Radon Zones and Who Should Test

Why this matters

Radon is the one indoor hazard where a map makes people less careful. An owner hears "we're not in a high radon area" and concludes the house is fine, and a contractor tightening that house or finishing its basement repeats the same conclusion. EPA says the opposite: test every home, whatever the map says. A tech who knows what the zone map is for, what EPA actually recommends and when a job changes the picture can give the one-sentence advice that matters, at the moment it matters, without overstepping into a measurement nobody asked them to make.

What radon is, in one paragraph

Radon is a radioactive gas produced by the decay of uranium in soil and rock. It has no color, odor or taste. It enters a house mainly from the soil, drawn in through cracks, joints, sump openings, utility penetrations and crawlspaces by the slight negative pressure most houses run at relative to the ground. EPA identifies it as the leading cause of lung cancer among nonsmokers. Indoor levels are measured in picocuries per liter of air (pCi/L). How a mitigation system removes it is covered in Radon Mitigation Basics Reference; this card is about who should test and when.

What the zone map is, and what it is not

EPA's Map of Radon Zones assigns every U.S. county to one of three zones based on geology, soil, housing and measurement data:

• Zone 1: predicted average indoor screening level greater than 4 pCi/L.
• Zone 2: predicted average between 2 and 4 pCi/L.
• Zone 3: predicted average less than 2 pCi/L.

The key word is average. A county average says nothing about one house, because radon levels vary enormously between neighboring houses on the same street depending on the soil directly beneath them, the foundation type, the cracks and openings, and how the house is operated. Elevated levels have been found in all three zones.

EPA states that the map is meant to help governments and organizations target radon programs and building codes, and that it is not to be used to decide whether a particular home needs testing. Its stated guidance is that every home should be tested regardless of zone. The Surgeon General and EPA recommend testing all homes below the third floor.

So the map has two legitimate field uses. In Zone 1 it adds urgency to a test you would recommend anyway. In any zone it is a reason to mention radon, never a reason not to.

The number EPA uses

EPA's action level is 4 pCi/L: at or above it, EPA recommends fixing the home. Between 2 and 4 pCi/L, EPA recommends that owners consider fixing it, because there is no known safe level and risk rises with concentration.

That federal guidance is a recommendation, not a regulation that binds a homeowner. The state layer sits on top and varies widely:

• Some states license or certify radon testers and mitigators, among them Illinois, New Jersey and Pennsylvania, and in those states only a credentialed person may perform measurements or mitigation for a client.
• Some states require radon disclosure or an informational pamphlet in a home sale, which Illinois does through its radon awareness law.
• Many states have neither, and rely on voluntary national proficiency programs to credential professionals.

Check your own state's radon program, usually in its health or environmental agency, before you describe what a tester or mitigator must hold.

How a test is done, so you can explain it

EPA describes two kinds of test. A short-term test runs from 2 to 90 days and gives a quick reading. A long-term test runs more than 90 days and gives a better picture of the year-round average, because radon rises and falls with weather, season and how the house is operated. Short-term tests under four days are run under closed-house conditions: windows and outside doors kept closed except for normal coming and going, for 12 hours before and throughout the test.

The device goes in the lowest level that is lived in or could be, about 20 inches or more above the floor, away from drafts, exterior walls, kitchens and bathrooms. A finished basement used as a bedroom is the right place; the far corner of an unfinished storage room is not, unless that room is going to be finished.

EPA's guidance for what to do with a result runs like this: if a short-term test reads 4 pCi/L or higher, do a follow-up test, either a long-term test for a better average or a second short-term test if the answer is needed quickly. Fix the home if the long-term result is 4 or higher, or if the average of the two short-term results is 4 or higher.

Your role in that sequence is to recommend it and explain it. Unless you are credentialed where your state requires it, you do not place devices for a customer or interpret the result beyond reading out what the lab reported.

When a job should trigger the conversation

The recommendation to test applies to every house all the time, which in practice means it gets said at no particular moment and heard by nobody. A field trade has better moments, because some work changes how radon gets in or who breathes it:

• Air sealing and window replacement. A tighter house exchanges less air with the outdoors, so radon that enters stays longer. It can raise indoor levels.
• New or larger exhaust. A high-capacity range hood, a new bath fan or a dryer in a tight house deepens the negative pressure that draws soil gas in.
• Basement and crawlspace work. Finishing a basement moves people into the level with the highest radon. Encapsulating a crawlspace or adding a sump changes the openings to the soil, and an open sump pit is a direct radon path unless it has a sealed lid.
• HVAC changes. Return ducts or an air handler in a basement, leaky return plenums near the slab, and combustion or makeup air changes all affect the pressure balance.
• Never tested. Any house where the owner cannot remember a test, or tested before a major renovation.
• After mitigation. EPA recommends a follow-up test after a system is installed and retesting on a regular interval, which its guidance puts at every two years, and after significant renovation.

The honest framing is "this work can change radon levels, and the only way to know is a test," never "this work will raise your radon." A sealed house with an open sump and a 400 CFM range hood is a likely candidate; a house that tested at 0.8 pCi/L last year and is getting two new windows is not a priority.

Worked example: a weatherization job with a basement bedroom

A crew is replacing windows and air sealing the attic and rim joists of a two-story house with a basement, in a Zone 2 county. The owners mention they are moving their teenager into a bedroom they are finishing in the basement. The house has never been tested.

Before the work. The crew lead tells the owners that tightening a house can raise radon, that the basement bedroom puts someone in the level where radon is highest, and that EPA recommends testing every home regardless of zone. He suggests a short-term test in the basement before work starts, from a qualified lab or a credentialed tester. The owners test and get 3.1 pCi/L.

Reading the first result. 3.1 is below the 4 pCi/L action level and inside the 2 to 4 range where EPA says to consider fixing. Nothing requires action. The lead suggests retesting after the work is done, because the house is about to change.

After the work. A second short-term test, again in the basement under closed-house conditions, reads 4.6 pCi/L. That is 1.5 pCi/L higher than before, a rise of 1.5 divided by 3.1, about 48 percent.

What the rise does and does not prove. It is tempting to say the air sealing raised radon by half. Two short-term tests weeks apart, in different weather, can differ by that much with no change to the house at all. What the result does do is cross EPA's follow-up trigger: a short-term reading of 4 or higher calls for a follow-up test. The owners choose a long-term test over the next 90 days, because the bedroom is not yet occupied and they want the better average.

The decision. The long-term test averages 4.2 pCi/L. That is at or above 4, so EPA's guidance is to fix the home. The owners hire a mitigator, credentialed where their state requires it, and after the system is installed a post-mitigation test confirms the level has come down. The crew's job was never to diagnose; it was to say the right thing before the work and to suggest the second test after it.

What would change the call. In a Zone 1 county the advice is identical; only the expectation of a high result is stronger. In a Zone 3 county the advice is still identical, because the zone does not remove the recommendation. If the first test had read 0.6 pCi/L and the work were a single window, retesting after the job would be reasonable but not something worth pressing. And if the house were in a state that licenses radon measurement, the crew lead's suggestion has to point to a licensed tester for anything beyond the owner's own kit.

How to verify you handled it right

The job record should say the radon conversation happened and what was recommended: "Owner advised to test basement before and after air sealing; basement bedroom planned; Zone 2; never tested." It should not record a result as if the crew measured it unless the crew is credentialed to. When the owner later asks why the shop raised it, the note shows the advice was given at the moment the work created a reason to give it.

References

• EPA, Map of Radon Zones and its accompanying guidance that all homes be tested regardless of zone
• EPA, A Citizen's Guide to Radon: action level, test types, follow-up testing and retesting
• Your state radon program, for licensing and disclosure rules (for example Illinois, New Jersey, Pennsylvania)
• See related: Radon Mitigation Basics