Radon Zone Map and Radon Construction Rules by State
Applies to new residential construction and to existing-home radon mitigation. Whether a passive or active radon system is legally required for your project depends on which state and municipality you're in — this guide separates the EPA's national zone map (informational) from the handful of places that turned part of it into a legal requirement.
Radon is a naturally occurring radioactive gas that seeps up from soil and rock, and the EPA has mapped every county in the country into one of three zones by how much of it is likely to show up indoors. But the map itself is not a law — it's a planning tool. The International Residential Code has its own radon appendix, and like most IRC appendices, it does nothing on its own until a state or city specifically adopts it.
Almost every state leaves that decision to individual cities and counties, which is why national estimates of "which states require radon-resistant construction" vary so much online. A handful of states didn't leave it to a patchwork of local ordinances — they wrote a requirement directly into state law or the state building code, and each one did it differently: Illinois requires it everywhere in the state regardless of the EPA map; Minnesota requires it statewide for all new homes; Massachusetts requires it only in its highest-radon counties but explicitly bans requiring a test; and Iowa passed a law to require it statewide that, as of this writing, isn't enforceable yet because the agency responsible hasn't adopted the implementing rule.
This guide separates what's genuinely national (the EPA zone system and the IRC's optional construction methods) from what only applies where a state chose to make it law, with the exact statute or code section for each.
Radon mitigation is a slab and foundation issue — whether your area requires (or already has) a passive system changes what a basement finishing project needs to account for. Free, no signup.
The EPA Map of Radon Zones: The National Baseline
The EPA, working with the U.S. Geological Survey, assigned every one of the 3,141 counties in the United States to one of three zones based on indoor radon measurements, geology, aerial radioactivity, soil parameters and foundation types. The map dates to 1993 and has not been replaced — EPA still links it directly, though it cautions that it "should not be used to determine if individual homes need to be tested" and should be supplemented with local data. Zone 1 counties are the code-relevant ones: they're the trigger IRC Appendix AF uses to decide where radon-resistant construction is worth requiring.
| Zone | Predicted average indoor radon level | What it means |
|---|---|---|
| Zone 1 (highest potential) | Greater than 4 pCi/L | The trigger zone referenced by IRC Appendix AF; most state and local RRNC mandates key off this zone |
| Zone 2 (moderate potential) | 2 to 4 pCi/L | No national construction trigger, but testing is still worthwhile |
| Zone 3 (low potential) | Less than 2 pCi/L | Lowest priority under the EPA map, but individual homes can still test high |
IRC Appendix AF: Optional Unless Your Jurisdiction Adopts It
Appendix AF (renumbered Appendix BE in the 2024 IRC) is the model code's radon chapter, and like every IRC appendix, "the provisions contained in this appendix are not mandatory unless specifically referenced in the adopting ordinance." The appendix itself contains a code-published table listing every Zone 1 county in the country by state — Iowa is listed as "All Counties," for example, while Massachusetts has only three (Essex, Middlesex, Worcester). A jurisdiction decides whether to adopt the appendix using that table or its own local data.
| Requirement | What it says | Code section |
|---|---|---|
| Gas-permeable layer under the slab | A layer such as 4 inches of clean aggregate beneath the slab or floor | 2021 IRC AF103.2 |
| Soil-gas-retarder membrane | 6-mil polyethylene (or 3-mil cross-laminated) sheeting, lapped at least 12 inches | 2021 IRC AF103.3 |
| Sealed entry routes | Floor openings, slab joints, sumps, foundation walls and duct penetrations sealed against soil gas | 2021 IRC AF103.4 |
| Vent pipe | Minimum 3-inch gas-tight pipe from the sub-slab layer through the roof, terminating at least 12 inches above it | 2021 IRC AF103.6 |
| Future-fan provisions | Vent pipe and electrical accessible so a fan can be added later to convert to an active system | 2021 IRC AF103.8 |
The Testing and Fan-Installation Trigger
Where Appendix AF is adopted with its testing section, the code lays out exactly what happens after a radon test: a result under 4 pCi/L just gets reported to the code official, no further action required. A result of 4 pCi/L or greater requires installing the fan the passive system was already roughed in for, per the code's own fan-installation sections. If it's still 4 pCi/L or higher after that, the system has to be modified and retested. Tests run at least 48 hours with windows and exterior doors closed.
| Test result | Required action | Code section |
|---|---|---|
| Below 4 pCi/L | Report the result to the code official; no further work required | 2021 IRC AF104 |
| 4 pCi/L or greater | Install the fan on the radon vent pipe | 2021 IRC AF103.9, AF103.12 |
| Still 4 pCi/L or greater after the fan | Modify the system and retest | 2021 IRC AF104 |
States That Require Radon-Resistant Construction by Law
Outside of local adoption, we found four states with a genuine statewide (or state-defined-zone) legal requirement, each built differently. Illinois goes furthest: it doesn't reference the EPA zone map for its mandate at all — the requirement applies everywhere in the state regardless of radon potential.
| State | Statewide requirement? | Trigger | Law or code |
|---|---|---|---|
| Illinois | Yes — all areas | None; applies everywhere in the state regardless of the EPA zone | Radon Resistant Construction Act, 420 ILCS 52/20 |
| Minnesota | Yes — all new homes | None; applies to all new residential structures statewide | Minn. R. 1303.2400–1303.2403 |
| Massachusetts | Yes — in its Zone 1 counties only | High radon potential counties named in the code's own table (Essex, Middlesex, Worcester) | 780 CMR, Appendix AF101.1 |
| New Jersey | Zoned | Only in designated "Tier 1" municipalities; optional elsewhere | N.J.A.C. 5:23-10 |
| Iowa | Passed, not yet enforceable | Was set to apply statewide (Iowa is entirely EPA Zone 1) starting July 1, 2026 | HF 2297 / Iowa Code § 103A.8E — see caveat below |
Illinois: Statewide, With No Zone Exception At All
Illinois's Radon Resistant Construction Act doesn't reference the EPA zone map for its actual mandate — it just requires passive radon-resistant construction in every new home in the state, full stop: "All new residential construction in this State shall include passive radon resistant construction." The Act also restricts who can do the work: installing radon resistant construction can be done by a general residential contractor, but installing an active mitigation system, a radon vent fan, or upgrading a passive pipe to active requires a licensed radon contractor.
Minnesota: All New Residential Structures, Statewide
Minnesota's rule is similarly broad: "The purpose of parts 1303.2400 to 1303.2402 is to establish minimum requirements for passive radon control systems that apply to all new residential structures." Upgrading to an active system later still has to meet the same passive-system requirements underneath it, per Rule 1303.2403.
Massachusetts: Mandatory in Its Highest-Radon Counties, But No Test Required
Massachusetts takes a narrower geographic approach than Illinois or Minnesota, but a stricter one within its scope. It requires passive radon construction in its Zone 1 counties "regardless of the radon levels at the site" — meaning an individual lot can't test its way out of the requirement. Unusually, the state also goes out of its way to say testing is NOT required at any point: not before construction, not on the completed system, and not after the home is finished, and "such testing shall not be a condition of issuing a certificate of occupancy." Active construction techniques (rather than passive) are allowed where approved, and builders may alternatively use the ANSI/AARST standard for radon-resistant new construction.
New Jersey: Only in Designated Tier 1 Municipalities
New Jersey's Radon Hazard Subcode requires radon-resistant construction for Use Groups R and E, but only within municipalities the state has specifically designated "Tier 1" in its own appendix. Outside those municipalities, radon-resistant construction is optional under state code, though a local jurisdiction could still choose to require it independently.
Iowa: A Statewide Law That Isn't Enforceable Yet
Iowa passed a law in 2026 (HF 2297) directing the state's Building Code Commissioner to require passive radon-resistant construction statewide for one- and two-family homes — a significant move, since Iowa is the one state the EPA classifies as entirely Zone 1. But according to guidance issued by the Iowa Department of Inspections, Appeals and Licensing shortly after the bill passed, the statute "does not independently create any enforceable passive radon mitigation requirement" until the Commissioner formally adopts the implementing rule. In practical terms: as of this writing, Iowa builders are not yet legally required to comply, even though the law is on the books. Confirm current status with the Iowa Building Code Commissioner before assuming this has taken effect.
States With No Statewide Mandate
Several large states we checked have explicitly said, on their own agency websites, that they do NOT require radon-resistant construction statewide — the decision is left entirely to local governments, which may or may not have adopted it.
| State | State agency position |
|---|---|
| Pennsylvania | "The statewide building code does not require radon-resistant new construction. However, local municipalities may have adopted this portion of the building code." (PA Department of Environmental Protection) |
| Florida | "Currently these standards are voluntary. It would take a majority of the building permit issuing agencies in any county to adopt them for the standards to become a required code within that county." (FL Department of Health) |
| Colorado | No statewide requirement found; some individual jurisdictions have adopted it locally |
| Ohio | No statewide requirement found in the Residential Code of Ohio |
Existing-Home Mitigation vs. New-Construction Prevention
The EPA recommends fixing an existing home if it tests at 4 pCi/L or higher, and considering a fix between 2 and 4 pCi/L — but for an existing home, this is a recommendation, not a general legal requirement (it becomes binding only where a specific rule adopts it, like the IRC's own post-test fan trigger above). An active sub-slab depressurization (ASD) system uses a fan to pull soil gas from beneath the slab and exhaust it above the roof; a passive system relies only on natural convective airflow. Retrofitting a mitigation system into a home with no passive rough-in already in place costs more than converting one that already has the vent pipe roughed in from new construction — EPA has published rough figures for both, though the exact numbers are dated and should be treated as a rough range, not a quote.
| Scenario | Rough cost range | Source |
|---|---|---|
| Radon-resistant construction, built into a new home | $250–$750 (potentially under $250 if moisture-control techniques are already used) | EPA, Radon-Resistant New Construction for Home Buyers |
| Retrofitting an existing home with no rough-in | $800–$2,500 | EPA, Improving Air Quality in Your Community: Radon |
Standards & Citations
| Standard | Code / Section | Requirement |
|---|---|---|
| EPA Map of Radon Zones | EPA Map of Radon Zones (1993) | Assigns every U.S. county to Zone 1 (>4 pCi/L), Zone 2 (2-4 pCi/L) or Zone 3 (<2 pCi/L) based on indoor measurements, geology, radioactivity and soil. |
| IRC Radon Control Methods Appendix | 2021 IRC Appendix AF | Optional appendix; not mandatory unless the adopting jurisdiction specifically references it in its ordinance. |
| Illinois Radon Resistant Construction Act | 420 ILCS 52 Sec. 20 | All new residential construction in Illinois must include passive radon-resistant construction, statewide, with no zone exception. |
| Minnesota Radon Control Systems | Minn. R. 1303.2400–1303.2403 | Minimum passive radon control system requirements apply to all new residential structures statewide. |
| Massachusetts Radon Control (Zone 1 Counties) | 780 CMR Appendix AF101.1 | Mandatory passive radon construction in high-radon-potential counties, regardless of site-specific radon level; testing is not required or permitted as a condition of occupancy. |
| New Jersey Radon Hazard Subcode | N.J.A.C. 5:23-10 | Radon-resistant construction required for Use Groups R and E in designated Tier 1 municipalities only. |
Apply These Requirements
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Frequently Asked Questions
Does my state require a radon-resistant system in new construction?
Only a few states require it by state law regardless of local adoption: Illinois (statewide, no zone exception), Minnesota (statewide, all new residential structures), Massachusetts (mandatory in its designated high-radon counties only), and New Jersey (only in designated Tier 1 municipalities). Iowa passed a statewide law in 2026, but as of this writing it isn't enforceable until the state's Building Code Commissioner adopts the implementing rule. Everywhere else, it depends on whether your specific city or county has adopted the IRC's optional Appendix AF (or an equivalent) — check with your local building department.
What is a Zone 1 radon area?
Zone 1 is the EPA's highest radon-potential classification: counties where the predicted average indoor radon screening level is above 4 pCi/L. It's based on indoor radon measurements, geology, aerial radioactivity, soil parameters and foundation types. The IRC's own radon appendix publishes a full, code-adopted table of every Zone 1 county in the country, which is what most state and local construction mandates are keyed to.
Do I have to test my new home for radon if it has a passive system?
It depends where you are. Where the IRC's testing section (Appendix AF104) is adopted, a test below 4 pCi/L just gets reported and a result at or above 4 pCi/L requires activating the fan on the already-installed vent pipe. Massachusetts is the opposite: it explicitly does NOT require testing at any stage and says testing cannot be made a condition of occupancy, even in its mandatory counties.
What's the difference between a passive and active radon system?
A passive system relies on natural convective airflow through a vent pipe connected to a gas-permeable layer under the slab — no fan, no power. An active system (also called active soil depressurization, or ASD) adds an in-line fan to mechanically pull soil gas out and exhaust it above the roof. New construction in an area that requires radon control typically installs the passive components and leaves the pipe and an electrical box accessible so a fan can be added later if a test comes back high.
Can I install my own radon mitigation system?
It depends on your state and the type of system. In Illinois, for example, a general residential contractor can install passive radon-resistant construction, but only a licensed radon contractor may install an active mitigation system, a radon vent fan, or upgrade a passive pipe to active. Check your state's specific licensing rules — several states (Minnesota, Ohio, Florida, and others) license radon testers and mitigation contractors separately from general contractors.
Where do the requirements on this page come from?
The EPA zone system, the base IRC Appendix AF text, the Illinois statute, and the Massachusetts amendment were all read directly from the primary source (ICC Digital Codes, the Illinois General Assembly's own statute site, and the Minnesota Revisor's Office). A few states (Iowa, New Jersey) are reported from a research pass whose sources were checked but not all independently re-pulled — those sections say so explicitly rather than presenting every claim with equal confidence.
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