What Is Radon?
Radon is a colorless, odorless, tasteless radioactive gas, and it is the only drinking-water contaminant on this site where the water itself is rarely the exposure pathway that matters. Radon-222 forms continuously underground as uranium and radium decay inside rock and soil, and it does not stay put — it dissolves into groundwater passing through fractured bedrock, and it seeps upward into the air inside homes built on the same ground. Because it comes from two directions at once, radon is really one hazard with two entry points, and the water pathway is the smaller of the two.
That distinction is the whole story. Ingesting radon in a glass of water carries some risk, mainly to the stomach, but the dominant threat is inhalation — radon gas escaping from water into indoor air every time someone showers, runs a dishwasher, or does laundry, then getting breathed in along with whatever radon has already seeped through the foundation from the soil. The National Academy of Sciences estimated that of the roughly 168 cancer deaths attributable to radon in drinking water each year, 89% come from inhaling radon released to indoor air and only 11% from direct ingestion. A water treatment system that only filters the water people drink — a pitcher, a countertop unit, a single kitchen tap — does nothing about the air people breathe throughout the rest of the house.
Radon is measured in picocuries per liter (pCi/L), a unit of radioactivity, not the parts-per-billion or milligrams-per-liter used for chemical contaminants elsewhere on this site. It is a decay-chain sibling of two other radionuclides covered here — uranium and radium — all three tracing back to the same uranium-238 series in bedrock. Radon is overwhelmingly a groundwater and private-well issue: surface water sources like rivers and reservoirs are open to the atmosphere, so radon off-gasses naturally before it ever reaches a treatment plant or a tap.
How Radon Gets Into Drinking Water
Bedrock Geology
Radon’s source is geologic, not human. Uranium and radium occur naturally in rock and soil everywhere, but concentrations run highest in granite, granitic gneiss, and other crystalline or metamorphic bedrock — the fractured rock formations common across New England, the Appalachian Piedmont, and parts of the Rocky Mountain West. Groundwater moving through those fractures picks up radon gas as it flows, and because the gas is chemically inert, nothing in the aquifer removes it before it reaches a well.
Private Wells vs. Public Systems
This is fundamentally a groundwater problem. Water drawn from deep, fractured bedrock wells has direct contact with radium-bearing rock and no chance to off-gas before it is pumped into a home. Public water systems that draw from surface water are essentially exempt — reservoirs and rivers release radon to the atmosphere on their own. Large public groundwater systems fare better too, because they often store water in open tanks or aerate it during treatment, both of which vent radon before delivery. The people with the highest exposure are on small groundwater systems and private wells, where water often runs straight from a sealed well to the tap with no aeration step in between — and, unlike regulated public systems, nobody tests a private well unless the owner arranges it.
Regional Hotspots
Radon-in-water concentrations vary by orders of magnitude depending on local bedrock, and a handful of regions stand out:
| Region | What’s driving it |
|---|---|
| New Hampshire / New England | USGS modeling found 55% of New Hampshire is underlain by groundwater with a 50%+ probability of exceeding 2,000 pCi/L, concentrated in granite terrain |
| Appalachian Piedmont (NC, GA, SC) | Fractured granite above the Fall Line routinely exceeds 8,000 pCi/L; historical Georgia outliers have measured up to 140,000 pCi/L |
| Southeastern Pennsylvania | Crystalline schist bedrock shows median groundwater radon near 4,300 pCi/L |
| Mountain West / Rockies | Uranium-rich granitic and metamorphic formations produce similarly elevated well readings |
Regional averages only describe the bedrock, not any individual well — two wells a few hundred feet apart, drilled into different fracture zones, can differ by a factor of ten.
Health Effects
Radon is the second leading cause of lung cancer in the United States after smoking, and the EPA attributes roughly 21,000 lung cancer deaths a year to radon exposure overall — the large majority from radon seeping directly into homes from soil gas, not from water. Waterborne radon adds to that airborne burden rather than replacing it: the same gas, entering the same lungs, from a second source.
Lung Cancer From Inhalation
When radon-laden water is agitated — a shower running, a washing machine agitating, a dishwasher spraying — dissolved radon gas releases into the air, where it can be inhaled along with radon that has already migrated up through the foundation from soil. Once inhaled, radon and its short-lived decay products lodge in lung tissue and emit alpha radiation directly into cells, which is the mechanism behind its carcinogenicity. There is no known safe threshold; risk rises with cumulative exposure, and it compounds sharply with tobacco use — the EPA’s risk tables put the lifetime lung cancer death risk at 4 pCi/L of indoor air at roughly 7 per 1,000 for people who have never smoked, versus roughly 62 per 1,000 for smokers.
Stomach Cancer From Ingestion
Swallowing radon-contaminated water irradiates the stomach lining directly, and this is the smaller, secondary risk — the National Academy of Sciences attributes about 11% of radon-related water deaths to this pathway. The epidemiological evidence for ingestion-driven stomach cancer specifically is thinner than for inhalation; several cohort studies, including a Finnish case-cohort analysis, found no statistically significant association at typical exposure levels, though EPA’s own risk modeling still assigns it a small, non-zero contribution based on absorbed radiation dose rather than direct epidemiology.
Children and Pregnant Women
There is no evidence that children or fetuses face a categorically different radon-water risk pathway than adults — the mechanism is the same inhalation-and-ingestion exposure. But children breathe more air relative to body size and spend comparable time in the same bathrooms and kitchens, so a household with elevated water radon delivers a proportionally larger dose to a smaller body. No radon-specific prenatal guidance exists beyond the general recommendation that any household with elevated indoor radon — water-sourced or soil-sourced — should mitigate regardless of who lives there.
Roughly 89% of radon-related deaths tied to drinking water come from breathing radon gas released into indoor air during showering, laundry, and dishwashing — not from drinking the water itself. A pitcher or faucet filter treats what you drink, not what your whole house breathes.
EPA Regulation and Limits
Radon is unusual among the contaminants covered on this site: there is no enforced federal MCL for radon in drinking water. The EPA proposed one in 1999 under authority granted by the 1996 Safe Drinking Water Act amendments — but the rule was never finalized, and more than a quarter-century later it still hasn’t been.
The 1999 proposal offered water systems two paths, designed to account for the fact that most radon exposure comes from soil gas, not water:
| Standard | Value | Status |
|---|---|---|
| Proposed MCL | 300 pCi/L | Applies to systems in states without an approved indoor-air Multimedia Mitigation (MMM) program — never finalized, not enforceable |
| Proposed Alternative MCL (AMCL) | 4,000 pCi/L | Available to systems in states that run an EPA-approved MMM program addressing indoor radon broadly — never finalized, not enforceable |
| MCLG (health goal) | Zero | EPA’s standard default for a confirmed carcinogen |
| EPA indoor-air action level | 4 pCi/L (air, not water) | The recommendation to mitigate a home’s indoor air — separate scale, separate unit basis |
| WHO guidance | No specific drinking-water guideline; addressed primarily as an indoor-air hazard | Reflects the same inhalation-dominant risk model |
The logic behind the two-tier proposal was the 10,000-to-1 rule: because roughly 10,000 pCi/L of radon in water contributes about 1 pCi/L to indoor air, a state that was already tackling indoor radon broadly — sealing foundations, venting soil gas — could accept a higher number in the water (4,000 pCi/L) since it was addressing the larger exposure source directly. States without such a program would have to hold water utilities to the stricter 300 pCi/L instead. No state ever built the required MMM infrastructure at scale, the rule stalled, and the EPA has not proposed a replacement since. The result: public water systems are not legally required to test for or remove radon, and there is no number a private well owner can check against a federal standard — only these unenforced 1999 proposed values and each state’s own advisory guidance, several of which use 4,000 pCi/L informally as an action level for private wells.
How Widespread Is Radon?
Radon in drinking water is not part of routine EPA compliance monitoring — because there is no finalized MCL, public water systems are not required to test for it, and it does not appear in the EPA’s regulated-contaminant SDWIS violation data the way arsenic or lead do. What data exists comes from state radon surveys, USGS groundwater assessments, and voluntary well testing, and it paints a consistent picture: radon is a groundwater problem concentrated by bedrock geology, not a uniform national hazard.
USGS modeling in New England found radon exceeded EPA’s proposed 300 pCi/L standard in roughly a third of wells tested across the region, and separate New Hampshire mapping put more than half the state’s groundwater at a 50% or greater probability of exceeding 2,000 pCi/L — a level nearly seven times the proposed MCL. New Hampshire alone has roughly 40% of its population on private wells, meaning a large share of residents have no regulatory backstop and no automatic testing. Similar patterns repeat wherever crystalline or granitic bedrock sits close to the water table: the Appalachian Piedmont, southeastern Pennsylvania, and pockets of the Mountain West.
Because private wells fall entirely outside SDWIS reporting, the true national prevalence of radon in drinking water is not comprehensively tracked — the data that exists is regional and voluntary, and millions of well owners in radon-prone bedrock have simply never tested.
How WaterVerge Tracks Radon
Because there is no enforced federal MCL, radon does not generate the compliance violation records that regulated contaminants like lead or nitrate do, so it is largely absent from the SDWIS violation data that powers most of WaterVerge’s city pages. It is also not part of UCMR 5, which focuses on PFAS and lithium monitoring for public systems already subject to enforceable standards.
Where WaterVerge can surface radon risk, it does so through geologic and regional context — bedrock type, regional USGS survey results, and state program data — rather than a per-system violation count, since that per-system data largely doesn’t exist for this contaminant. This is a real limitation: unlike a regulated chemical with thousands of reported measurements, radon’s true occurrence in any specific home’s water is unknown unless that home has been tested. If you are on a private well, especially in a state or county with granite or crystalline bedrock, WaterVerge’s city-level information should be treated as a starting point for awareness, not a substitute for a water sample. Pair it with our well water testing guide, and separately, test your home’s indoor air for radon — the two tests measure different exposure pathways and neither one substitutes for the other.
How to Remove Radon
What doesn’t work first: point-of-use pitcher filters, faucet-mounted filters, and refrigerator filters do nothing meaningful for radon. Even if one modestly reduced radon in the water passing through it, the dominant exposure pathway — gas released into indoor air from every other tap, shower, and appliance in the house — would be completely untouched. Radon-in-water is a whole-house problem, and it requires a whole-house, point-of-entry solution installed where water enters the home, before it reaches any fixture.
| Method | Removal Rate | Certification | Best For |
|---|---|---|---|
| Aeration (air stripping) | 99%+ | EPA-recognized best available technology | Point-of-entry, whole-house — the preferred method |
| Granular activated carbon (GAC), point-of-entry | Up to 99% | Widely used, though radon-specific NSF/ANSI certification is limited | Point-of-entry where aeration isn’t practical |
| Pitcher / faucet / countertop filters | Negligible impact on whole-house air exposure | N/A | Not a solution for radon |
| Boiling | Does not address the exposure pathway | N/A | Irrelevant — the risk is inhalation during normal water use, not drinking hot water |
Aeration is the better long-term choice for most homes. A point-of-entry aeration system bubbles air through incoming water and vents the released radon gas outdoors through a fan and stack, physically removing the gas before it ever reaches a faucet or showerhead — and, critically, before it can off-gas inside the house. Properly designed aeration systems routinely remove 99% or more of radon in water.
GAC works, but it has a disposal wrinkle unique to a radioactive contaminant. Granular activated carbon adsorbs radon effectively — up to 99% removal — but the radon doesn’t vanish; it decays in place, on the carbon, into a chain of short-lived radioactive decay products that accumulate on the filter media over the unit’s service life. That buildup can create a low-level gamma radiation source inside the home and can mean the spent carbon needs to be handled and disposed of more carefully than an ordinary spent filter, sometimes with utility or state guidance on disposal. Aeration avoids this issue entirely because it vents the gas rather than trapping it.
Because both systems are point-of-entry equipment, they are typically installed and sized by a licensed water treatment professional rather than a DIY under-sink swap — see our best whole-house water filters guide for how whole-house systems are evaluated generally, and pair any water treatment decision with a separate indoor air radon test, since soil-gas infiltration is usually the larger overall source and requires its own foundation-sealing or sub-slab depressurization fix.
Frequently Asked Questions
Does a water filter pitcher remove radon?
No, and it would not meaningfully reduce your exposure even if it did. Radon’s dominant risk comes from gas released into indoor air during showering, laundry, and other water use throughout the house — a pitcher only treats the small fraction of water you drink directly. Radon in water requires a point-of-entry aeration or activated carbon system installed where water enters the home.
Is there a legal limit for radon in drinking water?
No. The EPA proposed an MCL of 300 pCi/L and an alternative limit of 4,000 pCi/L in 1999, but the rule was never finalized and remains unenforced today. Public water systems are not legally required to test for or remove radon, and there is no federal standard a private well owner can check results against.
How much radon in water is dangerous?
There is no bright line, since risk scales with concentration and duration of exposure and no threshold is considered fully safe. Many states informally treat levels above the EPA’s proposed 4,000 pCi/L as reason to install treatment, and some use the stricter 300 pCi/L figure. As a rough guide, every 10,000 pCi/L in water adds about 1 pCi/L to a home’s indoor air.
Should I test my air or my water for radon?
Both, and they measure different things. Water testing tells you what’s coming from your well; a separate indoor-air radon test, done with an air detector over several days, tells you your home’s total exposure from both soil gas and any water contribution combined. A water fix alone will not resolve elevated indoor-air radon coming from the foundation.
Can boiling water remove radon?
No. Radon exposure from water comes from breathing gas released during normal use — showering, washing, running appliances — not from drinking it hot or cold. Boiling does not address that exposure pathway and offers no protective benefit against radon specifically.
Check Your City
Radon risk is dictated almost entirely by what’s under your foundation, not by which city you live in — two neighborhoods a few miles apart can sit on completely different bedrock and carry very different exposure. Search your city on WaterVerge for regional context on groundwater and radon-prone geology, and if you’re on a private well, treat testing — both water and indoor air — as the only way to know your actual numbers rather than relying on a regional average.