EPA Radon Zone 1 counties have a predicted average indoor radon screening level greater than 4 pCi/L, Zone 2 counties fall between 2 and 4 pCi/L, and Zone 3 counties are predicted below 2 pCi/L — those are the verbatim cutoffs the EPA published in EPA-402-R-93-071, the 1993 Map of Radon Zones. The zone is a county-level planning tool for code officials and lenders; it is not a measurement of your house, and the EPA explicitly recommends every US home be tested regardless of zone.
What the EPA Map of Radon Zones actually is
The document people call “the EPA radon map” is formally titled EPA Map of Radon Zones, document number EPA-402-R-93-071, published by the US Environmental Protection Agency in 1993. It assigns every county in the United States, plus Indian lands and US territories, to one of three zones based on the predicted average indoor radon screening level for that county.
EPA produced the map in cooperation with the US Geological Survey and state radon offices. The goal was not to tell individual homeowners what their basement reads — the EPA has always been emphatic that only an in-home test can do that — but to give building-code officials, public-health planners, real-estate disclosure regimes, and federal agencies (HUD, USDA, VA) a defensible map of where radon-resistant new construction techniques and disclosure requirements should be prioritized.
Three input layers fed the classification:
- Indoor radon measurements. The 1991 EPA National Residential Radon Survey (EPA 402-R-92-011) gave a statistically representative dataset of ~5,700 short-term screening measurements across US homes.
- Geology. USGS aeroradiometric uranium data and surficial geology maps identified rock and soil units (granites, black shales, glacial till over uranium-bearing bedrock, phosphatic limestones) known to generate radon at depth.
- Soil-gas radon and permeability. Field measurements of soil-gas radium and soil permeability indicated how much radon could realistically migrate from soil into building substructures.
Each county was then evaluated against those layers and assigned a predicted indoor screening average. Where the layers conflicted, EPA documented the rationale in the state-by-state technical chapters that accompany the map.
The three zone definitions, verbatim
EPA defines the three zones in EPA-402-R-93-071 using the predicted average indoor radon screening level, expressed in picocuries per litre (pCi/L) of air:
| Zone | Predicted average indoor screening | EPA-implied planning posture |
|---|---|---|
| Zone 1 (highest) | Greater than 4 pCi/L | Radon-resistant new construction strongly recommended; prioritise public-health outreach and disclosure. |
| Zone 2 (moderate) | 2 to 4 pCi/L | Test every home; consider radon-resistant new construction in higher-end-of-range areas. |
| Zone 3 (lowest) | Less than 2 pCi/L | Test every home anyway; individual houses can and do exceed 4 pCi/L even in Zone 3. |
The shorthand “low / medium / high” loses an important nuance: the cutoffs are tied to 4 pCi/L, which is the same number used as EPA’s recommended action level for indoor air. That alignment is deliberate — Zone 1 counties are the counties where the average home is, on a population basis, expected to be at or above the action level.
The 4.0 pCi/L action level — and why WHO uses 2.7
The EPA Citizen’s Guide to Radon (EPA-402-K-12-002) recommends that homeowners take corrective action when a long-term radon test indicates an annual average at or above 4.0 pCi/L (148 Bq/m³). The World Health Organization’s 2009 Handbook on Indoor Radon proposes a lower reference level of 100 Bq/m³ (approximately 2.7 pCi/L) and notes that countries should adopt no higher than 300 Bq/m³ (approximately 8 pCi/L) where the lower level is not practical. The EPA also encourages homeowners to “consider taking action” between 2 and 4 pCi/L. The zone tiers in the map line up with both the EPA action level and the EPA “consider action” band.
What “predicted average indoor screening level” technically means
Three pieces of jargon do a lot of work here. Predicted means modelled, not measured: most counties did not have enough indoor measurements in 1991–1993 to compute a true county average, so EPA used the geology and soil-gas inputs to predict what the indoor average should look like. Average means the central tendency across all homes in the county — not the maximum, not the worst basement on the worst street. Indoor screening level refers to a short-term (typically 2–7 day), lowest-livable-area measurement following the EPA screening protocol, which generally over-reads compared to a year-round occupied-area annual average.
The practical implication is enormous and gets misunderstood constantly: the EPA zone is a statement about a county’s population of homes, not about your home. A Zone 3 county can and does contain houses that test above 20 pCi/L — usually because of geology beneath that specific lot, a sub-slab pressure differential the HVAC creates, or a building feature that channels soil gas into the living space. Equally, a Zone 1 county contains plenty of houses that read well below 2 pCi/L. The only way to know what your house is doing is to test it. See our DIY radon testing guide for the EPA-recommended protocol.
Which states fall mostly into which zone
The 1993 map is published at county granularity, so any state contains a mix of zones — but there are clear regional patterns. We will name those general patterns; we will not invent county-level pCi/L numbers, because the EPA county classifications are categorical (Zone 1/2/3) rather than numerical, and any specific county pCi/L figure should come from a state survey, not from us.
| General pattern | States typified by it | Geological driver |
|---|---|---|
| Predominantly Zone 1 | Iowa, North Dakota, eastern Pennsylvania, parts of Colorado, Minnesota, Ohio | Glacial till over uranium-bearing bedrock; Reading Prong granites; phosphatic shales. |
| Mixed Zone 1 / Zone 2 | Kentucky, Tennessee, Virginia, West Virginia, Wisconsin, Michigan, Illinois, Indiana, New York, Wyoming, Montana, Idaho, Washington | Appalachian black shales; Karst limestones; uranium-bearing granitic terranes. |
| Predominantly Zone 2 / Zone 3 | Most of California, Oregon, Texas, the Carolinas, Florida, the Gulf Coast | Younger sedimentary cover, lower-uranium bedrock, deeper water tables. |
| Predominantly Zone 3 | Louisiana, much of Mississippi, much of Florida, coastal South Carolina, southern Texas, Hawaii | Thick coastal-plain sediments, low-uranium parent rock. |
For a county-by-county look at where the highest indoor screening averages have actually been measured, see our highest-radon US counties guide, which pulls from state survey datasets rather than the 1993 EPA categorical map.
The 1993 vintage problem — and why state surveys override the map
EPA-402-R-93-071 is now more than 30 years old. EPA has never formally re-issued it. Three things have changed underneath it since 1993:
- State radon programs have built far larger indoor datasets. Colorado’s CDPHE, Iowa’s state radon program, Pennsylvania DEP, Minnesota Department of Health, and a number of others now publish county-level indoor radon averages drawn from tens or hundreds of thousands of in-home tests — orders of magnitude more than the 1991 NRRS could provide.
- Building-stock has turned over. Tighter building envelopes, sealed crawlspaces, conditioned basements, and HRV/ERV systems all change how soil gas behaves once it enters a home.
- Reading Prong and Appalachian basin updates. Subsequent USGS mapping has refined what we know about radon-source geology, particularly along the I-95 corridor in the mid-Atlantic.
EPA itself acknowledges that the 1993 map should be supplemented by current state-program data when available. On any property transaction, code review, or mitigation decision, the order of authority is: (1) an in-home test of your house, (2) the current state radon program’s county dataset, (3) the 1993 EPA zone. We carry all three layers in our methodology and identify where they disagree.
What the zone does not tell you
The single most common misuse of the EPA zone is treating it as a prediction for an individual address. It is not. Here is what the zone does not tell you:
- House-to-house variance. Neighbouring houses on the same street routinely differ by a factor of 5 or more, driven by foundation type, slab cracks, sump configuration, and HVAC pressure balance.
- Basement vs. slab-on-grade vs. crawlspace. A finished basement bedroom and an upstairs office in the same building will read very differently; EPA testing protocol specifies the lowest livable level for a reason.
- Seasonal variation. Indoor radon is typically highest in winter (stack effect plus closed windows) and lowest in summer; a single short-term test can mis-read by a factor of 2 in either direction. See short-term vs. long-term testing for the EPA protocol.
- Post-renovation behaviour. Adding a basement bathroom, sealing a crawlspace, or installing a high-CFM range hood can move a previously safe house above 4 pCi/L. See retesting after renovation.
- Mitigation feasibility. The zone says nothing about whether a sub-slab depressurization system will work on your foundation type. That is a contractor question; see sub-slab depressurization explained.
- Lung-cancer risk for you specifically. EPA risk estimates are population-level; smoking status changes them by roughly an order of magnitude. See radon and lung cancer risk numbers.
What to actually do based on your zone
The EPA recommendation, repeated in EPA-402-K-12-002, is unchanged across all three zones: test every home. The zone changes only the urgency and the cadence, not the question.
If you are in Zone 1
Assume your house is above 4 pCi/L until a test proves otherwise. Run a long-term (90+ day) alpha-track or electret test, ideally over a heating season, in the lowest livable level. If the annual average comes back at or above 4 pCi/L, the EPA recommends mitigation; in Zone 1 the prior probability is high enough that many homeowners just budget for a sub-slab depressurization system on the assumption it will be needed. New construction in Zone 1 counties should follow the AARST CC-1000 radon-resistant new construction standard.
If you are in Zone 2
The base rate is roughly even, so testing is the only sane way to know. EPA recommends a short-term screening test (2–7 days, closed-house conditions); if it reads above 4 pCi/L, follow up with either a second short-term test or a long-term test before mitigating. Re-test every 2 years and after any major renovation, HVAC change, or foundation modification.
If you are in Zone 3
Test once with a short-term kit so you have a baseline. If the result is below 2 pCi/L, retest every 5 years or after a renovation. If the result is between 2 and 4 pCi/L, the EPA suggests considering mitigation. If the result is above 4 pCi/L — which absolutely does happen in Zone 3 — treat it like a Zone 1 result and confirm with a second test.
How EPA radon zones interact with real-estate disclosure
Many states require radon disclosure on property transfers, and some — Illinois, Florida, Maine, New Jersey, Pennsylvania — have specific disclosure statutes that reference the EPA zone or state-equivalent data. A few jurisdictions require radon-resistant new construction in Zone 1 counties via local building code, sometimes referencing the IRC Appendix F radon-control provisions. The EPA zone is not federal law in itself; its legal weight comes from being adopted into state statute or county code. Our sources page lists which states cite the EPA map in current code.
One sentence answer
If you only remember one thing: Zone 1 = predicted county-average indoor screening above 4 pCi/L, Zone 2 = 2 to 4 pCi/L, Zone 3 = below 2 pCi/L — and the EPA says test every home regardless. Everything else is detail.
Sources
- EPA Map of Radon Zones — EPA-402-R-93-071 — The original 1993 federal document defining the three-tier classification and the predicted indoor screening cutoffs used throughout this guide.
- A Citizen’s Guide to Radon — EPA-402-K-12-002 — Source for the 4.0 pCi/L action level and the “consider action” band between 2 and 4 pCi/L.
- EPA Assessment of Risks from Radon in Homes — EPA-402-R-03-003 — Source for EPA’s lung-cancer risk model used to justify the 4 pCi/L cutoff that aligns with the Zone 1 threshold.
- National Residential Radon Survey — EPA 402-R-92-011 — The 1991 dataset of indoor screening measurements that fed the 1993 county classifications.
- WHO Handbook on Indoor Radon — World Health Organization, 2009 — Source for the 100 Bq/m³ (2.7 pCi/L) reference level and the contrast with the EPA action level.
- Colorado Radon Program — Colorado Department of Public Health and Environment — Example of a state-level dataset that supplements the 1993 EPA county map with current in-home measurements.
Related guides
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