Radon in New Hampshire: The Definitive Homeowner’s Guide

If you live in New Hampshire, the odds are roughly one in three that your home has elevated radon. That is more than double the national average, and it is a direct consequence of the granite bedrock that gave the Granite State its name. Radon does not announce itself — it has no color, no odor, no taste — yet it is the second leading cause of lung cancer in the United States and the leading cause among people who have never smoked.

This guide explains why NH has a radon problem, how to find out whether your home has one, what your options are if it does, and how to choose a mitigation contractor without getting it wrong. It is written by AARST/NRPP-certified technicians who have been installing radon mitigation and water-radon treatment systems across Southern, Central, and Coastal New Hampshire for more than 25 years.

Why New Hampshire has a radon problem

Radon forms naturally as uranium in soil and rock breaks down. New Hampshire's bedrock is heavy in granite and other crystalline rock that contains comparatively high concentrations of uranium. As that uranium decays, radon gas is released into the soil — and from there into homes and into well water drawn from the same bedrock.

The EPA divides U.S. counties into three radon-risk zones. Zone 1 is the highest. Most of New Hampshire — including all of the Southern, Central, and Coastal regions ARM serves — falls into Zone 1 or Zone 2. The state's own data, which the New Hampshire Department of Environmental Services regularly publishes, supports the often-cited estimate that about one in three NH homes has indoor radon at or above the EPA action level of 4.0 picocuries per liter (pCi/L). Nationally, the equivalent figure is closer to one in fifteen.

The takeaway is simple: in New Hampshire, radon should be assumed and tested for, not assumed away.

What radon is — and why it matters

Radon is a radioactive noble gas. As you breathe it in, its decay products release small bursts of radiation in the lining of your lungs. Over years or decades, that exposure damages cells and meaningfully raises lung cancer risk.

The EPA estimates that radon contributes to roughly 21,000 lung cancer deaths in the United States every year. The risk multiplies sharply for people who also smoke, but radon is the leading cause of lung cancer in non-smokers. In water, ingested radon also contributes a smaller but real risk to internal organs.

Indoor radon is measured in picocuries per liter (pCi/L). The EPA's action level — the level at which mitigation is recommended — is 4.0 pCi/L. The agency notes that there is no "safe" level of radon, and recommends considering action even between 2 and 4 pCi/L. Radon in water is measured in the same units, but the meaningful thresholds are very different (covered below).

How radon enters New Hampshire homes

Two pathways matter in NH, and either is enough on its own to push a home above the action level.

Soil gas (the dominant pathway). Radon-rich gas in the soil migrates upward and finds the path of least resistance — usually your foundation. It enters through cracks in slabs, gaps around pipe penetrations, sump pits, dirt floors in crawl spaces, and the joint where the slab meets the foundation wall. Newer, well-built homes are not exempt; in fact, the air-tightness that makes a modern home efficient can also concentrate any radon that does enter.

Well water (the NH-specific pathway). If your home draws water from a private bedrock well — as more than 40% of NH homes do — that water can carry dissolved radon directly into your house. Every time someone showers, runs the dishwasher, fills the washing machine, or boils water on the stove, dissolved radon is released into the indoor air. The rule of thumb used by USGS and EPA is that for every 10,000 pCi/L of radon in your water, your indoor air radon will rise by roughly 1 pCi/L.

Because both pathways feed the same indoor air, NH homes on private wells should plan to test both.

How to test for radon in your NH home

Testing is straightforward, inexpensive, and the only way to know what you actually have. There are three common testing approaches:

• Short-term tests (2 to 7 days). Charcoal canisters or electronic monitors placed in the lowest livable level of the home. Inexpensive and fast, useful for a first read or for real estate transactions. Because radon levels fluctuate with weather and HVAC use, short-term results are a snapshot, not the final word.
• Long-term tests (90+ days). Alpha-track detectors that average exposure over a full season, smoothing out the fluctuations that make short-term tests noisy. The most accurate picture of your true average exposure.
• Continuous radon monitors (CRMs). Professional-grade electronic devices that record hourly readings. Used for diagnostic work, post-mitigation verification, and AARST-protocol real estate testing.

For radon in water, you collect a water sample (typically from a tap close to the well) using a sealed kit and send it to a certified lab — radon dissipates quickly once water is exposed to air, so the sampling protocol matters.

New Hampshire DES recommends testing for radon in your home, and re-testing roughly every two years or after any major renovation or HVAC change. For private wells, NH DES recommends at least two radon-in-water samples taken at least one month apart before making a treatment decision, because well-water radon can swing meaningfully over time.

If you are buying or selling an NH home Radon testing during a real estate transaction follows a specific protocol — the AARST MAH-2019 standard — that is stricter than a casual short-term test. Use a certified measurement provider and follow the closed-house conditions in the protocol so the result holds up under inspection-contingency review.

Reading your results

Indoor air radon. Below 2 pCi/L is considered low. Between 2 and 4 pCi/L, the EPA recommends considering mitigation; many NH homeowners do mitigate at this range because the cost-to-benefit is favorable and lung cancer risk is dose-dependent. At or above 4 pCi/L, mitigation is recommended outright.

Radon in water. There is no enforceable federal limit for radon in private well water. The EPA proposed (but never finalized) a Maximum Contaminant Level of 4,000 pCi/L. As a practical matter, most NH water-treatment professionals recommend treating well water that exceeds 5,000–10,000 pCi/L, and many treat above 4,000 pCi/L when paired with elevated indoor air. The 10,000:1 rule means a well at 30,000 pCi/L is meaningfully contributing to indoor air radon all by itself.

Mitigating radon in air

Air radon mitigation is an engineering problem with a small number of standard solutions. The right one depends on your foundation type.

Sub-slab depressurization (SSD). The most common system in NH and the gold standard for homes on a slab or with a basement. A pipe is installed through the slab, connected to a quiet inline fan, and routed up and out above the roofline. The fan creates a small vacuum under the slab so that soil gas is pulled out and vented before it can enter the living space. Properly installed SSD systems routinely take a 10–20 pCi/L home down to under 1 pCi/L.

Sub-membrane depressurization. For crawl spaces. A heavy polyethylene membrane is sealed to the walls and floor of the crawl space and a vent pipe pulls soil gas from beneath the membrane.

Drain-tile and block-wall depressurization. Used in specific foundation conditions, often in combination with SSD. Applied where the home has interior drain tile or hollow concrete block walls that can themselves be a radon pathway.

All of these systems must be installed to ANSI-AARST SGM-SF-2017 (the consensus standard for radon mitigation in single-family homes) or the equivalent multifamily standard. They include a manometer (a small U-tube pressure gauge) so you can verify at a glance that the system is running. Costs in NH typically range from about $1,000 to $2,500 for a standard residential install; complex foundations, multiple vent points, or aesthetic routing requirements can push higher.

Mitigating radon in well water

Two technologies dominate, and the right one depends on how high the water-radon level is.

Aeration (Bubble-Up® and similar designs). Water enters a sealed tank where small bubbles agitate it, releasing the dissolved radon, which is then vented outside. EPA identifies aeration as the Best Available Technology for radon-in-water reduction, and well-designed systems remove 95–99% of dissolved radon. This is the right choice for higher water-radon levels and for homes where indoor air radon contribution must be brought down decisively.

Granular activated carbon (GAC). Water flows through a tank of activated carbon that adsorbs radon. Lower up-front cost and effective for lower water-radon levels. The trade-off: the carbon accumulates radioactivity over time and must be handled and replaced carefully — a real consideration that an experienced installer will plan for.

If a radon-in-water number is borderline, an experienced installer should walk you through the specific tradeoffs in your situation. (For a deeper dive specifically on water radon, see Radon in Well Water Treatment: What NH Homeowners Need to Know.)

Choosing an NH radon mitigation contractor

Radon work is a specialty. The certifications and standards matter — they are how you separate professionals from generalists.

• AARST/NRPP certification. The American Association of Radon Scientists and Technologists administers the National Radon Proficiency Program. Certified mitigators have demonstrated knowledge of radon physics, system design, and the relevant ANSI standards. Ask any contractor for their NRPP number and verify it.
• AARST multifamily certification. Required for any work on multifamily, commercial, or school buildings, and a useful proxy for the depth of a contractor's training even on residential work.
• NH state recognition. Confirm the contractor is current on the NH-listed certified radon professionals roster.
• Standards adherence. ANSI-AARST SGM-SF-2017 for single-family mitigation; MAH-2019 for measurement during real estate. Ask which standards a contractor builds to.
• Post-mitigation testing. A reputable mitigator will retest after installation to confirm the system actually achieved the target reduction. If a quote does not include post-test, ask why.
• Workmanship warranty and fan warranty. Ask for both. Quality fans carry 5-year manufacturer warranties; quality installers stand behind workmanship for at least that long.

Why ARM Advanced Radon Mitigation & Water Treatment is AARST/NRPP certified for both radon mitigation and AARST multifamily-certified for commercial and multifamily projects. Our technicians build to the current ANSI-AARST standards and include post-mitigation testing on every install. Twenty-five years of NH-only work means we have seen virtually every foundation type, well configuration, and home age the state has to offer.

Maintaining your system over time

A radon mitigation system is not install-and-forget, but it is close. The handful of things to do:

• Re-test indoor air radon every two years, and any time you make significant changes to the foundation, HVAC system, or living-space layout.
• Glance at the manometer (the U-tube on the vent pipe) every few months. If the two columns are even — instead of offset — the fan is not pulling and needs service.
• Plan for a fan replacement around the 8–10 year mark. Quality inline radon fans are reliable but not eternal.
• If you have a water radon system, follow the manufacturer and installer's service interval. Aeration tanks need periodic cleaning; GAC tanks need media changes on a schedule that depends on radon level and water use.

Frequently asked questions

How long does radon mitigation take to install?

A standard sub-slab depressurization system in a single-family NH home typically installs in a single day. More complex foundations, water-radon systems, or multifamily projects take longer.

How quickly will my radon levels drop after mitigation?

Within 24 hours of system startup, indoor radon usually falls to its new baseline. We recommend a follow-up test 24–48 hours after installation and a longer-term test 30 days later for confirmation.

Will mitigation work in my older NH home?

Yes. We have installed systems in NH homes that pre-date the Civil War and in homes built last year. The diagnostic work to choose the right approach is what matters, not the home's age.

Does opening windows fix radon?

Temporarily, partially, and not in winter. Ventilation helps in the short term but is not a substitute for an engineered system, especially in a state where windows are closed for half the year.

Is radon worse in basements?

Generally yes — soil gas concentrates closest to the soil. But mitigated systems protect the entire home, and you should test in the lowest livable level you actually use.

Can I install a radon system myself?

It is legal to do so in NH, but unwise. The diagnostic work to choose the right system, the standards work to install it correctly, the electrical work to wire the fan, and the post-test verification all matter. Most DIY systems we see in the field underperform — sometimes by a lot.

Does homeowner's insurance cover radon mitigation?

Almost never. Mitigation is treated as a home improvement, not a covered loss.

Get a radon assessment

If you have not tested your NH home in the last two years — or ever — start there. If you have a result above 2 pCi/L in air, or a private well that has not been tested for radon, that is the next step. Advanced Radon Mitigation & Water Treatment offers testing, mitigation system design, and full installation across Southern, Central, and Coastal New Hampshire.

Call 603-644-1207 to schedule a radon assessment, or request a quote at radonh2o.com/contact.