Independent New Hampshire Journalism · Northfield, NH

More Than a Third of New Hampshire Homes Tested for Radon Came Back at or Above the Federal Action Level. Here Is What the State’s Own Data Show.

State health officials estimate the average indoor radon level in New Hampshire at 5.9 picocuries per liter, against a national average of 1.3. The federal government counts radon as the second leading cause of lung cancer. And in a Dartmouth-led survey, about one in eight New Hampshire and Vermont adults who had not tested, or did not know, and were offered information on getting a free kit sent one back for analysis.

Radon has no smell, no color and no taste. It is a radioactive gas that comes from the breakdown of uranium in rocks and soil, the state says, and it can seep into a house through the ground or come up with water from a deep well. The only way to know how much is inside a particular home is to test it.

New Hampshire has a lot of test results to go on. In a data brief published in August 2023, the state Department of Health and Human Services and the Department of Environmental Services reported on 32,747 radon samples collected in New Hampshire homes from 2007 to 2021. About 37 percent came in at 4.0 picocuries per liter or higher, the level at which the U.S. Environmental Protection Agency says a home should be fixed. That is roughly 12,000 of the samples. The figures describe homes whose owners chose to test; the brief does not present them as a random sample of the state’s housing.

The numbers, in the state’s terms

Radon is measured in picocuries per liter of air, written pCi/L. The EPA puts the average indoor level in the United States at 1.3 and the average outdoor level at 0.4. The state brief estimates New Hampshire’s average indoor level at 5.9, about four and a half times the national figure and above the federal action level of 4.0.

The brief also looked at the results by place. Its town map shows the greatest apparent risk in the southeastern and eastern parts of the state, and it says Carroll, Rockingham and Strafford counties “have the highest percentage of homes with elevated levels.” Among homes at 4 pCi/L or higher, the brief says, the median concentration ranged from 6.0 to 9.1 pCi/L by county. It gives a figure for each of the three top counties: 9.1 pCi/L in Carroll, 7.9 in Rockingham and 7.7 in Strafford. The brief’s sentence describes each figure as the level that half of the homes tested met or exceeded; read with the 6.0-to-9.1 range, the figures appear to describe homes already at or above the action level, not every home tested. Values above 100 pCi/L have been reported in every county in New Hampshire.

The federal map is older than the test data

The EPA drew its Map of Radon Zones in 1993 to help governments target their work, and it says the map should not be used to decide whether an individual home needs a test. For New Hampshire it placed Carroll County in Zone 1, the highest potential, where the predicted average indoor screening level is above 4 pCi/L. It placed the other nine counties in Zone 2, moderate potential, with a predicted average from 2 to 4 pCi/L. No county was placed in Zone 3, the low-potential zone.

Rockingham and Strafford, both moderate on the 1993 map, are two of the three counties the state’s test data put at the top for the share of homes at or above the action level. The map’s own text answers the question that raises: “All homes should be tested, regardless of zone designation.”

What the risk looks like

The EPA estimates that radon is responsible for about 21,000 lung cancer deaths in the United States every year, with an uncertainty range of 8,000 to 45,000, and that about 2,900 of those deaths occur among people who never smoked. By the agency’s estimates, radon is the leading cause of lung cancer among people who do not smoke and the second leading cause overall, after smoking. The risk climbs with the level of exposure, and the EPA describes the effects of radon and smoking together as synergistic.

The World Health Organization sets a lower mark, according to a peer-reviewed study of New Hampshire and Vermont residents discussed below. The study notes that the WHO’s action threshold for mitigation is 2.7 pCi/L and that the EPA’s 4 pCi/L action level was based on the technical limits of reducing radon inside homes.

Who tests, and who does not

In February and March 2022, the Dartmouth Cancer Center’s community outreach team worked with the University of New Hampshire Survey Center to survey adults on population-based panels in New Hampshire and Vermont. The results were published in the journal Environmental Health in 2026 by a team whose first author is in the thoracic surgery division at Brigham and Women’s Hospital in Boston and whose co-authors include staff of the New Hampshire Radon Program and the Vermont Department of Health. Of 1,717 people who responded, 513 said they had tested their home for radon, 767 said they had not, and 437 did not know. The paper’s abstract reports those groups as 29.7 percent, 44.7 percent and 25.5 percent of respondents.

Among 108 respondents whose earlier test had shown 2.0 pCi/L or more, 59 had installed a new mitigation system, 55 percent. People who had not tested were more likely to have less formal education, less likely to own their homes, and more likely to express negative attitudes toward cancer prevention.

The researchers then offered information on getting a free test kit to the 996 respondents with a steady place to live who had not tested or did not know whether they had. Here is how many followed through.

Return rates differed by state, the authors wrote: 40 percent in New Hampshire, which sends a short-term kit that takes a few days, and 35 percent in Vermont, which sends a long-term kit that takes at least 90 days.

Fixing the air

The state’s advice follows the federal numbers. At 4.0 pCi/L or higher, the Department of Health and Human Services recommends calling a certified radon mitigation contractor. Between 2.0 and 4.0, both the EPA and the department recommend considering it. A working system, the department says, should bring radon below 2 pCi/L, and homeowners should retest every two years. The 2023 brief puts the cost of a mitigation system at $1,000 to $3,000.

Fixing the water

Radon dissolves in groundwater, and a private well can carry it into the house. The state treats water on a separate scale, in picocuries per liter of water. The Department of Environmental Services strongly recommends treatment when a private well tests at or above 10,000 pCi/L. Between 2,000 and 10,000, the state says treating the water may be advisable if the air in the home is above 4 pCi/L.

Because radon is volatile, the department says it has to be treated where water enters the house, not at the kitchen sink. Whole-house aeration is the preferred method, and the 2023 brief puts its cost at $4,000 to $6,000. At or below 10,000 pCi/L, a whole-house granular activated carbon filter may be used if it is changed every one to two years, to keep gamma radioactivity from building up in the filter. The department recommends routine testing of a private well every three to five years.

What the law requires at a sale

New Hampshire law makes radon part of a home sale. Under RSA 477:4-a, before a purchase and sale contract is signed for property that includes a building, the seller or the seller’s agent must give the buyer a written notice. The radon paragraph reads: “Radon, the product of decay of radioactive materials in rock, may be found in some areas of New Hampshire. Radon gas may pass into a structure through the ground or through water from a deep well.” It adds that testing can establish whether radon is present and that equipment can remove it from air or water. The same section carries notices on arsenic, lead, PFAS and flooding, and the buyer signs to acknowledge receiving them.

The section requires a notice, not a test. And it says that it does not create “any liability with the seller or seller’s agent for failure to provide the notification.” The section dates to a 1990 law that took effect Jan. 1, 1991; the section was last amended in 2024, effective Jan. 1, 2025. This is a summary of the statute’s text, not legal advice; buyers and sellers with questions about a specific sale should consult a New Hampshire attorney.

Getting a test

The state radon program has offered free test kits for the air in a home, and the Department of Environmental Services says the program receives funding from the EPA through its State and Tribal Indoor Radon Grant Program. The 2023 brief directs residents to request a kit at aelabs.com/nh. The program can be reached at (603) 271-1708 or radon@dhhs.nh.gov. For help understanding a well-water result and treatment options, the state lists the Department of Environmental Services at (603) 271-1513 or dwgbinfo@des.nh.gov.

The 37 percent figure describes homes whose owners went looking. The survey shows how many never get that far.

— Granite State Report

Fact check

Editor’s note. Test-result, cost and treatment figures are drawn from the New Hampshire Department of Health and Human Services and Department of Environmental Services radon data brief (August 2023), the DHHS “Radon in the Home” handout (October 2021) and DES fact sheet DWGB-3-11. Health-risk figures are from the U.S. Environmental Protection Agency. Testing behavior is from Hasson and others in Environmental Health (2026). The sale notice is from RSA 477:4-a. Percentages and ratios the sources did not state are Granite State Report arithmetic from their figures.

Corrections: Granite State Report corrects verified errors promptly and appends a dated note identifying what changed and when. The corrections policy and the corrections register are public, and correction notes are never removed.