Radon Testing in Massachusetts: What Every Homebuyer Should Know
- Stephen Gaspar
- Jul 20
- 10 min read
Updated: Jul 24
Here is the first thing I tell people when we start talking about radon testing in Massachusetts: take a breath. You just breathed some radon. So did I.
Radon is everywhere. It is outside, it is inside, and every one of us is breathing a certain amount of it right now. There is no such thing as a house with absolutely no radon in it, so that is not what we are looking for when we test a home.
What we are trying to find out is whether the house is allowing radon to collect at a level where the people living there would be better off reducing their long-term exposure. That is the whole conversation. We measure it, we look at the number, and if the number is higher than it ought to be, we deal with it.
I have been explaining radon this way for 17 years because the two extremes do not help anybody. Pretending radon is nothing is irresponsible. Talking about an elevated result as though the house is radioactive and nobody should ever go inside it is not useful either. Radon is a real environmental condition, we have a reliable way to measure it, and in most homes we have a very straightforward way to lower it.
What Is Radon?
Radon is a naturally occurring radioactive gas. Uranium and radium exist in rock and soil. As those materials break down through a radioactive decay chain, radon is one of the stages they pass through. At that point it is a gas, which means it can move through the soil and find its way into a building.
In a Massachusetts house, radon can enter through cracks in the slab, sump pits, floor drains, utility penetrations, gaps around the foundation and exposed soil in a crawl space. That does not mean the foundation is bad. Soil gas will use whatever pathways are available, including openings that are common in otherwise well-built homes.
The gas itself continues to decay after it enters the house. That process produces very small radioactive particles that can be inhaled. The health concern comes from lung tissue being exposed to that radiation repeatedly over a long period of time, especially when the indoor concentration is elevated.
One breath is not the issue. The level in the house and the amount of time a person spends breathing that air are what we are evaluating.
The Sunlight Comparison I Use With Clients
I often explain the basic idea by comparing it to the sun. It is not a perfect scientific comparison, and I am not saying radon is beneficial. I use it because people already understand that exposure can be ordinary and still become something we should manage when the intensity or duration goes up.
The sun is there every day. When the UV index is high, we do not panic because sunlight exists. We reduce the exposure. We spend less time in direct sun, wear protection and try not to get burned.
Radon works the same way in that limited, practical sense. The gas is already part of our environment. If it is accumulating inside a house at an elevated level, and somebody is going to live and sleep there for years, the sensible response is to lower the concentration. We are not trying to create a radiation-free house. We are reducing an exposure we do not need.
Why Radon Testing Still Matters in Massachusetts
Radon has no smell, color or taste. I cannot walk into a basement, look at the stone foundation or concrete floor and tell a buyer what the radon result is going to be. I can identify places where soil gas may enter. I can tell you whether a mitigation system is installed. I still need a test to know the level.
Massachusetts estimates that about 650,000 homes, roughly one out of every four, have radon above the EPA action level of 4 picocuries per liter. The state also estimates that approximately 34,000 homes are above 20 pCi/L. Elevated readings have been found in every Massachusetts county.
Those numbers are enough reason to test, but they are not a reason to guess. A fieldstone basement does not guarantee a high result. New concrete does not guarantee a low one. I have seen people rely on a neighbor's test or a radon-zone map, but neither tells us what is happening in the house being purchased. Two houses standing next to one another can test very differently.
Radon testing is also not automatically part of a standard Massachusetts home inspection. A home inspection is a visual evaluation of the accessible building and its systems. Radon measurement is a separate environmental service that needs to be requested and included in the inspection agreement.
How Does a Radon Test Work During a Home Purchase?
Most radon tests used during a real-estate transaction run for at least 48 hours. The testing device is generally placed in the lowest level of the home that is occupied or could reasonably be occupied. If the basement is finished, that is usually where it goes. An unfinished basement that could become living space may also be the correct location.
The house needs to be under closed-building conditions. Windows and exterior doors should be closed for at least 12 hours before testing begins and kept closed during the test except for normal entry and exit. Nobody has to stop living in the house. We simply do not want somebody leaving the basement windows open and artificially diluting the air we are supposed to be measuring.
Device placement matters. It should not be sitting next to an exterior door, a heating outlet, a fireplace, a draft or an unusually damp area. Major storms and severe weather may affect a short-term test as well. A continuous radon monitor records measurements over time, which gives me information I can use when I review whether the test conditions appear reasonable.
I care about the conditions because an impressive-looking report does not make a questionable test reliable. If the data or the situation gives me a reason not to trust the result, I would rather explain that and repeat the test than give a buyer false confidence in a precise number.
What Do Massachusetts Radon Test Results Mean?
Radon in air is reported in picocuries per liter, abbreviated pCi/L. The EPA uses 4 pCi/L as its action level, and Massachusetts follows that guidance. I explain the results as ranges because exposure does not suddenly change from harmless to dangerous when a monitor moves from 3.9 to 4.0.
Below 2 pCi/L
This is generally a lower result. It does not mean the house contains no radon, and it does not guarantee that the level will never change. The EPA recommends testing again periodically because the building, the weather and the way the lower level is used can change over time.
Between 2 and 3.9 pCi/L
The EPA and Massachusetts advise homeowners to consider reducing the level in this range. When a short-term result lands close to 4, a longer-term test may also help an owner understand the home's average concentration over a longer period.
4 pCi/L or Higher
At or above 4 pCi/L, the recommendation is to have the home mitigated by a qualified radon professional. That result deserves attention, but it does not automatically make the house a bad purchase. It gives us a measured condition and a reason to obtain a proper mitigation plan.
I do not describe 3.9 as completely safe and 4.0 as a catastrophe. I explain what the number means, what the recommendations are and what the buyer can do with the information.
What a Radon Mitigation System Actually Does
A radon mitigation system does not eliminate radon. It lowers the amount that is building up inside the house.
The most common residential approach is active soil depressurization. A contractor installs piping through or below the foundation and uses a continuously operating fan to pull soil gas from beneath the slab. The gas is discharged outside before as much of it can enter the living space.
Depending on the house and the design, the system may also involve sealing selected openings or connecting more than one foundation area. The EPA reports that some systems can reduce indoor radon by as much as 99 percent. The actual reduction in any individual house has to be proven by testing after the work is completed.
That last part is important in a very practical way. A pipe and a fan are pieces of equipment. The post-mitigation test tells us whether those pieces of equipment lowered the radon level the way they were supposed to.
A Radon System Does Not Eliminate the Need to Test
When I see a mitigation system already installed, I generally view that as evidence that somebody found an issue and took action. I do not view it as proof of the current radon level.
The small gauge mounted on many systems is a pressure indicator. It tells us whether the fan is creating suction in the piping. It is not measuring radon. A fan can be running while the system needs adjustment, and a renovation or addition can change the building enough that the original layout no longer serves every foundation area.
I recommend testing a home even when the system is operating. Ask for the original result, the result after mitigation, the installer's information and any later tests. Massachusetts recommends retesting a mitigated home every year or two. That is how the owner confirms the system is still accomplishing what it was installed to do.
What About Radon in Private Well Water?
Radon in indoor air and radon dissolved in private well water are two separate tests. A monitor in the basement does not analyze the water, and a normal water-quality panel does not necessarily include radon unless somebody specifically orders it.
Groundwater can carry dissolved radon into the house. Showering, washing clothes and using the water can release some of that gas into the indoor air. Massachusetts recommends that homes with private wells test the water for radon in addition to testing the air.
When somebody tells me the well water has already been tested, I ask to see the laboratory report. “Water tested” can mean many different things. The report tells us exactly which analyses were performed and which were not.
Do New Massachusetts Homes Need Radon Testing?
Yes. New construction can include radon-resistant features, but those features do not tell us the indoor concentration.
Since 2015, qualifying new one- and two-family homes and townhouses in Essex, Middlesex and Worcester counties have been required to include passive radon-control systems. That is useful infrastructure. If testing later shows an elevated level, the passive pipe can often be activated by adding a fan.
What I would not do is look at the pipe and assume the house has passed a radon test. Massachusetts does not require the builder to hand the buyer a completed radon result simply because the passive system is present. Test the finished house and use the measurement, not the label, to make the decision.
When Should a Massachusetts Home Be Retested?
A radon test is a measurement from a particular house under particular conditions at a particular time. I would consider retesting when:
The previous result is more than two years old or nobody can document how the test was conducted
Renovations change the foundation, ventilation or configuration of the lowest level
A basement becomes a bedroom, office, playroom or another space where people will spend more time
Periodic testing also makes sense after a lower result, and a mitigated home should generally be checked every year or two. Houses are not static. Foundations, equipment and occupancy change, and the radon history should stay with the property just like the service history for the heating system.
Common Questions About Radon Testing in Massachusetts
Are we breathing radon right now?
Yes. Radon is naturally present in outdoor and indoor air. The national outdoor average is about 0.4 pCi/L and the average indoor level is about 1.3 pCi/L. The purpose of testing is to find out whether the individual house is allowing a higher concentration to build up.
Does a result below 4 pCi/L mean there is no risk?
No. The EPA does not identify a completely risk-free radon level. Four pCi/L is the point at which the EPA recommends taking corrective action, and reducing levels between 2 and 4 should also be considered.
Can opening windows change a short-term radon test?
Yes. Open windows can dilute the indoor air and produce a result that does not represent normal closed-building conditions. It can also artificially elevate indoor levels when windows in the upper levels of the home are opened, and the pressure draws increased air from the basement level. That is why the house must remain closed except for ordinary entry and exit before and during the test.
How long does a radon test take during a home purchase?
Most short-term real-estate tests run for a minimum of 48 hours. The schedule also needs to account for the closed-building period before the test begins.
Can I rely on the neighbor's radon result?
No. Nearby houses can have very different radon levels. Test the house you are buying.
Does a house with a mitigation system still need testing?
Yes. The fan and pressure gauge tell us something about the equipment. Only a radon test tells us the concentration in the air.
My Practical Advice to Massachusetts Buyers
If a radon result comes back elevated, slow the conversation down and deal with the actual information in front of you. Confirm that the test was conducted properly. Understand the measured level. Get a qualified mitigation contractor involved when the result calls for it, and make sure the agreement includes testing after the system is installed.
Most elevated radon conditions are manageable. Whether the seller pays, the buyer takes a credit or the work happens after closing is a real-estate decision. My job is to make sure the buyer understands the building condition and what needs to happen next.
I do not make a big deal out of radon in the sense of trying to scare people. I make a big deal out of testing it correctly. We are all breathing some radon now and we will continue to breathe some amount of it wherever we live. The test tells us whether too much is accumulating in the house. If it is, a properly designed system lowers the level, we test again to confirm the reduction, and the owner keeps monitoring it over time.
That is how I talk about radon at an inspection. Respect the health concern, get a reliable number and respond in proportion to what the number says. There is no benefit in dismissing it, and there is no benefit in turning a condition we routinely measure and reduce into a reason to panic.
Inspections Plus provides professional radon testing in Massachusetts as an additional service with residential home inspections throughout Greater Boston, Southern Massachusetts and Eastern Massachusetts. Radon testing of private well water can also be coordinated when appropriate.
Official Sources
Facts verified July 20, 2026.
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