New Hampshire Radon Risk Explained: What Homeowners Need to Know in 2026
New Hampshire has the second-highest average indoor radon levels in the United States, behind only Iowa. Roughly 40% of homes in the state test above the EPA’s recommended action level of 4.0 picocuries per liter (pCi/L), and certain areas — particularly the granite-rich bedrock regions of southern and central New Hampshire — consistently produce readings of 10, 20, or even 50+ pCi/L. Radon is a colorless, odorless radioactive gas produced by the natural decay of uranium in rock and soil. It seeps into homes through cracks in foundations, gaps around pipes, sump pits, and any opening where the building contacts the ground. Long-term exposure to elevated radon is the second leading cause of lung cancer in the United States, responsible for an estimated 21,000 deaths annually. The good news: testing is cheap ($125-$200), fast (48 hours), and if levels are high, mitigation systems reduce radon by 90-99% for $800-$1,500. If you’re buying a home in New Hampshire, radon testing should be treated as mandatory — not optional.
Why New Hampshire Has Such High Radon
New Hampshire’s geology is the culprit. The state sits on some of the oldest rock in North America — predominantly granite, gneiss, and schist that are naturally rich in uranium. As uranium decays through its radioactive chain, it produces radium, which in turn produces radon gas. The gas migrates through soil and fractured bedrock, entering homes through the path of least resistance: the foundation.
Several factors make New Hampshire particularly radon-prone:
- Granite bedrock: The Granite State earned its name — the same stone that gives the state its identity also produces radon. Southern and central NH have the highest uranium concentrations.
- Fractured bedrock: NH’s geology includes extensive fracture systems that provide pathways for radon to travel from deep underground to the surface. Homes built on fractured granite can have extremely high levels.
- Tight building envelopes: Energy-efficient homes and winterized older homes are sealed against cold air — but that same sealing traps radon inside. Winter months typically produce the highest indoor readings because homes are closed up with less ventilation.
- Well water: About 40% of NH homes use private wells that draw from bedrock aquifers. Radon dissolved in well water is released into the air when you shower, run the dishwasher, or use any faucet. Waterborne radon can add 1-4 pCi/L to indoor air levels.
- Basements: Most NH homes have basements — the lowest level of a home has the highest radon concentrations because it’s closest to the source.
Radon Levels Across New Hampshire
The EPA classifies radon risk by county zone. All of New Hampshire falls into Zone 1 (highest risk) or Zone 2 (moderate risk). No county in the state is classified as low risk.
| County | EPA Zone | Avg Indoor Radon (pCi/L) | % of Homes Above 4.0 pCi/L |
|---|---|---|---|
| Hillsborough (Manchester, Nashua) | Zone 1 | 7.2 | 48% |
| Rockingham (Portsmouth, Exeter, Salem) | Zone 1 | 5.8 | 42% |
| Merrimack (Concord, Bow, Hooksett) | Zone 1 | 6.5 | 45% |
| Strafford (Dover, Rochester) | Zone 1 | 5.1 | 38% |
| Belknap (Laconia, Gilford) | Zone 1 | 6.8 | 46% |
| Grafton (Hanover, Lebanon, Plymouth) | Zone 1 | 5.5 | 40% |
| Sullivan (Claremont, Newport) | Zone 1 | 5.9 | 42% |
| Cheshire (Keene) | Zone 2 | 4.2 | 32% |
| Carroll (Conway, Wolfeboro) | Zone 2 | 4.0 | 30% |
| Coos (Berlin, Lancaster) | Zone 2 | 3.8 | 28% |
Even in the “lower risk” northern counties, nearly one in three homes tests above the action level. In Hillsborough County (Manchester, Nashua), almost half of all homes exceed the 4.0 pCi/L threshold. Testing is the only way to know your home’s level — radon concentrations can vary dramatically between adjacent houses depending on the underlying geology and foundation characteristics.
How to Test for Radon
Short-Term Testing (2-7 days)
Short-term tests provide a snapshot of radon levels and are the standard method used during home purchases. A charcoal canister or continuous radon monitor (CRM) is placed in the lowest livable level of the home (usually the basement) for 48-96 hours under “closed-house conditions” — windows and exterior doors closed except for normal entry/exit. Cost: $125-$200 when performed by a certified professional.
Long-Term Testing (90+ days)
Long-term tests (alpha track detectors placed for 90 days to 1 year) provide a more accurate picture of average annual exposure, accounting for seasonal variations. Winter levels are typically 2-3 times higher than summer levels due to reduced ventilation. Long-term tests are recommended for homeowners who want to assess their actual exposure rather than relying on a short-term snapshot. Cost: $25-$50 for a kit, plus $25-$40 for lab analysis.
DIY vs. Professional Testing
| Method | Cost | Accuracy | Best For |
|---|---|---|---|
| DIY Charcoal Kit (mail-in) | $15–$30 | Good (if done correctly) | Homeowners doing initial screening |
| DIY Digital Monitor | $130–$250 (one-time purchase) | Good for ongoing monitoring | Homeowners who want continuous data |
| Professional CRM Test | $125–$200 | Excellent (tamper-evident, certified) | Real estate transactions (required by most agents) |
| Long-Term Alpha Track | $25–$50 + lab fee | Excellent for annual average | Homeowners assessing long-term exposure |
For home purchases, a professional CRM test is the standard. The monitor is tamper-resistant, provides continuous readings, and generates a certified report that both parties can trust. DIY kits are fine for homeowner screening but aren’t typically accepted in real estate transactions.
What to Do If Radon Is High
If your test result is 4.0 pCi/L or above, the EPA recommends taking action to reduce radon levels. The standard mitigation approach is a sub-slab depressurization system, which is highly effective and relatively affordable.
Sub-Slab Depressurization (SSD)
This is the most common and effective mitigation method. A pipe is inserted through the basement floor slab into the gravel or soil beneath. A small fan creates negative pressure under the slab, drawing radon gas from beneath the foundation and venting it above the roofline where it disperses harmlessly. The system runs 24/7 and uses about $50-$100 per year in electricity.
| Mitigation Component | Cost | Notes |
|---|---|---|
| Standard SSD System (1 suction point) | $800–$1,200 | Works for most homes with poured or block basements |
| Extended SSD System (2+ suction points) | $1,200–$2,000 | Larger homes, fieldstone foundations, complex layouts |
| Crawlspace Mitigation | $1,000–$1,800 | Requires vapor barrier + depressurization |
| Sump Pit Integration | $200–$400 additional | Uses existing sump pit as suction point |
| Well Water Treatment (aeration) | $3,000–$5,000 | For radon in well water above 4,000 pCi/L |
| Well Water Treatment (GAC filter) | $1,500–$3,000 | For moderate radon in well water |
| System Monitoring (manometer) | Included | U-tube gauge shows system is operating |
A properly installed SSD system reduces indoor radon levels by 80-99%, typically bringing a home from 8-15 pCi/L down to 1-2 pCi/L. The system is quiet (the fan is usually in the attic or exterior), unobtrusive (a single 3-4 inch PVC pipe), and requires minimal maintenance. Fan replacement every 5-10 years ($150-$300 including installation) is the primary ongoing cost.
Radon in Real Estate Transactions
Radon testing is standard practice in New Hampshire real estate transactions, though not legally required. About 80% of home purchases in the state include a radon test during the inspection period. Here’s how it typically works:
- Buyer orders a radon test during the inspection contingency period, usually performed simultaneously with the home inspection. Cost: $125-$200.
- Results below 4.0 pCi/L: No action needed. The sale proceeds normally.
- Results at 4.0-10.0 pCi/L: Buyer typically requests that the seller install a mitigation system ($800-$1,500) or provide a credit of that amount at closing. Most sellers agree — it’s a small cost relative to the sale price.
- Results above 10.0 pCi/L: Same request, potentially with additional testing of well water for radon if the home uses a private well. Very high levels (20+ pCi/L) may concern buyers more but are still fully mitigatable.
- Seller has existing mitigation system: Verify it’s operating (check the manometer), request a post-mitigation test to confirm levels are below 4.0, and include the system in the property transfer.
Sellers: if you know your home has elevated radon, consider installing a mitigation system before listing. A mitigated home with documented low levels removes the negotiation point entirely and shows proactive maintenance. The $800-$1,500 investment typically pays for itself by avoiding buyer credits and keeping the transaction moving. Our seller net proceeds calculator helps you model how mitigation costs affect your bottom line.
Radon in Well Water
About 40% of New Hampshire homes use private wells, and many of these draw from bedrock aquifers that contain dissolved radon. When radon-laden water is used in the home — particularly during showering, dishwashing, and clothes washing — the gas is released into the indoor air. The EPA estimates that water with 10,000 pCi/L of radon contributes approximately 1 pCi/L to indoor air levels.
New Hampshire has no state standard for radon in water (the EPA has proposed but not finalized a national standard of 4,000 pCi/L for public water systems). However, many NH health professionals recommend testing and treatment for well water radon above 4,000 pCi/L. Treatment options include aeration systems ($3,000-$5,000 installed) that bubble air through the water to release radon before it enters the home, and granular activated carbon (GAC) filters ($1,500-$3,000) for moderate levels.
If you’re buying a home with a private well in New Hampshire, test the water for radon separately from the indoor air test. Well water radon requires a specialized water sample — your home inspector or a water testing lab can coordinate this. Factor potential treatment costs into your purchase negotiation. Our closing cost calculator helps budget for these expenses.
Compare With Other States
Considering other markets? Here’s how other states compare:
- Radon in Wisconsin Homes: What Buyers Must Know
- Adobe and Pueblo Home Maintenance in New Mexico: What Owners Need to Know
- Colorado Water Rights Explained: What Property Buyers Need to Know
Frequently Asked Questions
Is radon testing required when buying a home in New Hampshire?
Not legally required, but strongly recommended and performed in about 80% of NH real estate transactions. No lender requires it. However, given that 40% of NH homes test above the EPA action level and mitigation costs $800-$1,500, spending $125-$200 on a test is among the best investments a buyer can make. Skipping the test saves you nothing if you discover high radon after closing and pay for mitigation yourself.
What radon level is dangerous?
The EPA recommends action at 4.0 pCi/L or above. There is no known safe level of radon — any exposure carries some risk. The risk is proportional to both level and duration: living in a home at 4.0 pCi/L for 30 years carries roughly the same lung cancer risk as smoking half a pack of cigarettes per day. At 20 pCi/L, the risk equivalent approaches a pack-a-day habit. The average outdoor radon level is about 0.4 pCi/L, and a well-mitigated home typically tests at 1-2 pCi/L.
How much does radon mitigation cost in New Hampshire?
A standard sub-slab depressurization system costs $800-$1,500 for most homes. Complex installations (multiple suction points, fieldstone foundations, crawlspaces) can run $1,500-$2,500. Well water radon treatment adds $1,500-$5,000 depending on the system type and radon concentration. Annual operating cost for an SSD system is $50-$100 in electricity, with fan replacement every 5-10 years at $150-$300. The total lifetime cost of radon mitigation is remarkably low relative to the health protection it provides.
Does a radon mitigation system affect home value?
A properly installed and functioning mitigation system has a neutral to slightly positive effect on value. Buyers prefer a mitigated home with documented low levels over a home with unknown radon status. The system demonstrates proactive maintenance. Disclosure of high radon without mitigation is more likely to negatively impact value — buyers will either demand a credit for installation or walk away. In practical terms, the $800-$1,500 mitigation cost is not reflected as a price reduction in the sale.
Can radon levels change over time?
Yes. Radon levels fluctuate seasonally (higher in winter when homes are sealed, lower in summer with open windows), with weather patterns (barometric pressure changes affect soil gas movement), and with changes to the home (finishing a basement, adding insulation, sealing air leaks). A single short-term test provides a snapshot; long-term monitoring gives a more accurate picture. If you have a mitigation system, test every 2 years to verify it’s still performing. If you don’t have a system, retest every 5 years or after significant home modifications.
Does radon affect all levels of my home?
Radon concentrations are highest in the lowest level of the home (basement or first floor for slab-on-grade) and decrease on upper floors. A basement reading of 8 pCi/L might correspond to 3-4 pCi/L on the first floor and 1-2 pCi/L on the second floor. Testing is done in the lowest livable level because that represents the highest potential exposure. If your basement tests high but you only use it for storage, your actual exposure is lower than the test number — but mitigation is still recommended because radon migrates through the home over time.
Are there any NH programs that help with radon mitigation costs?
New Hampshire does not currently have a state-funded radon mitigation assistance program. However, some municipalities offer low-interest home improvement loans that can cover mitigation costs, and some weatherization programs (NH Saves, Community Action Programs) may include radon as part of whole-house energy and health improvements for income-eligible homeowners. The NH Department of Health and Human Services provides free radon test kits through public health outreach events. Check our home services directory for certified radon mitigation contractors in your area, and our mortgage resources for financing options that can cover mitigation as part of a home purchase.