Radon at home: how to test, what the numbers mean, and how fixes work

Most of the air pollutants I used to measure for a living had the courtesy to announce themselves. Ozone stings the eyes on a hot afternoon. Wood smoke you can smell from the end of the driveway. Radon gives you nothing: no color, no odor, no taste, no cough. It seeps up out of the ground into basements and ground floors, and the only way to know how much is in your house is to measure it.

The good news is that measuring it is cheap and easy, and when the number comes back high, the fix is a well-understood piece of plumbing rather than a mystery. The part people find confusing is the middle: what the units mean, whose number you are comparing against, and what to do when two tests disagree. That middle is what this piece is for.

Where radon comes from

Uranium occurs in small amounts in most rock and soil. As it decays it passes through a chain of elements, including radium, which decays into radon-222. Radon is a noble gas, so it binds to nothing and drifts freely through soil pores and rock cracks, and its half-life of about 3.8 days gives much of it time to reach the surface.

Outdoors it dilutes into open air. Indoors it can build up, and the real trouble is what it turns into: solid radioactive decay products that cling to dust, settle in the lungs and release alpha particles, a short-range radiation that does its damage right where it lands. The U.S. EPA describes radon as the leading cause of lung cancer among people who have never smoked and the second leading cause overall, and notes that smokers exposed to radon face a much higher risk than from either factor alone.

Radon became a household word in the mid-1980s, when a house on eastern Pennsylvania's uranium-rich Reading Prong turned out to hold enough of it to set off radiation alarms at the nuclear plant where the owner worked. High readings turn up in parts of the Appalachians, New England, the Upper Midwest, the northern Plains and the Rockies, but regional maps only describe odds. Two houses on the same street can test very differently.

How it gets in

A house in cold weather behaves a bit like a chimney. Warm air rises and leaks out through the upper floors and attic, which leaves the lowest level at slightly lower pressure than the soil around it. That small difference pulls soil gas in through whatever openings it can find: cracks in the slab, the joint where floor meets wall, hollow block walls, sump pits, floor drains, gaps around pipes, and the bare dirt of a crawlspace.

That is why readings tend to be highest on the lowest level, higher in the heating season than in summer, and sometimes higher while big exhaust fans or a clothes dryer are running. It also explains why old drafty houses and new tight ones can both have radon.

The numbers, and whose numbers they are

In the U.S., radon is reported in picocuries per liter of air (pCi/L). Most of the rest of the world uses becquerels per cubic meter (Bq/m³). The conversion is easy to keep in your head: 1 pCi/L equals 37 Bq/m³.

Here are the reference points you are most likely to meet, and whose they are. These are other people's numbers. We are explaining them, not setting them, and guidance gets revised, so check the current version at the source before you make a decision.

None of these numbers is a line between safe and unsafe. Risk rises with concentration and with years of exposure, and the EPA's guidance is that any radon exposure carries some risk. The WHO adds that where 100 Bq/m³ cannot be reached, levels should still stay under 300. And the EPA's 4 pCi/L marks the point where it recommends fixing, not a promise that anything lower is harmless, so a 3.8 is not good news just because it starts with a 3.

In the U.S., the official starting point is the EPA's radon pages, which also list state radon contacts. Elsewhere, check your own national health or radiation agency, since reference levels differ.

Testing: kits, monitors and timing

Short-term tests usually run two to seven days, though the EPA counts anything up to 90 days as short-term. Most are small charcoal canisters or similar passive detectors that you open, leave in place, reseal and mail to a lab. They are cheap and quick, but they capture only a snapshot of a gas that swings with weather and season.

Long-term tests run for more than 90 days, often with an alpha-track detector, and give a much better picture of what you breathe across a year. The trade-off is patience.

Digital continuous monitors show a running average on a screen. They are excellent for watching trends and keeping an eye on a mitigation system over months, but they need a day or two to settle and their accuracy varies by model. For a big decision, pair one with a lab-analyzed test or a certified tester's calibrated equipment.

For any short-term test, the conditions matter as much as the device. EPA guidance calls for "closed-house conditions": windows and outside doors kept shut apart from normal coming and going, starting 12 hours before the test and lasting until it ends. Kit instructions typically add a few more points:

• Use the lowest level you live in regularly, such as a finished basement or a ground-floor bedroom.
• Keep it out of kitchens, bathrooms and laundry rooms, and away from sump pits, exterior walls, drafts, vents and direct sun.
• Set it at about knee height or a little higher, somewhere it will not be bumped or moved.
• Skip short tests during severe storms or very high wind.
• Record exact start and end times and mail the kit the day you finish, since the lab corrects for decay in transit.

Reading your result

The EPA's A Citizen's Guide to Radon lays out a sequence, and it is worth reading the current edition yourself. In outline, it suggests that if a first short-term test comes back at or above 4 pCi/L, you follow up with a second test: a long-term one if you want the best estimate of your yearly average, or another short-term one if you need an answer sooner. If the long-term result, or the average of the two short-term results, is 4 pCi/L or higher, the guide recommends fixing the house. It also suggests considering a fix for results between 2 and 4.

My own advice, after too many hours staring at instrument logs: treat a single short-term result as a reason to measure again, not as a verdict. Two readings that roughly agree are worth far more than one. If the first number is several times the action level, though, skip the months-long wait and run a quick second short-term test while you gather quotes.

Our explainer on the Air Quality Index on smoky days covers the outdoor half of the air you breathe, and our guide to public maps and data tools points to where radon zone maps live. Treat those maps as background reading; they cannot tell you what is in your basement.

How fixes work

The standard repair for houses with a basement or slab is called active soil depressurization, and the idea is simple. An installer drills through the slab, scoops out a small pit underneath, and seals in a pipe that runs to an in-line fan and vents above the roofline. The fan creates a slight vacuum under the slab, so soil gas takes the easy path into the pipe and out the top instead of up through your floor.

A few details tell you whether a system has been put together well:

• A pressure gauge on the pipe. Most systems carry a small U-tube manometer with colored liquid in it. Uneven levels mean the fan is pulling; level ones mean something has stopped.
• The fan outside the living space, in an attic, a garage or on an exterior wall, because the pipe beyond the fan is under positive pressure and any leak there belongs outdoors.
• Sealed openings. Sump pits get airtight lids and large cracks get sealed so the fan works efficiently. Sealing on its own, without the fan, is generally not enough to bring a high reading down.
• Crawlspaces get heavy plastic sheeting laid over the dirt and sealed at the edges, with the suction pipe beneath.

Costs depend on the foundation, the soil and the pipe route, so get more than one quote and ask about the installer's certification; the EPA's pages point to the state contacts and national proficiency programs that track it. The fan runs around the clock, which means a small electricity bill, a little extra heat loss in winter and, after some years, a replacement fan.

Once the system runs, retest. Wait at least a day after it starts, then test again every couple of years or after big changes such as finishing the basement. In granite country, New England especially, radon can also arrive in private well water and escape into the air from showers and faucets. Water needs its own test and different fixes, such as aeration or activated-carbon filters.

Short answers to common questions

My neighbor tested low. Do I still need to test? Yes. Soil and construction vary from house to house, and your neighbor's number tells you very little about yours.

I live on the fourth floor of an apartment building. Does this apply to me? Levels are generally lower the higher you go, because radon comes from the ground. The EPA recommends testing homes below the third floor, so ground-floor and basement units come first.

Can't I just open the windows? Ventilation lowers radon while the windows are open, and it climbs back once they are shut. That works for a mild week in May and is not much of a plan for January.

What about granite countertops? Some stone gives off a little radon, but in nearly every home the soil under the house is a far bigger source. Test the air, and do not tear out the kitchen over it.

If you have never tested, do it before the nights warm up and the windows open for the season. Set the kit on a basement bookshelf, stick a note with the start time on the furnace, and by next week you will know something about your own air that you have been breathing blind for years.