDC, 2008-2017)

The three state programs each measured differently, which is why we keep them apart above. But one dataset measured every county in all three states the exact same way: the CDC Tracking Network’s lab-test data, pre-mitigation, 2008-2017 [CDC EPHT]. This is the apples-to-apples county picture. Share of tests that came back at or above the 4.0 action level, worst to best:

A few things jump out. York, Maine and Carroll, New Hampshire top the whole region, essentially tied near 48%. Every county we work in runs high: Rockingham at 41.9% and Strafford at 42.5% in New Hampshire, York at 48.4% in Maine, and Essex at 28.7%, the Massachusetts county right on the New Hampshire line. And the pattern from the state averages holds all the way down: New Hampshire and Maine counties fill the top of the list, while even the highest Massachusetts county, Worcester at 33.5%, only reaches the middle. (For New Hampshire, these CDC 2008-2017 numbers run a little higher than the older 1988-2011 state table further down; different years and samples, same story.)

Find your town’s exact number

A county average is a starting point, not your house. For the exact share of homes over 4.0 in your own town or county, each state runs a live, searchable tool. These update as new tests come in, so they beat any table we could freeze here:

• New Hampshire: NH DHHS WISDOM radon map, https://wisdom.dhhs.nh.gov/EPHT
• Massachusetts: MA EPHT residential radon dashboard, by town and county, https://matracking.ehs.state.ma.us/Environmental-Data/radon/index.html
• Maine: Maine Tracking Network radon, town, county, and state, https://data.mainepublichealth.gov/tracking/radon
• All three at once: CDC Tracking Network radon Data Explorer, https://ephtracking.cdc.gov/DataExplorer/

The only number that decides anything is your own. Test your home.

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New Hampshire, county by county

New Hampshire has published a full county table exactly once, from its 1988-2011 tracking data (n=25,476), where statewide exceedance was about 30% [NH EPHT 2016]. It’s the most complete county breakdown the state has released. The newer 2007-2021 brief (37% statewide) didn’t republish the full county table [NH DHHS 2023]. The two windows are different samples, so treat the 30% and 37% as two snapshots, not a trend.

Even Sullivan, the lowest county in the state, still had 1 in 8 tests over the action level, and readings over 100 pCi/L happen there too [NH DHHS 2023]. There’s no “safe town” here.

One number that gets misread constantly

The 2023 brief says the three highest-risk counties, Carroll, Rockingham and Strafford, have county medians of 9.1, 7.9, and 7.7 pCi/L [NH DHHS 2023]. That is not the typical home. That’s the median among homes that already tested elevated (4.0 or higher). The median of all tested homes is much lower. Statewide it was about 2.3 pCi/L in the 1988-2011 data; Rockingham’s median across all tests was 2.7, Strafford’s 3.0 [NH EPHT 2016]. So when you see “half of homes are at 7.9 or higher,” read the fine print. That’s half of the already-high homes, not half of all homes. Big difference, and it’s the single easiest radon stat to get wrong.

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Maine, county by county

Maine reports radon mostly through regions and county means, not a clean percent-over-4.0 table by county. The exact per-county and per-town percentages live inside the Maine Tracking Network’s interactive dashboard, so for those we point you to that tool for your county [Maine Tracking]. What the state does say out loud: in southwest Maine and in Hancock and Aroostook Counties, more than half of tested homes are elevated [Maine Tracking].

12 of Maine’s 16 counties are EPA Zone 1, the highest tier. Only Knox, Sagadahoc, Waldo and Washington are Zone 2, and none are Zone 3 [EPA]. Notice Cumberland: it’s Zone 1 like most of the state, yet it posts the highest measured mean in Maine. The geologic prediction and the actual measurements point the same direction there, hard.

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Massachusetts, county by county

Massachusetts is the newest piece here, because there’s a lot less published radon data for it than for its northern neighbors. The state’s headline numbers are modeled. Per-county and per-town test figures sit in the MA EPHT dashboard, linked above for your county [MA EPHT]. What’s solid: the EPA zones and the building-code line that runs off them.

The three Zone 1 counties, Essex, Middlesex, and Worcester, carry a rule the others don’t: since 2015, new one- and two-family homes and townhouses built in those counties must include a passive radon-resistant system [MA DPH]. There’s a gap worth knowing, too: Massachusetts has no rule protecting owners of existing homes [MA DPH]. If your house was already standing, testing is on you, and it’s voluntary.

Two of those three Zone 1 counties, Essex and Middlesex, sit right along the New Hampshire border. The same granite doesn’t check for a state line.

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EPA Zones vs what testing actually found

The EPA zone map is a coarse 1993 prediction, and around here it consistently understates the measured risk [EPA].

• In New Hampshire, only Carroll is painted Zone 1. But “Zone 2” Strafford (41%), Coos (39%), and Rockingham (36%) all test as high as or higher than the map’s top tier [NH EPHT 2016].
• In Maine, Cumberland is Zone 1 like most of the state, yet it posts the single highest measured county mean [ALA/CDC 2022].

The lesson for a homeowner is simple. Don’t trust the color on the map. Trust a test in your own basement. The map tells you the odds. A test tells you your house.

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Hotspot towns near us

New Hampshire and Maine don’t publish a ranked list of “worst radon towns.” What they publish is county data plus an interactive town-probability map, and the town-level percentages have to be read off that map [NH DHHS WISDOM; Maine Tracking]. So the town table below anchors each town to its county’s published number, and for the town-specific percentage it points you to the state map. We won’t freeze a town figure the state keeps live in its own dashboard.

Everything below is squarely in our service area. Rockingham and Strafford in New Hampshire, York County across the line in Maine.

Rockingham also holds New Hampshire’s largest test sample, more than 5,600 tests, so its 36% rests on solid ground [NH EPHT 2016]. And Strafford ties Carroll for the highest elevated share in the state. If you’re in Dover, Rochester, Exeter or Portsmouth, you’re in the thick of it. Across the bridge in Kittery, York or the Berwicks, you’re in Maine’s high-radon southwest corner. Same granite. Same odds.

For the towns we’ve actually measured ourselves, see our separate Seacoast radon report (in preparation, links here once published).

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Why northern New England runs so high

It’s the rock. Radon is a decay product of uranium-238, and northern New England sits on bedrock that’s naturally high in it [Maine CDC MEG; USGS]. In New Hampshire that’s the Conway Granite and the White Mountain plutons. In Maine it’s the Sebago granite of the southwest. The gas works its way up out of the ground and into the lowest level of your house [MA DPH]. That’s why the age of the home barely matters. A 1790 farmhouse and a brand-new build on the same ledge can both read high. The source is under the slab.

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A separate map: radon in your well water

Radon in your air and radon in your water are two different maps with two different standards, and they don’t line up.

In New Hampshire, indoor-air radon is worst in the southeast, the Seacoast and the eastern belt [NH DHHS 2023]. But the USGS says the highest groundwater radon probability is up in the White Mountains and north-central New Hampshire, and it estimates about 55% of the state is more likely than not to have elevated radon in its groundwater [USGS]. So a Seacoast home can be an air concern without a water problem, and a White Mountains well can carry very high water radon even where the air map looks calm. Don’t assume one from the other.

The water standards are advisories, not enforceable rules, and every New England state picked a different flag level: New Hampshire 2,000, Maine 4,000 (lowered from 20,000 back in 2006), Massachusetts 10,000, Connecticut 5,000, Rhode Island 4,000 pCi/L [Maine CDC MEG]. Each state just drew its own line. The physics link is worth knowing: roughly 10,000 pCi/L in water adds about 1 pCi/L to your indoor air [Maine CDC MEG, citing NAS]. Water radon mostly reaches you through the air when you shower, run the dishwasher, or do laundry.

Private-well reliance changes the stakes too. Nearly half of New Hampshire households are on a private well, and Maine’s share is higher still, while Massachusetts is overwhelmingly on public water (exact figures in the Sources notes) [CDC NH; Maine CDC; MA]. The rule of thumb from every state: test the air first, and only chase the water if you’re on a private well or your air comes back high. The waterproofing-and-radon overlap is covered on our waterproofing with a radon system page.

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What the numbers mean for your health (in the EPA’s words)

• The EPA says there’s no safe level of radon. Act at 4.0 pCi/L, and consider fixing between 2 and 4 [EPA].
• Radon is the second leading cause of lung cancer overall, and the number-one cause among people who never smoked [EPA].
• The EPA attributes about 21,000 lung-cancer deaths a year in the U.S. to radon [EPA]. At the state level, the estimates are roughly 100 deaths a year in New Hampshire [NH DHHS] and 628 a year in Massachusetts [MA DPH].
• Risk climbs with both the level and how long you’re exposed, and it’s sharply higher for smokers [EPA].

That’s the case for testing. A number you can’t see, smell or taste, sitting in the lowest level of the house, that you can actually measure.

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Plain what-to-do

1. Test the air first. A short-term test tells you fast whether you’ve got a problem; a longer or follow-up test confirms it. Start there before you touch anything else.
2. Read your result against the county data above. Under 2 is low. 2 to 4 is the “consider it” band. 4 and up is the EPA’s line to act. And remember the median trap: those 7-to-9 county figures are the elevated homes, not the typical one.
3. If you’re on a private well, test the water too, in that order, air then water.
4. Don’t lean on your EPA zone color. Around here it undersells the real risk. A test in your basement beats a 1993 map every time.

Where to go from here:

• Confirm a reading or get set up to test: radon testing.
• The specific questions (“is 3.0 concerning?”, “is 2 pCi/L in a bedroom okay?”, “does it change at resale?”, “summer vs winter?”): our radon answers hub.
• How a system works and what the mechanics are: radon mitigation systems.
• What it runs to fix: our radon mitigation cost guide.
• The basics on what radon even is: what is radon.

One note on us: 603 is certified for radon measurement and testing (NRSB RMS-113966). This page is about reading the data. When it comes to fixing a high reading, we’ll point you the right way.

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Talk to a real person about your number

If you’ve got a reading in hand and you’re not sure what it means, or you just want the air tested, 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 take a look. We cover New Hampshire, Maine, and Massachusetts, home base right here on the Seacoast.

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Sources and methodology notes

Every dataset here, with its year, sample, and what it measures. Where a figure lives only inside a state’s interactive tool, we point you to that tool instead of freezing a copy.

• [NH DHHS 2023]: NH DHHS, “Radon Risk in New Hampshire” data brief, Aug 2023. Test results 2007-2021, n=32,747, self-selected. Statewide 37% ≥4.0; est. average 5.9 pCi/L; elevated-home county medians 6.0-9.1; ~100 NH lung-cancer deaths/yr. https://www.dhhs.nh.gov/sites/g/files/ehbemt476/files/documents2/radon-data-brief-final.pdf
• [NH EPHT 2016]: NH DHHS EPHT, “Spotlight: Radon Risk,” Jan 2016. Test results 1988-2011, n=25,476. The only full NH per-county table (% ≥4.0, median-all-tests, median-elevated). Statewide 30% in this window. http://tinymce.nhwis.net/plugins/moxiemanager/data/files/docs/Radon_Jan2016_Final.pdf
• [NH DHHS WISDOM]: NH DHHS WISDOM EPHT portal / town-probability map. Town-level percentages readable here for per-town lookups. https://wisdom.dhhs.nh.gov/EPHT
• [Maine Tracking]: Maine Tracking Network (Maine CDC), radon topic. 57,000+ air tests, mostly 2013-mid-2019, service-provider reported, mitigated homes excluded, self-selected. SW Maine + Hancock + Aroostook >50% elevated; ~1 in 3 households have tested. https://data.mainepublichealth.gov/tracking/radon
• [Maine DHHS]: Maine DHHS / Maine Radon Program fact sheet. ~1/3 of homes elevated (closer to 2/3 in some areas); statewide average above 4.0. https://sosradon.org/sites/sosradon/files/ME%20Radon%20Facts_0.pdf
• [ALA 2025]: American Lung Association, “State of Lung Cancer,” Maine. 36.5% of Maine test results ≥4.0 (share of tests, not homes); ME ranks 37th. https://www.lung.org/research/state-of-lung-cancer/states/maine
• [CDC EPHT]: CDC National Environmental Public Health Tracking Network, radon Data Explorer, “Radon Test Data from Labs,” pre-mitigation, county level, 2008-2017. Percent of tests ≥4.0 pCi/L by county, measured the same way in every state (the same-method county anchor). Pulled 2026-07: MA 14 counties, NH 10, ME 16. https://ephtracking.cdc.gov/DataExplorer/
• [AARST 2026]: Indoor Environments Association / AARST, 2026 Radon Report Cards (NH, ME, MA), built on CDC EPHT 2008-2017 test data. Pre-mitigation tests on record: NH 15,608, ME 11,825, MA 234,152. New-home radon-system requirement: NH none, ME all counties, MA Zone 1 counties only. https://aarst.org/report-card/
• [ALA/CDC 2022]: American Lung Association / Univ. of Iowa, “Radon Testing Disparities,” 2022, CDC tracking data 2008-2017, Bayesian-smoothed. The only same-method cross-state anchor: est. means ME 5.6 / NH 5.5 / MA 3.2 pCi/L; tests per 1,000 units ME 15.7 / NH 24.3 / MA 79.9. ME: Cumberland highest county mean 7.8, Penobscot lowest 2.2. MA: Worcester highest 4.5, Suffolk lowest 2.0, Middlesex 3.3 (most tests). https://www.lung.org/getmedia/3516e485-7ebe-45ed-8154-3a6f2aa4c79f/Radon-Testing-Disparity-report-for-Maine-2022-08-14.pdf and https://www.lung.org/getmedia/13dfde49-a345-4550-8fc6-68a8933bb035/Radon-Testing-Disparity-report-for-Massachusetts-2022-08-14.pdf
• [MA DPH]: Mass.gov / MA DPH, “Radon – get the facts” and “Radon and New Construction.” Modeled estimates (re-verified against the MA DPH 2022 radon fact sheet): ~650,000 homes >4.0, ~34,000 >20, ~1 in 4 homes, 628 MA lung-cancer deaths/yr. Fact sheet: https://sosradon.org/sites/sosradon/files/State_Fact_Sheet_Originals/2022%20Massachusetts%20Radon%20Fact%20Sheet.pdf . New-construction passive-system mandate (2015) in Essex, Middlesex, Worcester; no rule for existing homes; radon enters via floor-wall joints/cracks/sump/slab, highest in the lowest level, any age of home. https://www.mass.gov/info-details/radon-get-the-facts and https://www.mass.gov/info-details/radon-and-new-construction
• [MA EPHT]: MA Environmental Public Health Tracking, Residential Radon dashboard. Certified-lab test results by town and county; exact per-county/per-town figures live in the dashboard (linked for lookups). https://matracking.ehs.state.ma.us/Environmental-Data/radon/index.html
• [EPA]: U.S. EPA, “A Citizen’s Guide to Radon,” “Health Risk of Radon,” and “EPA Map of Radon Zones” (1993 geologic prediction). No safe level; action at 4.0, consider 2-4; ~21,000 US deaths/yr; U.S. indoor avg ~1.3, outdoor ~0.4 pCi/L. NH: Carroll is the only Zone 1 county, other 9 Zone 2. ME: 12 of 16 counties Zone 1. MA: Essex/Middlesex/Worcester Zone 1, Suffolk Zone 3, rest Zone 2. https://www.epa.gov/radon/health-risk-radon and https://www.epa.gov/radon/epa-map-radon-zones-and-supplemental-information
• [USGS]: USGS New England Water Science Center (Dec 2022 NH groundwater maps) and Yang et al. 2014 (Maine well water). ~55% of NH more-likely-than-not elevated groundwater radon, highest in White Mountains / north-central; Maine well-water uranium/radon clusters near granitic intrusions, ~324,000 rural Mainers at risk >4,000 pCi/L. Geology: uranium-238 in Conway Granite (NH) and Sebago granite (ME). https://www.usgs.gov/centers/new-england-water-science-center/news/new-maps-predict-areas-elevated-radon-uranium-new
• [Maine CDC MEG]: Maine CDC, “Maximum Exposure Guideline for Radon in Drinking Water,” Oct 2006. State water advisories (NH 2,000 / ME 4,000 / MA 10,000 / CT 5,000 / RI 4,000 pCi/L); ~10,000:1 water-to-air transfer ratio (NAS); no finalized federal enforceable water standard. https://www.maine.gov/dhhs/mecdc/sites/maine.gov.dhhs.mecdc/files/MEG%20analysis%20paper.pdf
• [CDC NH]: CDC New Hampshire Environmental Public Health Tracking. ~44% of NH households on private wells; ~half of NH bedrock wells have radon of concern. https://www.cdc.gov/environmental-health-tracking/php/our-work/about-new-hampshire-tracking.html
• [Maine CDC / private wells]: About half of Maine residents rely on private wells (the highest of the three states); Massachusetts is ~90-93% on public water, leaving roughly 7-10% on private wells. (Figures moved here from the body to keep the well-water section readable.) https://data.mainepublichealth.gov/tracking/radon

Deliberately excluded: Commercial lead-gen aggregators (radonresources.com, radonbyzipcode.com) publish inflated, internally inconsistent “town averages” built on as few as 1 to 6 self-submitted tests, and one falsely claims New Hampshire “has no active radon program.” Those figures are not authoritative and are not used anywhere on this page.

No pricing appears on this page by design; for cost, see the radon mitigation cost guide. No 603 test data appears here; our own measured readings live in a separate Seacoast radon report we’re finishing now.