There’s a particular kind of hazard that makes me take a breath and slow down when I’m talking to clients about it — not because it’s dramatic or rare, but because it’s completely invisible, genuinely dangerous, and sitting undetected in a lot of homes in this region right now. Radon is that hazard.
I test for radon on inspections regularly across southern New Hampshire and Vermont, and I can tell you that a meaningful chunk of homes I test come back elevated. This isn’t a scare tactic. It’s just the geology of where we live. And the frustrating thing is that most people have heard of radon but don’t really know what it is, whether their home has it, or what they’d do about it if it did.
So let’s fix that.
What Is Radon, Actually?
Radon is a naturally occurring radioactive gas. It forms when uranium in soil and rock decays — uranium breaks down into radium, radium breaks down into radon, and radon is a gas, so it moves. It seeps up through the ground and into buildings through foundation cracks, floor drains, utility penetrations, and sometimes even through poured concrete itself.
It has no smell, no color, no taste. You can’t detect it with any of your senses. The only way to know if it’s in your home is to test for it.
The reason it matters is that when you breathe radon, the radioactive particles it’s in the process of decaying into — called radon decay products — can get trapped in your lungs. Over time, that radiation damages lung tissue. Radon is the second leading cause of lung cancer in the United States, behind only smoking. The EPA estimates it’s responsible for about 21,000 lung cancer deaths per year. And unlike some health risks that require decades of extreme exposure, radon risk accumulates with everyday living — sleeping, cooking, watching TV — all in a house with elevated radon.
Why New Hampshire and Vermont Specifically?
The geology of New England — and the Granite State’s name isn’t accidental — creates elevated radon potential across a wide swath of the region. Granite and other crystalline bedrock are naturally higher in uranium content than many other rock types. As that uranium slowly breaks down underground, radon works its way toward the surface.
The EPA divides the country into radon zones. Zone 1 carries the highest predicted average indoor radon levels (above 4 pCi/L). New Hampshire has significant Zone 1 coverage, particularly in areas with granitic bedrock, and Zone 2 coverage across much of the rest. Vermont’s radon profile is similarly elevated in many areas, especially central and northern Vermont.
The Monadnock Region, the Upper Valley, the Lakes Region, the Champlain Valley — these aren’t low-risk areas. They’re sitting on the same bedrock formations that push radon up into basements and crawl spaces across the Northeast.
The other factor is the types of houses we have around here. Older homes, tight homes, homes with slab-on-grade or partially below-grade basements — these all create conditions where radon can accumulate rather than dissipate. A well-sealed, energy-efficient home can actually trap more radon than a drafty old cape if there’s no mitigation in place, because the same air sealing that keeps your heat in keeps the radon in too.
Radon Testing in NH & VT
During a home inspection, radon testing is typically done with a continuous electronic monitor or with charcoal canisters placed at the lowest livable level of the home — usually the basement, or the first floor if there’s a slab or crawl space.
I use a continuous electronic monitor that logs readings hour by hour over a 48-hour test period. This is the more detailed approach: it captures fluctuations, timestamps readings, and produces a full report rather than just a single average number. You can see whether levels spiked at night, stayed consistent, or showed any unusual patterns. It gives you a more complete picture.
The test has to be conducted under closed-house conditions — windows shut, doors closed except for normal entry and exit — for at least 12 hours before the test begins and throughout the testing period. This is standard protocol and affects results if it isn’t followed, which is worth knowing as a buyer: if the seller has been airing out the basement the week before your inspection, that matters.
The result comes back in picocuries per liter (pCi/L). The EPA action level is 4 pCi/L — meaning if your result is at or above that, they recommend mitigation. The average U.S. indoor radon level is around 1.3 pCi/L. The average outdoor level is about 0.4 pCi/L. So anything coming back at 4 or above warrants attention, and anything coming back at 2–4 is worth keeping an eye on, especially in a home where people spend a lot of time in the basement.
What If the Test Comes Back Elevated?
This is where I think a lot of people get stuck, because the response to an elevated result can feel uncertain. But it doesn’t have to be. Radon is one of the more fixable hazards a home can have.
The standard mitigation approach is called sub-slab depressurization — a fancy name for a system that’s actually pretty elegant. A certified radon mitigation contractor installs a pipe through the slab (or through the foundation in a crawl space) and connects it to a small fan in the attic, exterior wall, or roof. That fan runs continuously and creates a slight negative pressure beneath the slab, drawing radon-laden soil gas up and out through the pipe before it ever enters the living space.
These systems work. I’ve seen homes that tested at 10, 12, even 15 pCi/L get mitigated down to under 1 pCi/L after installation. The systems are relatively inexpensive — though mitigation in New Hampshire and Vermont depends heavily on foundation type and system complexity — and they’re low-maintenance. The fan runs on a small amount of electricity and lasts many years. There’s a simple visual indicator (a U-tube manometer) that shows whether the system is operating correctly.
If you’re buying a home and the radon test comes back elevated, you have a few options. You can negotiate with the seller to have mitigation installed before closing. You can negotiate a credit and handle it yourself. Or if the result is borderline, you can install a system after you move in and re-test. For homes that already have a mitigation system, it’s worth re-testing periodically — every two years or so — to confirm the system is still performing.
What About Well Water?
This comes up a lot in our service areas in rural NH and VT, where private wells drilled into bedrock are the norm. Radon can also enter homes dissolved in well water — you can release it by showering, running the dishwasher, or doing laundry. Waterborne radon is generally a smaller contributor to indoor air levels than soil gas, but it’s not nothing, and in areas with high uranium bedrock, well water radon can be significant.
Testing your well water for radon is a separate test from an air test. If your home has a drilled well and elevated air radon, it’s worth testing the water too. Treatment options exist — aeration systems and granular activated carbon (GAC) filters are both used — and they’re installed either at the point of entry (treating all water coming into the house) or at the point of use.
This is one of the reasons I also bundle water testing into inspections when it makes sense for the property. A drilled well in the Monadnock Region, Keene area, or western Vermont isn’t just a radon conversation — it’s also an arsenic, bacteria, nitrates, and water quality conversation. It’s all related to the same geology and the same infrastructure situation.
Existing Homes and Long-Term Residents
Something worth saying clearly: radon isn’t just a home-buying issue. If you’ve lived in your home for years and have never tested, there’s no time like the present. Short-term test kits are available at hardware stores for under $30 and get mailed to a lab. Long-term kits give a more accurate average over 90+ days. Neither requires any special skills to deploy.
If you’re in New Hampshire or Vermont and you haven’t tested your home since you moved in — especially if you have a basement where people spend regular time, or a finished lower level — it’s worth doing. The risk isn’t theoretical. It’s slow, it’s cumulative, and it’s preventable.
Frequently Asked Questions
How long does radon testing take?
A standard short-term test runs 48 hours. For a home inspection, the monitor goes in at the start of the inspection period and gets picked up two days later. The closed-house conditions need to be in place for at least 12 hours before the test starts, so in practice the seller needs to button the house up the evening before the inspection. Results are typically available within a day of retrieval.
If you’re testing your own home outside of a transaction, long-term kits — which sit in place for 90 days or more — give you a more accurate annual average, since radon levels fluctuate day to day and seasonally. But for a real estate transaction, a 48-hour test under closed-house conditions is the standard.
Does radon change with the seasons?
Yes, and this is something worth understanding. Radon levels tend to be higher in winter than in summer. The reason is pretty intuitive once you think about it: in winter, houses are sealed up tight, ventilation is minimal, and the temperature differential between the warm house and the cold ground can actually increase the stack effect — the tendency for air to be drawn upward through the structure. That pull can draw more soil gas in through the foundation.
This doesn’t mean a summer test is useless, but it does mean a summer result might be somewhat lower than what you’d see in January. For real estate transactions happening in warmer months, that’s worth keeping in the back of your mind. A result that comes back at 3.2 pCi/L in July might look a little different in December.
Is radon worse in older homes?
Not necessarily — and this surprises people. You might assume that an old, drafty farmhouse would be worse because it’s less well-maintained and more porous. But a drafty house actually dilutes radon more effectively, because outside air is constantly infiltrating and mixing with indoor air. The homes that can really trap radon are the tight, well-insulated ones — newer construction or older homes that have been heavily weatherized. The same air sealing and insulation upgrades that make a house energy-efficient can inadvertently concentrate radon if there’s no mechanical ventilation or mitigation in place.
Foundation type matters a lot too. Homes with dirt-floor crawl spaces or rubble stone foundations have different risk profiles than homes with poured concrete slabs. The only way to actually know is to test.
Do I need to test if my home already has a mitigation system?
Yes. A mitigation system is not a permanent guarantee — it’s a mechanical system with a fan that can wear out, a pipe that can develop issues, and a pressure differential that can change over time if the home is modified (new addition, new HVAC system, resealing the slab). The EPA recommends re-testing every two years after a system is installed, and after any major renovation that affects the foundation or envelope of the house.
Most mitigation systems have a simple visual indicator — a small U-tube with colored fluid — that shows whether the fan is creating negative pressure. If that indicator looks wrong, that’s a sign to get the system checked. But even if it looks fine, periodic testing is the only way to confirm the system is actually keeping levels down to where they should be.
Can I test for radon myself, and when should I hire a professional?
You can absolutely test yourself. Charcoal canister kits are available at most hardware stores — Lowes, Home Depot, your local hardware store — for $15 to $30. You set the canister in the lowest livable level of the home, leave it for the specified time (usually 48–96 hours for short-term, 90+ days for long-term), then mail it to a lab. Results come back in a week or two. It’s a legitimate starting point and perfectly reasonable for existing homeowners who just want to know where they stand.
For a real estate transaction, you want a professional test — not because the DIY kits are inaccurate, but because the results need to be documented, the chain of custody needs to be clear, and there are protocols around closed-house conditions and monitor placement that matter for the result to hold up in a negotiation. A continuous electronic monitor also gives you more data than a passive canister, which can be useful if results are borderline.
Does radon only affect the basement?
Radon enters from the ground, so it’s most concentrated at the lowest level of the home. But it doesn’t stay there. It mixes with household air and gets distributed throughout the house — though levels do tend to drop as you go up. A first floor might have 50–75% of the basement concentration; upper floors less than that.
This matters for how you think about risk. If your basement is unfinished and nobody spends time down there, your exposure is lower than someone who has a finished basement office or playroom where the family spends hours a day. That context should factor into how urgently you respond to a given test result. A result of 5 pCi/L in a basement nobody uses is a different conversation than 5 pCi/L in a space where your kids play every afternoon.
The Bottom Line
Radon is one of those things where the gap between how serious it is and how much attention most people give it is genuinely large. It’s not exotic. It’s not rare in this region. It’s a real hazard, it’s testable, and when it’s found, it’s fixable.
If you’re buying a home in New Hampshire or Vermont and your inspector isn’t talking to you about radon testing, ask about it. If you’re a current homeowner who hasn’t tested, put it on the list. And if you’ve got a drilled well and live anywhere near granitic bedrock — which is most of southern NH and a good chunk of Vermont — consider getting the water tested while you’re at it.
The 48 hours it takes to run a test and the peace of mind that follows are worth it. And if the result comes back elevated, at least you know — and knowing is the only way to fix it.
Resonant Homes LLC provides radon testing as part of our home inspection services across southern New Hampshire and Vermont, including the Monadnock Region, Greater Keene area, the Cheshire County corridor, and across the Connecticut River into VT. We use continuous electronic monitors and provide full reports with hourly readings. Radon testing can be added to any inspection or scheduled as a standalone service. We also offer water quality testing for homes on private wells. Reach out if you have questions.
[…] Radon in Water — Yes, radon dissolves into groundwater, not just soil gas. This is a separate test from air radon and matters most if your water usage is high (long showers, dishwashers, etc., all release it into the air you breathe). […]