How much radon from well water ends up in indoor air?

The EPA estimates that for every 10,000 pCi/L of radon in water, about 1 pCi/L enters indoor air. The inhalation risk from that airborne radon is about 100 times greater than the ingestion risk from drinking the water directly. This means a well with high radon creates a meaningful indoor air quality problem, especially in bathrooms and kitchens where water is agitated.

What radon level in well water is considered high?

The EPA proposed an MCL for radon in drinking water at 300 pCi/L for utilities that also implement an indoor radon reduction program, and 4,000 pCi/L without such a program. These were never finalized. As a practical reference, many state programs treat above 4,000 pCi/L in private wells as the action threshold. Contact your state radon program for local guidance.

Does radon in well water affect everyone in the house?

The inhalation risk affects everyone in the home, since radon released during showering and dishwashing enters the air in enclosed spaces. The drinking water ingestion risk is lower and affects only those consuming the water. Improving ventilation in bathrooms and installing point-of-entry aeration to release radon before water enters the home are the two main protective strategies.

Can I test my well water for radon myself?

Yes. Water radon test kits are available for home use. You collect a water sample and mail it to a laboratory. Make sure the lab is certified by your state's radon testing program or the National Radon Proficiency Program (NRPP). Some state radon offices offer discounted testing kits. Test separately for waterborne radon and for airborne radon in your home.

Does a standard water filter remove radon?

Granular activated carbon (GAC) at the point of entry removes radon from water very effectively, typically 95%+. Aeration (releasing water through the air at the point of entry) also removes radon. Standard under-sink filters and pitcher filters don't address waterborne radon because the radon has already off-gassed by the time the water reaches those filters.

Radon in Well Water: Why Inhalation Is the Bigger Risk

Health disclaimer: This page provides general information about radon in well water for educational purposes only. It is not a substitute for professional testing, licensed radon mitigation services, or advice from your state radon program. If you have concerns about radon exposure, contact a certified radon professional.

About 89% of the health risk from radon in well water comes from breathing it in, not from drinking it. When water containing radon enters your home and you shower, wash dishes, or run a tap, radon gas escapes into the air. That airborne radon is what drives the real exposure risk, and it’s the reason radon in water is primarily an indoor air quality problem, not just a water quality problem.

Where Radon in Well Water Comes From

Radon is a naturally occurring radioactive gas. It forms from the decay of uranium and radium in rock and soil. It’s everywhere in the earth’s crust at some level, but certain geology produces far more of it.

Granite and granitic gneiss bedrock are the main sources. These rocks are uranium-rich, and as uranium decays, it produces radium, which produces radon. Water flowing through fractures in granite picks up radon and carries it to the surface through wells.

The highest-risk regions in the United States are the Northeast and Mid-Atlantic. Maine, New Hampshire, Vermont, Connecticut, Pennsylvania, and parts of New York sit on granite bedrock and have documented high radon levels in private wells. Parts of the Carolinas, Virginia, and the upper Midwest also have elevated radon geology. USGS mapping of radon potential by county is publicly available and a useful first check for your location.

This is not a pollution problem. Radon in well water has nothing to do with industrial contamination, agriculture, or human activity. It’s geology. And it’s almost exclusively a private well issue. Municipal water systems draw from surface water or treat their water extensively. Radon in surface water off-gasses before it reaches your tap. By the time city water reaches your house, radon levels are negligible. Private wells pulling from deep rock fractures are where the risk is.

Why Inhalation Is the Primary Concern

The EPA’s own analysis found that for every 10,000 picocuries per liter (pCi/L) of radon in water, roughly 1 pCi/L ends up in your indoor air. That sounds like a small number until you consider the math on risk.

The ingestion risk from drinking water with radon is real but relatively low compared to inhalation. The inhalation risk from breathing radon-contaminated air is about 100 times higher per unit of exposure. So a well running at 20,000 pCi/L is contributing about 2 pCi/L to your indoor air. The EPA’s indoor air action level is 4 pCi/L. You can see how a well with high radon gets you partway to that level from water use alone, before accounting for any radon entering through the foundation.

The release happens at any point where water is agitated. Showers are the biggest contributor. The spray breaks the water into small droplets, dramatically increasing the surface area from which radon can escape. Hot showers accelerate the release. Enclosed bathrooms with no ventilation concentrate the radon. Running dishwashers and washing machines also release radon, though the release is more diffuse.

Bathrooms and kitchens accumulate the most radon from water use. If you have a poorly ventilated bathroom and a high-radon well, you’re breathing elevated radon concentrations every time you shower.

How to Test for Radon in Water

Testing for radon in water requires a specific water test, separate from any air radon test you may have done for your home. Standard water quality panels don’t include radon. You need to ask specifically for a waterborne radon test.

Find a lab certified through your state’s radon testing program. The National Radon Proficiency Program (NRPP) also certifies labs nationwide. Your state radon program’s website will have a list. Some state programs offer discounted test kits directly through their offices.

Sample collection for radon differs from other water tests because radon off-gasses quickly. The lab will give you specific instructions, but the general approach is to collect the sample without agitating the water, keep the bottle completely full with no air space, and ship it to the lab as quickly as possible, usually overnight. Don’t collect the sample after running water for several minutes, because flushing releases radon before you can capture it.

If your water test shows elevated radon, test your indoor air separately. Indoor air radon testing kits are widely available. You want to know your baseline indoor air level before and after any water treatment, because soil radon entering through the foundation may also be contributing. Treating the water without addressing foundation entry won’t solve an indoor air problem driven by soil radon.

The Two Treatment Options

Point-of-entry aeration is the first option. Aeration systems spray water through the air in a vented chamber before it enters the house. Radon escapes into the vented air stream rather than into your home. The vent carries the radon-containing air outside. Aeration systems remove 95-99% of radon from water. They don’t use any chemical treatment. The tradeoff is the cost of installation and the need for a properly designed vent to the exterior.

Point-of-entry granular activated carbon (GAC) filtration is the second option. A large carbon tank at the point of entry traps radon very effectively, also in the 95%+ range. GAC systems are simpler to install than aeration and require no venting. The important consideration: the carbon bed accumulates radioactivity over time as it traps radon and its decay products. When the carbon is spent and needs replacement, it must be disposed of as low-level radioactive waste through a certified disposal process. Your water treatment professional should be able to handle this. It’s not something you can toss in the trash.

Point-of-use filters, including under-sink RO systems and pitcher filters, don’t address the inhalation problem. Even if they removed radon from drinking water at the faucet, the radon would have already off-gassed throughout your plumbing and into your indoor air before reaching that filter. If inhalation is the primary risk, treatment has to happen at the point of entry, before water enters any interior plumbing.

Putting It Together

If you live in a granite bedrock region and have never tested your well for radon, testing is worth doing. Maine, New Hampshire, Connecticut, Pennsylvania, and neighboring states have enough documented radon-in-water history that private well owners there should treat it as a standard part of their well testing checklist.

Test your water and your indoor air. Address indoor air radon from both sources. The well water testing guide covers how to build a complete testing plan for your well. If you want to look at mail-in options for water radon testing, the best mail-in water tests comparison includes labs that offer radon analysis. Your radon contaminant hub has additional context on treatment system selection and state program resources.

Contact your state radon program before selecting a treatment system. They can confirm the appropriate action threshold for your state and recommend certified installers. For a look at whole-house treatment options including aeration systems: best whole-house water filters.

Sources

U.S. Environmental Protection Agency. “Radon in Drinking Water.” https://www.epa.gov/radiation/radon-drinking-water
U.S. Environmental Protection Agency. “A Citizen’s Guide to Radon.” https://www.epa.gov/radon/citizens-guide-radon-guide-protecting-yourself-and-your-family-radon
U.S. Geological Survey. “Radon in Ground Water.” https://www.usgs.gov/mission-areas/water-resources/science/radon-ground-water
National Radon Proficiency Program. https://c.aaanrrp.org