Radon Testing in Massachusetts: When to Test and What the Number Means

Radon is the only serious problem a house can have that you cannot see, smell, or find on an inspection report unless somebody specifically goes looking for it. It is also the one that Massachusetts law does not require anyone to tell you about.

What it is and why New England

Radon is a radioactive gas produced by the decay of uranium in soil and rock. It seeps into buildings through foundation cracks, sump pits, slab penetrations, and the ordinary gaps around where the house meets the ground. It is the second leading cause of lung cancer in the United States after smoking, according to the EPA.

New England's geology is the reason it comes up here more than in some other regions. Granite and other uranium-bearing bedrock underlie much of Massachusetts, and the concentration varies substantially — not just between towns, but between neighbouring houses on the same street. This is genuinely important and routinely misunderstood: your neighbour's radon result tells you very little about your own. The pathway into the building depends on that building's foundation, its ventilation, and the specific soil beneath it.

The Massachusetts Department of Public Health's Radon Assessment Unit publishes results by community, which is useful for calibrating your expectations. It is not a substitute for testing the specific house.

What the number means

Radon is measured in picocuries per litre, written pCi/L.

• The EPA action level is 4.0 pCi/L. At or above that, the EPA recommends mitigation.
• Between 2.0 and 4.0 pCi/L, the EPA suggests considering mitigation. There is no threshold below which radon is known to be harmless; the risk is continuous.
• Outdoor air typically runs well under 1 pCi/L.

A result of 4.2 is not a crisis and a result of 3.8 is not a clean bill of health. It is a dose-response relationship, and the mitigation decision is about how much you want to reduce a long-term exposure.

Short-term versus long-term tests

This distinction decides how much your number is worth.

• A short-term test runs 2 to 7 days under closed-house conditions — windows and exterior doors kept shut except for normal entry and exit, for at least 12 hours before and throughout. It is what gets used in real estate transactions because it fits inside an inspection contingency.
• A long-term test runs 90 days or more and gives you something much closer to the annual average you actually live with.

Radon levels swing by season, by weather, and by how the house is being operated. They generally run highest in winter, when the house is sealed and the stack effect from heating pulls soil gas up through the foundation. A short-term test in January and a short-term test in July on the same house can differ substantially.

The practical consequence: a transaction test is a snapshot taken under deliberately unfavourable conditions, which is the right way to do it for a decision under time pressure. After you move in, run a long-term test. If you mitigate, test again afterwards to confirm the system is doing its job, and re-test every couple of years.

Massachusetts does not require disclosure

There is no Massachusetts statute requiring a seller to test for radon or to disclose a known result, and Massachusetts is not a broad seller-disclosure state to begin with — the notable exception is lead paint, covered in the Massachusetts Lead Law.

A seller who has a prior test result and is asked directly should not misrepresent it. But nobody is obliged to volunteer it, and nobody is obliged to test.

That puts the burden squarely on the buyer, and it means the radon test lives inside your inspection contingency. Order it as an add-on when you schedule the home inspection; most Massachusetts inspectors offer it and it is inexpensive relative to everything else you are paying for that week.

Testing the water, if you are on a well

Radon dissolves into groundwater and is released into the air when the water is agitated — showers, dishwashers, laundry. If the home draws from a private well, which is common in Dover, Sherborn, Carlisle and parts of the outer MetroWest towns, the water is worth testing separately from the air. Massachusetts DPH publishes guidance on radon in private well water.

Municipal water is treated and aerated before it reaches you, so this is a private-well question rather than a general one. If you are looking at a well property, it belongs on the same list as the Title 5 septic inspection — the two show up in the same houses for the same reason.

How mitigation works

The standard fix for an elevated air reading is active sub-slab depressurisation: a pipe through the slab into the gravel beneath it, an inline fan, and a vent that discharges above the roofline. The fan runs continuously and draws soil gas out before it can enter the living space.

It is a well-understood, undramatic piece of work — typically a day, sometimes less. Systems are quiet, they use a modest amount of electricity, and a properly installed one will usually take a house from well above the action level to well below it. Get quotes rather than assuming a figure; cost depends on foundation type, access, and how the discharge has to be routed, and a walk-out basement with an easy exterior run is a different job from a finished basement in a house with a complicated roofline.

Look for a mitigator certified through the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB). Massachusetts DPH publishes guidance and can point you at certified professionals.

Negotiating it

When a test comes back high, this is one of the more straightforward items in a Massachusetts negotiation, for a reason worth understanding: the problem is fully solvable, the solution has a known price, and the seller now knows about it. If this buyer walks, the next buyer's inspector will find the same number, and the seller will be having the same conversation two weeks later with less time on the clock.

The usual outcomes:

• The seller installs a system before closing and provides a post-mitigation test showing the result.
• The seller credits the buyer at closing and the buyer arranges the work.
• The parties split it.

Ask for the post-mitigation test either way. A system that was installed but never verified is not the same as a system that works. And if a house already has a mitigation system, ask when it was installed, whether the fan has been replaced, and for the most recent test — fans do fail, silently, and a dead fan on a mitigated house is worse than no system at all because everyone stopped worrying about it.

If you are staying put

None of this is transaction-specific. If you have owned your home for years and have never tested, a long-term test kit costs very little and takes one winter. The seller's guide touches on what is worth doing before a listing; a known-good radon result is a small, cheap piece of documentation that removes a question from a future buyer's inspection.

Action levels are the EPA's and are stated as of 2026. The Massachusetts Department of Public Health's Radon Assessment Unit and the EPA are the authorities on testing protocol and current guidance; this is general information, not a health assessment of any specific property.