Tap any US county to check its EPA radon zone, its predicted indoor radon level and well water test results where available.
EPA Radon Zones by County
Indoor air prediction (EPA, 1993). Zones are not a measurement of your home or your well. High indoor radon zone? Test your private well water too.
- Zone 1 highest potential: predicted average indoor radon above 4 pCi/L
- Zone 2 moderate potential: 2 to 4 pCi/L
- Zone 3 low potential: below 2 pCi/L
- Not listed no EPA zone
United States
1,070 of 3,141 counties are EPA Zone 1
Hover, tap or tab to a county.
Radon in well water
Radon dissolved in well water escapes into the air when you shower, wash dishes or do laundry. A standard air test does not measure it, so wells need their own water test.
Get a Radon in Water Test KitCompare Radon Aeration SystemsHow to Test Your WellProduct links open Amazon. We may earn a commission at no extra cost to you.
202 counties with well water radon data, ranked
| County | Median in well water | Wells |
|---|---|---|
| Merrimack, NHEPA Zone 2 | 4,200 pCi/L | 891986 to 2003 |
| Carroll, NHEPA Zone 1 | 3,800 pCi/L | 351986 to 2001 |
| Stevens, WAEPA Zone 1 | 2,540 pCi/L | 1091984 to 2000 |
| Howard, MDEPA Zone 1 | 2,310 pCi/L | 901988 to 2020 |
| Wayne, PAEPA Zone 2 | 2,090 pCi/L | 1272001 to 2022 |
| Rockingham, NHEPA Zone 2 | 2,000 pCi/L | 211986 to 2014 |
| York, PAEPA Zone 1 | 1,630 pCi/L | 621986 to 2021 |
| Billings, NDEPA Zone 1 | 1,420 pCi/L | 261984 to 1984 |
| Lewis and Clark, MTEPA Zone 1 | 1,390 pCi/L | 301992 to 2010 |
| Hunterdon, NJEPA Zone 1 | 1,360 pCi/L | 451986 to 2011 |
| Jefferson, MTEPA Zone 1 | 1,350 pCi/L | 791992 to 2010 |
| Carson City, NVEPA Zone 1 | 1,310 pCi/L | 661987 to 2003 |
| Jefferson, COEPA Zone 1 | 1,310 pCi/L | 231993 to 2005 |
| Pike, PAEPA Zone 2 | 1,150 pCi/L | 971991 to 2015 |
| Baltimore, MDEPA Zone 1 | 1,090 pCi/L | 1251994 to 2020 |
| Grafton, NHEPA Zone 2 | 1,070 pCi/L | 291986 to 2002 |
| Bergen, NJEPA Zone 2 | 1,040 pCi/L | 321986 to 1991 |
| Park, COEPA Zone 1 | 1,010 pCi/L | 941998 to 2013 |
| Hillsborough, NHEPA Zone 2 | 1,000 pCi/L | 341986 to 2003 |
| Lancaster, PAEPA Zone 1 | 1,000 pCi/L | 1551986 to 2021 |
| Sullivan, PAEPA Zone 1 | 960 pCi/L | 222012 to 2017 |
| Potter, PAEPA Zone 2 | 900 pCi/L | 482015 to 2017 |
| Bucks, PAEPA Zone 1 | 870 pCi/L | 1161986 to 2023 |
| Chester, PAEPA Zone 1 | 840 pCi/L | 6181986 to 2021 |
| Montgomery, PAEPA Zone 1 | 840 pCi/L | 481986 to 2023 |
| Morris, NJEPA Zone 1 | 790 pCi/L | 281987 to 2014 |
| Delaware, NYEPA Zone 1 | 780 pCi/L | 282001 to 2020 |
| Santa Barbara, CAEPA Zone 1 | 760 pCi/L | 212011 to 2011 |
| Custer, IDEPA Zone 1 | 750 pCi/L | 451991 to 1996 |
| Lycoming, PAEPA Zone 1 | 750 pCi/L | 832011 to 2022 |
| Douglas, NVEPA Zone 1 | 720 pCi/L | 1011986 to 2013 |
| Prince William, VAEPA Zone 2 | 720 pCi/L | 941990 to 2014 |
| Sussex, NJEPA Zone 1 | 720 pCi/L | 481998 to 2011 |
| El Dorado, CAEPA Zone 2 | 670 pCi/L | 501989 to 2008 |
| Middlesex, NJEPA Zone 2 | 660 pCi/L | 351985 to 2006 |
| Warren, NYEPA Zone 3 | 640 pCi/L | 242004 to 2022 |
| Bradford, PAEPA Zone 1 | 630 pCi/L | 792013 to 2022 |
| Litchfield, CTEPA Zone 2 | 620 pCi/L | 431988 to 2005 |
| Hillsborough, FLEPA Zone 2 | 590 pCi/L | 541995 to 2012 |
| San Luis Obispo, CAEPA Zone 2 | 590 pCi/L | 212005 to 2013 |
| Napa, CAEPA Zone 3 | 580 pCi/L | 252004 to 2012 |
| Santa Cruz, AZEPA Zone 2 | 580 pCi/L | 421996 to 2013 |
| St. Lawrence, NYEPA Zone 2 | 560 pCi/L | 422005 to 2020 |
| Clinton, PAEPA Zone 1 | 540 pCi/L | 602015 to 2023 |
| Mercer, NJEPA Zone 1 | 540 pCi/L | 691985 to 2012 |
| Madera, CAEPA Zone 2 | 530 pCi/L | 471993 to 2014 |
| Gunnison, COEPA Zone 1 | 520 pCi/L | 221996 to 1997 |
| San Diego, CAEPA Zone 3 | 520 pCi/L | 802004 to 2017 |
| Steuben, NYEPA Zone 1 | 520 pCi/L | 332003 to 2020 |
| Lemhi, IDEPA Zone 1 | 510 pCi/L | 241991 to 1994 |
| Carbon, WYEPA Zone 1 | 500 pCi/L | 461997 to 2010 |
| Grand, COEPA Zone 1 | 500 pCi/L | 291997 to 1999 |
| Ravalli, MTEPA Zone 1 | 500 pCi/L | 621992 to 2001 |
| Cattaraugus, NYEPA Zone 1 | 490 pCi/L | 331996 to 2021 |
| La Paz, AZEPA Zone 2 | 470 pCi/L | 912004 to 2013 |
| Harris, TXEPA Zone 3 | 450 pCi/L | 632002 to 2016 |
| Monterey, CAEPA Zone 2 | 450 pCi/L | 752005 to 2013 |
| Allegany, NYEPA Zone 1 | 440 pCi/L | 271996 to 2021 |
| Blaine, IDEPA Zone 1 | 440 pCi/L | 271989 to 1995 |
| Maricopa, AZEPA Zone 2 | 440 pCi/L | 2031989 to 2017 |
| Cochise, AZEPA Zone 2 | 430 pCi/L | 931996 to 2013 |
| Fresno, CAEPA Zone 2 | 430 pCi/L | 961993 to 2014 |
| Gila, AZEPA Zone 2 | 430 pCi/L | 212000 to 2013 |
| Lyon, NVEPA Zone 1 | 430 pCi/L | 391988 to 2013 |
| Sonoma, CAEPA Zone 3 | 430 pCi/L | 562004 to 2012 |
| Washoe, NVEPA Zone 2 | 430 pCi/L | 1021988 to 2013 |
| Chautauqua, NYEPA Zone 1 | 420 pCi/L | 321996 to 2021 |
| Broward, FLEPA Zone 3 | 410 pCi/L | 271996 to 2016 |
| Lassen, CAEPA Zone 3 | 410 pCi/L | 202010 to 2010 |
| Merced, CAEPA Zone 3 | 410 pCi/L | 231994 to 2013 |
| Lebanon, PAEPA Zone 1 | 400 pCi/L | 241993 to 2021 |
| Palm Beach, FLEPA Zone 3 | 400 pCi/L | 331998 to 2019 |
| Graham, AZEPA Zone 2 | 390 pCi/L | 602002 to 2013 |
| Pima, AZEPA Zone 2 | 390 pCi/L | 1391989 to 2017 |
| Greenlee, AZEPA Zone 2 | 380 pCi/L | 222001 to 2001 |
| Guilford, NCEPA Zone 3 | 380 pCi/L | 721996 to 2007 |
| Kern, CAEPA Zone 2 | 380 pCi/L | 501995 to 2018 |
| Utah, UTEPA Zone 2 | 380 pCi/L | 221998 to 2017 |
| Jefferson, WVEPA Zone 1 | 370 pCi/L | 311988 to 2013 |
| Orange, FLEPA Zone 3 | 370 pCi/L | 371999 to 2015 |
| Pinal, AZEPA Zone 2 | 370 pCi/L | 691990 to 2013 |
| Bonner, IDEPA Zone 1 | 350 pCi/L | 381991 to 1995 |
| Lewis, NYEPA Zone 2 | 340 pCi/L | 222005 to 2020 |
| Mendocino, CAEPA Zone 3 | 340 pCi/L | 281993 to 2012 |
| San Joaquin, CAEPA Zone 3 | 340 pCi/L | 261993 to 2005 |
| Shasta, CAEPA Zone 3 | 340 pCi/L | 282007 to 2010 |
| Tulare, CAEPA Zone 2 | 340 pCi/L | 271995 to 2014 |
| Yuma, AZEPA Zone 2 | 340 pCi/L | 222013 to 2016 |
| Kent, DEEPA Zone 3 | 330 pCi/L | 281988 to 2018 |
| San Bernardino, CAEPA Zone 2 | 330 pCi/L | 1032000 to 2016 |
| Stanislaus, CAEPA Zone 3 | 330 pCi/L | 481994 to 2006 |
| Clark, WAEPA Zone 1 | 320 pCi/L | 761988 to 1988 |
| Clinton, NYEPA Zone 2 | 320 pCi/L | 212004 to 2019 |
| Siskiyou, CAEPA Zone 3 | 320 pCi/L | 252010 to 2010 |
| Yavapai, AZEPA Zone 2 | 320 pCi/L | 941991 to 2018 |
| Ada, IDEPA Zone 2 | 310 pCi/L | 3231991 to 2001 |
| Cumberland, PAEPA Zone 1 | 310 pCi/L | 371994 to 2019 |
| Herkimer, NYEPA Zone 3 | 300 pCi/L | 251994 to 2021 |
| Mohave, AZEPA Zone 2 | 300 pCi/L | 1042000 to 2013 |
| Santa Cruz, CAEPA Zone 2 | 300 pCi/L | 372005 to 2013 |
| Chemung, NYEPA Zone 1 | 290 pCi/L | 202003 to 2018 |
| Orange, NYEPA Zone 1 | 290 pCi/L | 241994 to 2020 |
| Weld, COEPA Zone 1 | 290 pCi/L | 251993 to 2005 |
| Dorchester, MDEPA Zone 3 | 280 pCi/L | 231988 to 2012 |
| Doña Ana, NMEPA Zone 2 | 280 pCi/L | 601994 to 2015 |
| Franklin, WAEPA Zone 2 | 280 pCi/L | 481993 to 2016 |
| Park, WYEPA Zone 1 | 280 pCi/L | 251997 to 2001 |
| Payette, IDEPA Zone 2 | 280 pCi/L | 231991 to 1995 |
| Whitman, WAEPA Zone 2 | 280 pCi/L | 351993 to 2016 |
| Humboldt, CAEPA Zone 3 | 270 pCi/L | 202009 to 2010 |
| Maury, TNEPA Zone 1 | 270 pCi/L | 261989 to 2000 |
| Placer, CAEPA Zone 2 | 270 pCi/L | 251989 to 2008 |
| Bannock, IDEPA Zone 2 | 260 pCi/L | 281991 to 2003 |
| Elmore, IDEPA Zone 1 | 260 pCi/L | 311991 to 1995 |
| Canyon, IDEPA Zone 2 | 250 pCi/L | 1041991 to 1999 |
| Cassia, IDEPA Zone 2 | 250 pCi/L | 301990 to 2000 |
| Grant, WAEPA Zone 2 | 250 pCi/L | 641993 to 2016 |
| Latah, IDEPA Zone 1 | 250 pCi/L | 231991 to 2016 |
| Los Angeles, CAEPA Zone 2 | 250 pCi/L | 842000 to 2018 |
| Sacramento, CAEPA Zone 3 | 250 pCi/L | 211995 to 2005 |
| Iredell, NCEPA Zone 2 | 240 pCi/L | 242002 to 2014 |
| Marion, INEPA Zone 1 | 240 pCi/L | 231995 to 1995 |
| Pierce, WAEPA Zone 3 | 240 pCi/L | 261996 to 1996 |
| Salt Lake, UTEPA Zone 2 | 240 pCi/L | 931998 to 2013 |
| Sweetwater, WYEPA Zone 1 | 240 pCi/L | 322002 to 2017 |
| Lancaster, NEEPA Zone 1 | 230 pCi/L | 211994 to 1995 |
| Nye, NVEPA Zone 2 | 230 pCi/L | 271989 to 2013 |
| Power, IDEPA Zone 2 | 230 pCi/L | 261991 to 1995 |
| Scotts Bluff, NEEPA Zone 2 | 230 pCi/L | 951991 to 2001 |
| Caroline, MDEPA Zone 3 | 220 pCi/L | 301988 to 2012 |
| Orange, CAEPA Zone 3 | 220 pCi/L | 521999 to 2011 |
| Owyhee, IDEPA Zone 2 | 220 pCi/L | 251990 to 1995 |
| Thurston, WAEPA Zone 3 | 220 pCi/L | 601989 to 1996 |
| Erie, NYEPA Zone 1 | 210 pCi/L | 222006 to 2021 |
| Orange, NCEPA Zone 3 | 210 pCi/L | 531998 to 2007 |
| Salem, NJEPA Zone 2 | 210 pCi/L | 441988 to 2023 |
| Kent, MDEPA Zone 3 | 200 pCi/L | 771988 to 2012 |
| Morgan, COEPA Zone 1 | 200 pCi/L | 201994 to 1995 |
| New Castle, DEEPA Zone 2 | 200 pCi/L | 1441988 to 2018 |
| York, NEEPA Zone 1 | 200 pCi/L | 302003 to 2004 |
| Clark, NVEPA Zone 3 | 190 pCi/L | 601993 to 2015 |
| Santa Clara, CAEPA Zone 2 | 190 pCi/L | 212007 to 2011 |
| Cumberland, NJEPA Zone 2 | 180 pCi/L | 1001988 to 2023 |
| Lincoln, OKEPA Zone 3 | 180 pCi/L | 751982 to 2014 |
| Morgan, WVEPA Zone 1 | 180 pCi/L | 211995 to 2013 |
| Queen Anne's, MDEPA Zone 3 | 180 pCi/L | 331988 to 2012 |
| Twin Falls, IDEPA Zone 2 | 180 pCi/L | 581990 to 1995 |
| Valley, IDEPA Zone 1 | 180 pCi/L | 201991 to 1994 |
| Bernalillo, NMEPA Zone 1 | 170 pCi/L | 481993 to 2015 |
| Bingham, IDEPA Zone 2 | 170 pCi/L | 541989 to 1995 |
| Garrett, MDEPA Zone 2 | 170 pCi/L | 441992 to 2016 |
| Gloucester, NJEPA Zone 2 | 170 pCi/L | 941988 to 2023 |
| Pennington, SDEPA Zone 2 | 170 pCi/L | 371988 to 1992 |
| Pottawatomie, OKEPA Zone 3 | 170 pCi/L | 471987 to 2018 |
| Riverside, CAEPA Zone 2 | 170 pCi/L | 752000 to 2013 |
| Rockingham, VAEPA Zone 1 | 170 pCi/L | 341993 to 2013 |
| Sioux, NEEPA Zone 2 | 170 pCi/L | 691991 to 1999 |
| Suwannee, FLEPA Zone 3 | 170 pCi/L | 291995 to 2015 |
| Camden, NJEPA Zone 2 | 160 pCi/L | 431988 to 2023 |
| Lincoln, NEEPA Zone 3 | 160 pCi/L | 271994 to 2004 |
| Uvalde, TXEPA Zone 3 | 160 pCi/L | 241996 to 2017 |
| Anne Arundel, MDEPA Zone 2 | 150 pCi/L | 741990 to 2017 |
| Butte, CAEPA Zone 3 | 150 pCi/L | 211996 to 2010 |
| Essex, NYEPA Zone 3 | 150 pCi/L | 332004 to 2022 |
| Navajo, AZEPA Zone 2 | 150 pCi/L | 291991 to 2017 |
| Jefferson, IDEPA Zone 2 | 140 pCi/L | 371989 to 1995 |
| King, WAEPA Zone 3 | 140 pCi/L | 211991 to 1996 |
| Sussex, DEEPA Zone 3 | 140 pCi/L | 371988 to 2018 |
| Montgomery, NYEPA Zone 2 | 130 pCi/L | 251993 to 2021 |
| Otter Tail, MNEPA Zone 1 | 130 pCi/L | 231993 to 2013 |
| Huron, MIEPA Zone 3 | 120 pCi/L | 321988 to 1991 |
| Ocean, NJEPA Zone 3 | 120 pCi/L | 721990 to 2018 |
| Atlantic, NJEPA Zone 3 | 110 pCi/L | 341988 to 2012 |
| Iowa, IAEPA Zone 1 | 110 pCi/L | 451982 to 1998 |
| Oklahoma, OKEPA Zone 3 | 110 pCi/L | 921987 to 1990 |
| Shelby, TNEPA Zone 3 | 110 pCi/L | 301996 to 2014 |
| Suffolk, NYEPA Zone 3 | 110 pCi/L | 262012 to 2016 |
| Butte, IDEPA Zone 1 | 100 pCi/L | 551989 to 1995 |
| Cleveland, OKEPA Zone 3 | 100 pCi/L | 421987 to 2018 |
| De Witt, ILEPA Zone 1 | 100 pCi/L | 211996 to 2002 |
| Logan, OKEPA Zone 3 | 100 pCi/L | 201987 to 1989 |
| Minidoka, IDEPA Zone 2 | 100 pCi/L | 721989 to 1995 |
| Oakland, MIEPA Zone 2 | 100 pCi/L | 271996 to 2013 |
| Worcester, MDEPA Zone 3 | 100 pCi/L | 201988 to 2012 |
| Jefferson, PAEPA Zone 2 | 90 pCi/L | 421995 to 1997 |
| Burlington, NJEPA Zone 2 | 80 pCi/L | 221990 to 2012 |
| Kitsap, WAEPA Zone 3 | 80 pCi/L | 201996 to 2021 |
| Oneida, NYEPA Zone 2 | 80 pCi/L | 321994 to 2022 |
| Albany, NYEPA Zone 1 | 70 pCi/L | 241993 to 2021 |
| Bexar, TXEPA Zone 3 | 60 pCi/L | 341990 to 2017 |
| Dauphin, PAEPA Zone 1 | 60 pCi/L | 471993 to 2023 |
| Decatur, GAEPA Zone 3 | 60 pCi/L | 221995 to 2002 |
| Fayette, WVEPA Zone 2 | 60 pCi/L | 231993 to 2016 |
| Gooding, IDEPA Zone 1 | 60 pCi/L | 341989 to 1995 |
| Honolulu, HIEPA Zone 3 | 60 pCi/L | 312000 to 2000 |
| Jerome, IDEPA Zone 2 | 60 pCi/L | 551989 to 1995 |
| Medina, TXEPA Zone 3 | 60 pCi/L | 271996 to 2017 |
| Saratoga, NYEPA Zone 2 | 60 pCi/L | 511993 to 2022 |
| Lincoln, IDEPA Zone 2 | 50 pCi/L | 231989 to 1995 |
| Churchill, NVEPA Zone 2 | 40 pCi/L | 1451988 to 2004 |
| Kanawha, WVEPA Zone 3 | 40 pCi/L | 231993 to 2017 |
| Lafayette, LAEPA Zone 3 | 30 pCi/L | 232000 to 2013 |
The EPA Map of Radon Zones rates every county in the United States by its predicted average indoor radon level. It was built for the air inside homes, not for well water, but radon also dissolves into groundwater, and nobody tests a private well unless its owner does. This map shows each county’s EPA zone next to what well water samples from that county have found, where enough wells have been tested.
What the EPA Radon Zones Mean
EPA published the zones in 1993 from indoor radon measurements, geology, soil permeability and foundation types. Zone 1 counties have a predicted average indoor level above 4 pCi/L, Zone 2 counties fall between 2 and 4 pCi/L, and Zone 3 counties are predicted below 2 pCi/L. A zone describes a county average, so homes with high radon turn up in every zone. EPA recommends testing every home, whatever its zone.
Radon in Well Water
Radon in water matters mostly because it escapes into the air when water is sprayed or heated, in showers, dishwashers and washing machines. Private wells drawing from granite and other uranium-bearing rock can carry much more radon than surface water. An indoor air test does not tell you what is in your well, so the water needs its own test from a certified lab or a radon in water test kit. When levels are high, aeration systems are the most effective whole-house treatment, and granular activated carbon is sometimes used for lower levels.
Where the Numbers Come From
County zones come from the EPA Map of Radon Zones county list. Well water figures come from Radon-222 results for wells in the Water Quality Portal, the shared database of the USGS, the EPA and state agencies. We take the median of each well first, then the median across wells in the county, and show a county figure only where at least 20 wells were sampled. Many of these samples are decades old and come from studies that targeted particular aquifers, so treat them as a guide to local geology, not a measurement of your well.
What to Do Next
- Test your well water for radon. Our well water testing guide explains how to find a certified lab and what else to test for.
- Test the air in your home as well, especially in a Zone 1 or Zone 2 county.
- If your water result is high, compare aeration and carbon systems with a water treatment professional before you buy.