EPA Radon Zones: What Zone 1, 2, and 3 Really Mean

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The EPA Map of Radon Zones sorts every county in the country into one of three zones based on its potential for elevated indoor radon. It is one of the most searched radon resources online, and also one of the most misread. The EPA radon zones describe county-level potential, not the level inside any particular house, and the EPA itself says the map should not be used to decide whether a home needs testing. This guide explains what each zone means, how the map was built, what it is actually for, where Colorado and the Front Range counties fall, and how to move from a zone label to a real measurement. It summarizes EPA and CDC guidance current as of 2026; consult a physician about health concerns and a certified radon professional for testing and mitigation decisions.

Part of our Radon Levels guide — start there for the full picture →

What the EPA Radon Zones Mean

The EPA’s map legend assigns each county a color and a zone number tied to the predicted average indoor radon level for homes in that county. The EPA’s map document describes the three zones this way:

The unit, picocuries per liter (pCi/L), is the same one used on every residential radon test report. The EPA recommends fixing a home at 4 pCi/L or higher and suggests considering a fix between 2 and 4 pCi/L. Those action points are the same in every zone. A Zone 3 home that tests at 6 pCi/L needs mitigation just as much as a Zone 1 home that tests at 6 pCi/L.

For a fuller explanation of what these numbers mean on a test report, see our radon level chart and the guide to acceptable radon levels.

How the Map Was Built and What It Was Designed to Do

According to the EPA, the Map of Radon Zones was developed in 1993 to identify areas of the United States with the potential for elevated indoor radon levels. The agency lists five kinds of data that went into it:

• Indoor radon measurements
• Geology
• Aerial radioactivity
• Soil parameters
• Foundation types

Each county received a single zone based on how those factors combined. That is a reasonable way to rank broad regions, but it means the map paints large and geologically varied counties with one color.

The purpose statement on the EPA’s page is direct: the map exists to help national, state, and local organizations target their resources and implement radon-resistant building codes. It is a planning tool for governments, public health programs, and code officials. It was not designed as a consumer screening tool, and the EPA recommends that it be supplemented with any available local data to better understand radon potential for a specific area.

How to look up a county’s zone

The EPA’s zones page offers two formats. The map PDF shows the country colored red, orange, and yellow by county, which is useful for seeing regional patterns at a glance. A text version, published as a spreadsheet, lists each county with its state and zone number, which is easier for checking a specific county. The steps are simple:

1. Find the county the property sits in. Addresses near county lines, common around Denver, Aurora, and Broomfield, are worth confirming with the county assessor.
2. Look up that county in the text version or on the map.
3. Note the zone, then move straight to testing. The zone sets expectations; the test sets the plan.

The single most important sentence on the page

The EPA states that the Map of Radon Zones should not be used to determine if individual homes need to be tested, and that homes should be tested no matter where they are. The EPA’s map document adds that elevated levels of radon have been found in all three zones and that all homes should be tested.

That instruction is easy to miss when a buyer sees “Zone 3” and concludes that radon is someone else’s problem. It is the core reason this map should frame a conversation, not end one. The companion guides on the EPA map of radon zones and the EPA radon map walk through reading the map itself county by county.

Why a County Zone Cannot Predict One House

Radon is a radioactive gas that comes from the natural breakdown of uranium in soil and rock. The CDC notes that all outdoor and indoor air contains some radon, that it can build up inside homes and buildings, and that it can enter through small cracks or holes. How much gets in depends on conditions at a specific address, not the county average.

Several factors vary from one lot to the next:

• Soil and rock directly beneath the footprint. Uranium content and soil permeability can change over short distances, especially in areas with varied geology like the foothills west of Denver.
• Foundation type. Basements, crawl spaces, and slab-on-grade homes interact with soil gas differently. Many Front Range homes have full or garden-level basements with a large soil-contact area.
• Entry routes. Slab cracks, sump pits, floor drains, utility penetrations, and the joint where the floor meets the wall all give soil gas a path in.
• Pressure and airflow. Warm indoor air rising and leaking out the top of the house tends to draw soil gas in at the bottom, which is one reason winter readings can differ from summer ones.
• How the home is used and sealed. Renovations, new windows, finished basements, and HVAC changes can all shift indoor levels.

Because those factors are local, two neighboring houses can produce very different results. A zone label tells a buyer something about the region’s geology. Only a test tells them about the air in the home they are buying. Our guide to radon levels by ZIP code explains why even finer-grained maps have the same limitation.

Where Colorado and the Front Range Fall

Colorado is one of the states where Zone 1 dominates. In the county-level text version of the map that the EPA links from its zones page, the large majority of Colorado counties are listed as Zone 1, a smaller group of mostly mountain and San Luis Valley counties are listed as Zone 2, and no Colorado county is listed as Zone 3.

The Front Range metro counties are all listed as Zone 1 in that file, including:

• Denver, Adams, Arapahoe, Jefferson, and Douglas counties
• Boulder and Broomfield counties
• El Paso, Elbert, and Teller counties
• Larimer, Weld, and Pueblo counties

The counties listed as Zone 2 include Alamosa, Archuleta, Conejos, Costilla, Eagle, Hinsdale, Mineral, Rio Grande, Routt, Saguache, and San Juan. Even there, the EPA guidance is the same: test.

For a Front Range buyer or homeowner, the practical takeaway is simple. Zone 1 status confirms that radon is a regional concern worth planning for. It does not mean every house is high, and it does not mean any particular house is safe. It means testing should be routine, both for buyers during the inspection window and for long-time owners who have never checked. Our radon testing pillar covers Colorado testing in more depth, and the guide to normal radon levels explains what readings are typical indoors and outdoors.

Why Radon Matters Regardless of Zone

The health concern behind the map is lung cancer. The EPA states that exposure to radon causes lung cancer in non-smokers and smokers alike. The CDC describes radon as the second leading cause of lung cancer deaths in the United States after cigarette smoke, and notes that the EPA and the Surgeon General’s office estimate radon is responsible for more than 21,000 lung cancer deaths each year in the United States.

The risk builds with long-term exposure, which is why a home’s level matters more than a county’s average. A household spends years breathing the air in its own basement and bedrooms. That is also why the action level, not the zone, drives decisions: readings at or above 4 pCi/L call for mitigation, and readings between 2 and 4 pCi/L are worth considering for a fix. For readings that come back well above the action level, see our guide to high radon levels.

Using Zone Information for New Construction

New construction is where the map was meant to have the most direct effect. The EPA’s stated purpose includes helping jurisdictions implement radon-resistant building codes, and builders in high-potential areas often use the zone designation as a reason to include radon-resistant features from the start.

The EPA’s radon-resistant construction page lists five basic features builders should include:

1. Gravel. A 4-inch layer of clean, coarse gravel below the slab so soil gas can move freely underneath the house. The EPA notes alternatives such as perforated pipe or a collection mat are allowed in some regions.
2. Plastic sheeting or vapor retarder. Heavy-duty 6-mil polyethylene or a vapor retarder on top of the gravel.
3. A vent pipe. A 3-inch or 4-inch solid PVC Schedule 40 pipe running from the gravel layer through the conditioned space and roof, labeled as a radon system.
4. Sealing and caulking. Openings, cracks, and crevices in the foundation floor and walls sealed with polyurethane caulk.
5. A junction box. An electrical outlet in the attic so a vent fan can be added if testing later shows it is needed.

The EPA adds that new home buyers can ask a builder about these features and request them if they are not included, and that fixing an elevated level after move-in can cost the owner much more. A passive system built in during construction is not a guarantee of a low reading, so new homes still need a test after occupancy. If the result is elevated, the passive pipe can usually be converted to an active system by adding a fan. Our guide to the radon mitigation system explains how active systems work.

Questions to ask a builder in a Zone 1 county

• Is a radon vent pipe and sub-slab gas-permeable layer included, and where does the pipe exit the roof?
• Is there an electrical junction box in the attic near the pipe for a future fan?
• Has the slab and foundation been sealed at joints and penetrations?
• Will the builder test after completion, and what happens if the result is elevated?

From Zone Label to Real Number: Testing and Next Steps

The EPA describes radon testing as easy and inexpensive. There are two broad approaches.

Short-term tests

Short-term tests run for a few days and give a quick snapshot. They are standard during real estate transactions because of the inspection deadline. Test kits placed in the lowest livable level of the home, with windows and doors kept closed, are the typical setup. Our guide to choosing a radon test kit covers placement and closed-house conditions, and kits are available in our radon test kit shop.

Long-term tests and continuous monitors

Long-term tests run for months and better reflect the year-round average, smoothing out seasonal swings. Continuous electronic monitors give hour-by-hour readings and are often used by professionals during transactions and after mitigation to confirm results.

Reading the result

• Below 2 pCi/L: generally low; retest after major renovations or HVAC changes.
• 2 to 4 pCi/L: the EPA suggests considering a fix.
• 4 pCi/L or higher: the EPA recommends fixing the home.

If mitigation is needed, the most common approach in homes with basements or slabs is sub-slab depressurization with a fan and vent pipe. Costs vary by foundation type, layout, and contractor; our guide to radon mitigation system cost breaks down the main price drivers. After a system is installed, a follow-up test confirms whether it worked.

Every step in that process is the same regardless of zone. The only thing the zone changes is how surprised a homeowner should be by a high reading.

Radon Zones During a Home Purchase

Real estate transactions are where zone information gets used, and misused, most often. A listing agent might mention that a county is Zone 1 as a reason the seller already tested. A buyer might hear “Zone 2” and decide to waive the radon test to save a few days. Neither shortcut holds up.

For buyers

• Schedule the test inside the inspection window. Short-term tests need a few days of closed-house conditions, and the lab or monitor result has to arrive before the objection deadline. Booking the radon test alongside the general home inspection keeps the timeline realistic.
• Ask for prior results and mitigation records. A seller’s old test can be useful history, but it may predate a basement finish, new windows, or a furnace replacement. A current test reflects the home as it is now.
• Check any existing system. If the home already has a mitigation system, look for a working fan, a manometer showing suction, a labeled pipe, and a post-mitigation test result.
• Plan for the result. If a reading comes back at or above 4 pCi/L, buyers commonly negotiate a seller-paid system, a price credit, or an escrow holdback, depending on the contract and local practice.

For sellers

Sellers in Zone 1 counties can expect buyers to test. Testing before listing, and mitigating if needed, removes a common late-stage negotiation item. A documented system with a post-mitigation result tends to reassure buyers more than a zone label or an undated test does.

For long-time owners

Owners who bought before radon testing was routine, or who have remodeled since their last test, are often the people most surprised by a high reading. The EPA’s guidance to test no matter where a home is located applies to them too, and retesting after significant changes to the foundation, basement, or HVAC system is a reasonable habit.

Indoor Zones Versus Outdoor Radon

The zone map predicts indoor averages, not outdoor air. Radon is present in outdoor air everywhere, as the CDC notes, but it disperses quickly outside. The concern is what happens when soil gas is drawn into an enclosed building and accumulates. That is why two homes in the same zone, built on the same type of soil, can read very differently: the building itself shapes how much gas enters and how much is trapped.

It also explains why opening windows can temporarily lower an indoor reading and why closed-house conditions are required during a short-term test. A test taken with windows open would understate the level a household normally breathes. Our guide to outdoor radon levels explains how outdoor background compares with typical indoor readings and why the outdoor number is never a reason to skip an indoor test.

Common Misreadings of the Radon Zone Map

A handful of mistakes come up again and again.

• “Our county is Zone 3, so we can skip testing.” The EPA states that elevated levels have been found in all three zones and that all homes should be tested.
• “Zone 1 means the house definitely has radon.” Zone 1 means elevated averages are predicted for the county. Many individual Zone 1 homes test low.
• “The map shows our neighborhood.” The map works at the county level. It does not show neighborhoods, streets, or lots.
• “The map is current.” The zones were developed in 1993 from the data available then. The EPA recommends supplementing the map with local data.
• “The neighbor’s test applies to us.” Neighboring homes can have very different foundations, soils, and entry points. One home’s result does not transfer.
• “The zone sets a different action level.” The 4 pCi/L fix recommendation and the 2 to 4 pCi/L consideration range apply everywhere.

Used correctly, the map helps public health programs focus outreach and helps code officials decide where radon-resistant construction should be standard. Used as a personal risk calculator, it can lead to skipped tests and missed exposures. For more on how radon levels are organized and reported, visit our radon levels hub.

References

• The EPA Map of Radon Zones — U.S. Environmental Protection Agency
• EPA Map of Radon Zones (map and zone legend, PDF) — U.S. Environmental Protection Agency
• Health Risk of Radon — U.S. Environmental Protection Agency
• Radon-Resistant Construction Basics and Techniques — U.S. Environmental Protection Agency
• About Radon — Centers for Disease Control and Prevention

Buying in a Zone 1 county and want the radon question answered before the inspection deadline passes? Send us a quick message and we can connect you with a certified radon tester on the Front Range.

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We may earn commission from links on this page. Lead-form submissions are forwarded to local inspector partners. How we research and review.