Is radon in well water dangerous?

Yes, primarily because radon off-gasses from water during household use — showering, running the dishwasher, doing laundry — and enters the indoor air. Breathing radon gas is the primary risk, not drinking it. Radon is the second leading cause of lung cancer in the U.S. after cigarette smoke, causing approximately 21,000 deaths per year from all exposure pathways. Well water radon is an additional contributor to total indoor radon exposure on top of radon entering through foundation cracks.

How do I test my well water for radon?

Order a radon water test from a state-certified laboratory that has radon analysis capability — not all labs offer this test, so confirm before ordering. Samples must be collected carefully (typically by filling a bottle fully with no air gap to prevent off-gassing) and shipped overnight to the lab. In high-radon states, county extension programs often provide guidance. If your home air radon test shows levels above 4 pCi/L, testing your well water is the next diagnostic step to determine the water contribution.

How do I remove radon from well water?

Two methods are effective. Aeration — spraying water into the air or bubbling air through the water — removes 95–99% of radon and is the most effective treatment for high radon levels; it requires installation of a point-of-entry aeration system ($1,000–$4,000 installed). Granular activated carbon (GAC) filtration can also remove radon (typically 85–95%), but the carbon becomes radioactive over time and requires careful handling and disposal. Aeration is generally preferred for very high radon levels; GAC is used for moderate levels.

What radon level in water is dangerous?

The EPA proposed (but never finalized) an MCL of 4,000 pCi/L, with an alternative of 300 pCi/L. Most water quality professionals use 4,000 pCi/L as the treatment trigger. As a rough translation: every 10,000 pCi/L of radon in well water contributes approximately 1 pCi/L to indoor air levels (the EPA's indoor air action level is 4 pCi/L). So water above about 40,000 pCi/L could meaningfully elevate indoor air on its own — but combined with foundation radon entry, much lower water levels can push total air exposure above the action level.

Do public water systems have radon?

Public surface water systems (drawing from lakes and rivers) have essentially no radon — it off-gasses before reaching your tap. Public groundwater systems may have some radon at the source, but distribution through the water system and storage in tanks allows most radon to off-gas before it reaches homes. Private wells deliver groundwater directly to taps with no opportunity for radon to escape before use, which is why well water radon is a concern while public water radon generally is not.

All Contaminants

moderate risk levelRadioactive ContaminantsRelevant to well water

Radon in Well Water

Radon is a naturally occurring radioactive gas that dissolves into groundwater from uranium-bearing rock and soil. It is primarily a concern for private well owners — public water systems remove most radon before distribution. The main health risk is not from drinking radon-contaminated water, but from breathing radon gas that off-gasses when the water is used for showering, dishwashing, or laundry. Radon in water contributes to indoor air radon levels, which cause approximately 21,000 lung cancer deaths in the U.S. each year.

Quick Answer

Radon (Rn-222) is a colorless, odorless, tasteless radioactive gas produced by the natural decay of uranium in rock and soil. It is the second leading cause of lung cancer in the U.S. after smoking. Radon dissolves readily into groundwater as it passes through uranium-rich rock formations — particularly granite, gneiss, and shale. Well water radon typically represents a smaller fraction of total household radon exposure compared to radon entering through foundation cracks, but in high-radon geology it can be a significant contributor. Pennsylvania, New England states, and parts of the Upper Midwest and Mountain West have the highest uranium-rich bedrock and therefore the highest radon levels.

Why Is Radon in Drinking Water a Concern?

The EPA estimates that radon in private well water causes approximately 168 lung cancer deaths per year from water-to-air transfer, plus about 19 stomach cancer deaths per year from ingestion. While these numbers are smaller than air-pathway radon deaths (21,000/year), they are entirely preventable. Pennsylvania has the highest radon levels in the nation — the EPA estimates 40% of Pennsylvania homes have radon air levels above the action level of 4 pCi/L. Private well owners in granite and uranium-rich geology who have not tested their water are an underserved at-risk population. See the [Pennsylvania well water guide](/well-water/pennsylvania) and [New Hampshire well water guide](/well-water/new-hampshire) for state-specific context.

Private well owners in states with uranium-rich bedrock face the highest risk: Pennsylvania, Maine, New Hampshire, Vermont, Connecticut, New York, New Jersey, North Carolina, Montana, Wyoming, Colorado, and parts of the Pacific Northwest. Smokers who also have radon exposure face dramatically multiplied lung cancer risk — radon and tobacco smoke together are far more dangerous than either alone. Homes with active wells, lower ventilation, and more time spent indoors face higher cumulative exposure.

Health Effects of Radon in Drinking Water

Lung cancer — the primary risk, from breathing radon gas that off-gasses from well water during household use; radon causes approximately 21,000 U.S. lung cancer deaths annually from all exposure pathways

Smokers with radon exposure face a multiplicative risk: EPA estimates 1 in 3 lifetime lung cancer risk for smokers in high-radon homes vs. 1 in 20 for non-smokers

Stomach cancer — a minor additional risk from ingested radon in water; EPA estimates approximately 19 deaths per year from this pathway

Radiation dose from ingested radon contributes to total lifetime radiation exposure

No acute symptoms from radon exposure — all health effects are from long-term cumulative exposure

How Does Radon Get Into Drinking Water?

Natural decay of uranium and radium in granite, gneiss, shale, and other uranium-bearing rock formations

Groundwater moving through uranium-rich rock dissolves and carries radon

Deeper, confined aquifers often have higher radon than shallow, unconfined aquifers

Private wells are the primary concern — surface water and public water distribution systems release most radon before it reaches taps

Regulatory Limit

EPA Maximum Contaminant Level (MCL)

No finalized MCL

The EPA proposed an MCL of 4,000 pCi/L for radon in drinking water in 1999, with an alternative standard of 300 pCi/L available to states that implement a multimedia radon reduction program addressing both air and water pathways. Neither MCL was ever finalized due to cost concerns and regulatory challenges. The EPA's proposed action level of 4,000 pCi/L remains the practical guidance used by most state programs and water quality professionals. For private wells (which are not federally regulated regardless), testing and treating above 4,000 pCi/L is the standard recommendation. Some states — including Maine, New Hampshire, and Vermont — have their own well water radon guidance.

How to Test for Radon in Your Water

Radon in water is tested using a water sample sent to a certified laboratory with radon analysis capability — not all labs offer this test. Samples must be collected carefully to avoid off-gassing before analysis. Many state radon programs offer subsidized testing. Testing both air radon and water radon is recommended in high-risk geology — if air radon is elevated (above 4 pCi/L), water radon testing is the next diagnostic step to determine how much well water contributes. See [how to test well water](/guides/how-to-test-well-water) for general well testing guidance. Radon water tests typically cost $25–$75.

How to Remove Radon from Drinking Water

Best filter for Radon: Activated Carbon Filtration

These treatment methods have demonstrated effectiveness for Radon removal.

Activated Carbon

Activated carbon is the most widely used residential water treatment technology. It removes chlorine, taste and odor compounds, disinfection byproducts, many volatile organic compounds (VOCs), and — with NSF/ANSI 53 certification — lead and some PFAS. It is available as pitcher filters, under-sink units, and whole-house systems.

Chlorine and chloramine — virtually complete removalTaste and odor compounds — highly effective

Frequently Asked Questions

Activated Carbon Filtration

Removes: Radon

Uranium in Drinking Water

Radioactive Contaminants

Radium in Drinking Water

Radioactive Contaminants

Data Sources & Provenance

All data on this page is sourced from official U.S. government or public datasets.

EPA Drinking Water Contaminant InformationView source

ATSDR ToxFAQs / Toxicological ProfilesView source

EPA SDWIS — violation and detection dataView source

Last updated: 2026-04-30

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