Radon in well water
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private wells tested in Sullivan County, New Hampshire had radon above the limit, the highest share of any county, according to NH DHHS.
Radon is a radioactive gas released from bedrock into well water. It escapes into household air during showers and washing, where inhaling it raises lung cancer risk. EPA has no enforceable water MCL; New Hampshire recommends treatment above 2,000 pCi/L, and many states use 4,000 pCi/L.
- Health guideline
- 2,000 pCi/L
- New Hampshire level
- 2,000 pCi/L
- CAS number
- 14859-67-7
Check your radon number
Type the value from your lab report. It is compared to the EPA limit, the same way the full interpretation does it.
Test your own well for radon
Certified lab analysis of your own tap water, bottles and prepaid return shipping included. The Essential well panel covers arsenic, uranium, nitrate, lead, copper, manganese, iron, sodium and bacteria; radon and PFAS are add-ons.
Order the Tap Score well water test kit, from $209Already tested? Upload the report
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Getting it fixed
Book a free in-home water test with a licensed local company, or see the treatment guide for what removes radon.
Where radon is worst, by homeowner tests
Counties ranked by the share of private wells above the state guidance level in tests homeowners sent to the state lab. Shares combine town summaries, weighted by wells tested.
| County | Above state level | Wells tested | Highest |
|---|---|---|---|
| Sullivan County, New Hampshire | 148 | 35,000 pCi/L | |
| Strafford County, New Hampshire | 99 | 670,000 pCi/L | |
| Rockingham County, New Hampshire | 655 | 120,000 pCi/L | |
| Cheshire County, New Hampshire | 78 | 36,000 pCi/L | |
| Merrimack County, New Hampshire | 2,140 | 330,000 pCi/L | |
| Hillsborough County, New Hampshire | 695 | 380,000 pCi/L | |
| Grafton County, New Hampshire | 496 | 40,701 pCi/L | |
| Belknap County, New Hampshire | 577 | 71,000 pCi/L | |
| Carroll County, New Hampshire | 673 | 150,000 pCi/L | |
| Orleans County, Vermont | 214 | 33,500 pCi/L | |
| Orange County, Vermont | 51 | 20,900 pCi/L | |
| Windsor County, Vermont | 161 | 912,000 pCi/L | |
| Washington County, Vermont | 738 | 39,100 pCi/L | |
| Lamoille County, Vermont | 421 | 13,900 pCi/L | |
| Caledonia County, Vermont | 234 | 21,800 pCi/L |
By individual samples
Counties with at least 30 individual samples in public monitoring data, against the EPA limit.
| County | Above EPA limit | Samples | Detected | Median |
|---|---|---|---|---|
| Carroll County, New Hampshire | no limit | 35 | 100% | 3,800 pCi/L |
| Hillsborough County, New Hampshire | no limit | 36 | 100% | 1,350 pCi/L |
| Custer County, Idaho | no limit | 54 | 100% | 1,480 pCi/L |
| Elmore County, Idaho | no limit | 33 | 100% | 480 pCi/L |
| Jefferson County, Idaho | no limit | 41 | 100% | 230 pCi/L |
| Kootenai County, Idaho | no limit | 30 | 100% | 759 pCi/L |
| Latah County, Idaho | no limit | 30 | 100% | 465 pCi/L |
| Lincoln County, Idaho | no limit | 31 | 100% | 56 pCi/L |
| Minidoka County, Idaho | no limit | 92 | 100% | 160 pCi/L |
| Twin Falls County, Idaho | no limit | 65 | 100% | 320 pCi/L |
| Harris County, Texas | no limit | 86 | 100% | 635 pCi/L |
| El Dorado County, California | no limit | 68 | 100% | 1,290 pCi/L |
| Fresno County, California | no limit | 117 | 100% | 580 pCi/L |
| Kern County, California | no limit | 95 | 100% | 540 pCi/L |
| Los Angeles County, California | no limit | 134 | 100% | 360 pCi/L |
How to test
Use a state-certified lab and a proper sampling bottle; hardware-store strips cannot measure radon at the levels that matter. Collect from the kitchen tap before any treatment, and again after treatment if you install it.
How to treat
- Aeration (bubble-up or diffused) whole-house
- Granular activated carbon whole-house (lower levels only)