Acceptable Radon Levels and Guidelines by State

By Steve Crews · Last updated June 25, 2026

One standard, all 50 states

There is no patchwork of state-by-state action levels to memorize. The EPA action level of 4 pCi/L is the benchmark every US state radon program uses. Reach that level or higher, and the EPA and the Surgeon General say take action. The federal framework is consistent; the variation lies elsewhere.

The EPA also draws attention to the 2 to 4 pCi/L range. The agency explicitly recommends considering mitigation there, not just at 4. That guidance matters because risk is proportional to concentration and exposure time. A home that reads 3.5 pCi/L for decades carries meaningful cumulative risk, even if it technically sits below the action level. Lower is genuinely better, not just technically compliant.

One useful benchmark for comparison: the average indoor radon level across US homes is roughly 1.3 pCi/L. Anything meaningfully above that is worth paying attention to.

The EPA radon zone map

The EPA divides every US county into one of three zones based on predicted average indoor radon levels. These zones are geological estimates, not legal limits, and they help prioritize testing and outreach.

Zone 1 counties have a predicted average above 4 pCi/L. This is the highest-priority tier and covers large portions of Iowa, Minnesota, North Dakota, South Dakota, Nebraska, Kansas, Indiana, Ohio, Michigan, Pennsylvania, Colorado, Wyoming, Montana, and Idaho, among others.

Zone 2 counties have predicted averages between 2 and 4 pCi/L. Much of the mid-Atlantic coast, parts of the South, and broad swaths of the West fall here.

Zone 3 counties have predicted averages below 2 pCi/L. Hawaii, most of Florida, and portions of the Gulf Coast coast are in this tier.

The map is a useful guide, not a verdict on any individual house. Zone 1 homes test below the action level. Zone 3 homes test above it. Soil type, foundation design, and how tightly a home is sealed all matter more than the county average. The zone map tells you to be alert; only a test tells you where you actually stand.

States with notably elevated averages

A few states consistently produce high readings because of what is under their soil.

Iowa is the most frequently cited example. The state sits on thick layers of glacially-derived soil and uranium-bearing bedrock, and a substantial share of Iowa homes test above 4 pCi/L. State surveys have found elevated readings in nearly every county. If you live in Iowa and have not tested, that is a gap worth closing.

North Dakota, South Dakota, Minnesota, and Nebraska share similar glacial geology and see similarly elevated readings across wide areas. Montana, Wyoming, Idaho, and Colorado add uranium-rich bedrock exposed by geological uplift. In parts of those states, readings well above 4 pCi/L are common.

Pennsylvania has its own story. The Reading Prong, a band of uranium-bearing igneous and metamorphic rock, cuts through the southeastern corner of the state, taking in Bucks, Berks, and Montgomery counties. Readings in that area can run extremely high. Pennsylvania was actually one of the first places where unusually high residential radon became a national conversation, after a nuclear plant worker repeatedly triggered radiation detectors on his way into work, traced eventually to his home in Boyertown.

For a broader look at how radon concentrations break down across the country, our article on normal and average radon levels covers both national data and regional context.

How international guidelines compare

Two other major health bodies set their own reference levels, and comparing them to the EPA’s 4 pCi/L puts the US standard in perspective.

Health Canada uses 200 Bq/m3 as its guideline, which converts to roughly 5.4 pCi/L. Homes above that level are recommended for remediation. The higher number reflects cost-benefit considerations for Canadian housing, not a different read on the underlying science.

The World Health Organization recommends 100 Bq/m3, about 2.7 pCi/L, as its preferred reference level. The WHO acknowledges that some countries may not yet be able to achieve that target affordably, and suggests a maximum of 300 Bq/m3 as a ceiling in those cases, while pushing for the lower figure where achievable.

The EPA’s 4 pCi/L (roughly 148 Bq/m3) sits between the WHO and Health Canada numbers. Some researchers argue that the US action level should be closer to the WHO’s lower target. The EPA addresses this implicitly by recommending action consideration starting at 2 pCi/L, well below the 4 pCi/L line.

Real estate and disclosure: where states actually differ

This is where state variation becomes concrete. No state sets a different residential action level, but many impose radon-related requirements in home sales that others do not.

Some states require sellers to disclose known radon test results or the presence of a mitigation system. Others require buyers to receive radon education materials as part of the transaction. A handful have passed laws mandating testing under specific circumstances, such as when a home is sold or when a tenant requests it in rental housing. Some states have no specific radon disclosure statute beyond general property disclosure rules.

The takeaway for buyers and sellers: look up your state’s radon program or ask your real estate agent about the current disclosure requirements before closing. Sellers should document any past testing and any installed mitigation system, because a clean test result and a certified install are assets, not liabilities.

When buying a home that already has a mitigation system, there are specific things worth verifying, including whether the fan is functional and whether a post-installation retest confirmed the system is working. State-certified mitigators credentialed through AARST-NRPP (the national standard body for radon professionals) follow ANSI/AARST standards that give buyers confidence in the installation.

State programs and free testing

Every state has a radon program, usually housed within the health or environmental agency, and most offer practical resources that homeowners do not know to look for.

Common offerings include discounted or free short-term charcoal test kits mailed directly to residents, searchable databases of certified testers and mitigators, county-level radon data from past surveys, and guidance documents tailored to specific building types like schools or rental housing. The free and discounted kits in particular are worth checking before buying a kit at retail: some state programs charge only a small handling fee, making testing essentially free.

A quick web search for “[your state] radon program” turns up the right office in seconds. Our roundup of free radon test kit programs covers both state-level options and national programs for anyone whose state does not currently have kits available.

If you prefer to buy directly, a quality short-term charcoal canister test kit runs $15 to $30 and comes with prepaid lab analysis. Check price on Amazon

Testing your own home

The state average, the EPA zone, and the neighbor’s result are all context. Your home’s reading is the only number with consequences for you.

Test the lowest level you regularly use. If you have a basement bedroom or a finished lower level where someone spends hours daily, that space deserves the first test. Closed-house conditions are important for short-term tests: keep windows and doors shut for at least 12 hours before the test starts and throughout the testing period, which is usually 48 to 96 hours for a short-term charcoal kit.

If a first test comes back at or above 4 pCi/L, do a follow-up long-term test or a second short-term test before making decisions. Single results can be skewed by weather or testing conditions. A confirmed result above 4 pCi/L is the point at which the EPA and Surgeon General recommend taking action; a result between 2 and 4 is the range where reducing levels is worth serious consideration.

FAQ

Does my state set its own radon action level different from the EPA’s? No. Every US state radon program aligns with the EPA’s 4 pCi/L action level. States differ in the programs they run (free kits, certified contractor lists, public data), in real estate disclosure requirements, and in how aggressively they fund outreach, but there is no state with a separate residential standard that overrides the EPA’s threshold.

Why is radon so much worse in some states than others? Geology drives it. Uranium-bearing bedrock and glacially-deposited soils release more radon gas, so states like Iowa, North Dakota, Minnesota, and Pennsylvania see consistently higher indoor levels. Rocky Mountain states also see elevated readings in areas with uranium-rich geology from geological uplift. That said, high spots occur in every state because of local soil conditions and construction, so testing matters everywhere.

How does the US standard compare to what other countries use? The EPA’s 4 pCi/L is roughly 148 Bq/m3. Health Canada’s guideline is 200 Bq/m3 (about 5.4 pCi/L), and the WHO’s preferred reference level is 100 Bq/m3 (about 2.7 pCi/L). All three bodies agree that radon causes lung cancer and that risk increases with concentration. The differences reflect cost-benefit trade-offs in policy, not disagreement on the underlying science.

Should I test even if I live in a low-radon zone? Yes. Zone 3 designation means the predicted county average is below 2 pCi/L, not that every home in the county is safe. Individual homes test above the action level in Zone 3 counties because of localized soil conditions, foundation type, or ventilation. The EPA and Surgeon General recommend testing all homes regardless of geographic zone.