What the EPA radon zone tells you (and what it doesn't)

The EPA gives your county a radon zone from 1 to 3. It's a useful screening flag — and one of the most misread numbers in property research, because it describes a county, not your house.

Radon is a colorless, odorless, radioactive gas. It comes from the natural decay of uranium in soil and rock, seeps up through the ground, and can accumulate inside buildings. You can't see it, smell it, or taste it, and at the levels found in homes it produces no immediate symptoms at all. That combination — everywhere, invisible, and slow-acting — is exactly what makes it easy to ignore.

It shouldn't be ignored. The EPA and the U.S. Surgeon General identify radon as the second leading cause of lung cancer overall, and the leading cause among people who have never smoked. The EPA estimates it contributes to roughly 21,000 lung-cancer deaths in the United States every year. The risk is cumulative and comes from breathing elevated levels indoors over many years.

The EPA Map of Radon Zones

To help target testing and building codes, the EPA published the Map of Radon Zones, which sorts every U.S. county into one of three categories based on the predicted average indoor radon level for that county:

Those pCi/L units are picocuries per liter of air, the standard measure of radon concentration. Keep 4 pCi/L in mind — it reappears below as the number that actually matters for a specific home.

The number that matters: 4 pCi/L

The EPA's action level is 4 pCi/L. If a test on a specific home comes back at or above that, the EPA recommends taking steps to reduce it. The agency is also explicit that there is no known "safe" level — the risk simply declines as the concentration drops — and it suggests considering action between 2 and 4 pCi/L as well.

Notice that this threshold applies to a measured value in an individual building. The county zone is a planning tool that tells you how likely you are to be above it. The 4 pCi/L action level is what you compare an actual test result against. Conflating the two is the core mistake.

How radon gets into a house

Radon is drawn indoors by simple air pressure: a heated house is often at slightly lower pressure than the soil beneath it, so soil gas is pulled in through any opening in contact with the ground. Common entry points are:

• Cracks in a concrete slab or foundation walls
• Gaps around service pipes and utility penetrations
• Sump pits, floor drains and crawlspaces
• Construction joints and porous block walls

Because entry depends on the building and its soil contact, radon is a property of the individual structure — which is why a prediction about the county can only ever be a starting point.

Testing: cheap, and the only way to actually know

Here's the good news that follows from all of the above: since the only thing that settles the question is a measurement, and a measurement is inexpensive, the decision is easy.

• DIY test kits run about $15 – 25, available at hardware stores and online. A short-term kit sits in the lowest lived-in level of the home for a few days, then goes to a lab. Longer-term kits (90+ days) give a more representative annual picture.
• Professional testing costs more but is often used in real-estate transactions, where a neutral third party and a tamper-resistant continuous monitor matter.
• Test in the lowest level you'll regularly use, and follow closed-house conditions per the kit instructions for reliable results.

If a test comes back high: mitigation works

Elevated radon is a solved engineering problem. The standard fix is sub-slab depressurization — a fan and a vent pipe that draw soil gas from beneath the foundation and route it above the roofline before it can enter the living space. A typical residential system runs about $800 – 1,500 installed, and a well-designed one commonly cuts indoor levels well below the action level.

What LandBrief shows you

A LandBrief report pulls the EPA radon zone (1, 2 or 3) for your county from the EPA Map of Radon Zones and shows it as a screening indicator. When a property falls in Zone 1 — the highest predicted potential — the report raises it as a prominent flag and points you to the relevant state radon program guidance for your building type.

Be clear about what that is and isn't. It is the EPA's county-level prediction, reported faithfully. It is not a measurement of the specific house, it can't be, and it never replaces an actual radon test. It exists to tell you whether testing deserves a spot on your due-diligence checklist — and for radon, the honest answer is that it always does.

What to do before you buy

• Ask the seller for prior radon test results — and for records of any mitigation system, including whether the fan is still running.
• Make your offer contingent on a radon test, the same way you would a general inspection. It's a small line item against a large purchase.
• Re-test after any foundation work — a new crack, a sump installation, or a basement finish can change a home's radon profile.
• Don't let a Zone 2 or Zone 3 result talk you out of testing. A lower-risk county lowers the odds; it doesn't clear the individual house.
• If levels are high, price the mitigation, not the panic. An $800 – 1,500 system is often a negotiation point, not a deal-breaker.

Sources: EPA Map of Radon Zones (EPA-402-R-93-071) · EPA "A Citizen's Guide to Radon" (epa.gov/radon) · U.S. Surgeon General radon health advisory.