Radon in Well Water: What Your Result Means and When to Treat It

Radon in Well Water Is Mostly a Breathing Problem

Maybe a home inspection report listed a radon-in-water number in the thousands. Maybe a basement air test came back high and someone asked whether you're on a well. Either way, you're now looking at a figure in picocuries per liter (pCi/L) and wondering whether your water is the problem.

Start with what radon is. It's a naturally occurring radioactive gas, and the Vermont Department of Health is blunt about spotting it: "You cannot see, smell or taste radon." It dissolves into groundwater, which is why wells are where it shows up. The Connecticut Department of Public Health explains that radon in water "can enter the air in your home when it is agitated or heated," which can happen when you shower, run laundry or cook.

That's where most of the risk sits. The U.S. Environmental Protection Agency's Consumer's Guide to Radon Reduction says, "Most of your risk from radon in water comes from radon released into the air when water is used for showering and other household purposes." So a water result is really a question about the air in your house. Below, you'll see how to translate one into the other, why the air test comes first, what state guidance says about treating the water, and how the two whole-house treatments compare.

How Much Radon Your Water Adds to Indoor Air

Health agencies use a rule of thumb for this. The North Carolina Radon Program states it plainly: "As a general rule, 10,000 pCi/L radon in water will contribute 1 pCi/L radon to the air in the home." Connecticut uses the same ratio.

Why is the transfer so small? A National Research Council risk assessment found that "only about half of the radon in the household water supply escapes into the air," and what does escape is diluted into all the air inside the home. Now set that ratio next to EPA's indoor air guidance, which says to fix your home if radon is "confirmed to be 4 picocuries per liter, pCi/L, or higher." Suddenly a water number starts to mean something.

Treat those as a sense of scale. They come from one ratio, and the radon that actually reaches your air depends on how much water you run, the size of the house and how well it's ventilated. EPA's questions and answers on its 1999 proposal for public water used the same arithmetic, noting that 4,000 pCi/L "contributes about 0.4 pCi/L of radon to the air in your home."

Here's what the math does for you. Say your air test reads 8 pCi/L and your well carries 5,000 pCi/L. The water accounts for roughly half a picocurie of that 8. A water treatment system on its own would leave the house well above 4, because most of the radon is coming from somewhere else.

What About Drinking It?

Swallowing radon carries some risk, but it's much smaller. Connecticut's health department says radon in the water you drink "can contribute to a very small increase in your risk of stomach and other cancers," a risk it calls "almost insignificant compared to your risk of lung cancer from radon in air."

The National Research Council put rough numbers on it in its 1999 report. It estimated that about 19,000 lung cancer deaths a year in the United States resulted from breathing radon in the home, most of them among people who smoke. Of those, it attributed about 160 to radon released from household water. Drinking water with dissolved radon, it said, "would probably result in about 20 deaths annually" from stomach cancer. The pattern is clear enough: radon from water matters mainly because of the air.

Test the Air First, Then the Water

The state and university programs we checked all want an air result in hand, and two of them put the air test ahead of the water test. The University of Georgia's radon program says, "If you have a private well, we recommend testing your air first and if that result is high, then test your drinking water." North Carolina recommends that "you first test your air," and Vermont tells well owners to test for radon in air "before considering how to reduce radon levels." Maine's guide to treatment systems for radon in well water adds that if you've tested just your water, "test your air too before making treatment decisions."

EPA's consumer guide takes a slightly wider view. If you're concerned about radon and have a private well, it says to "consider testing for radon in both air and water," and notes that radon entering through soil "is usually a much larger risk" than radon arriving in water.

The logic is simple once you've seen the table above. An air result tells you whether you have a radon problem at all. A water result tells you how much of it is coming through the plumbing. Without the air number, you can't tell whether water treatment is the fix, a small piece of it, or money you don't need to spend.

Getting a Water Result You Can Use

Radon leaves water once it's agitated or heated, which is exactly how it gets into your air. That's also why the collection steps matter, so follow your kit's sampling instructions to the letter. Some states set their own rules here. Connecticut recommends "a qualified radon measurement professional and a lab approved by DPH" for radon in water, so check with your state radon office before you sample or order a mail-in kit.

Crystal Quest®'s Radon Water Test ($69) is analyzed by our partner lab, National Testing Laboratories, and you return the sample by guaranteed next-day delivery. If your well could also carry uranium or radium, the Radiological Water Test in its Standard tier ($219) adds radon-222 to a uranium, gross alpha and gross beta panel. If uranium is already on your report, our guide to uranium in well water explains how to read that result.

What State Guidance Says About Treating the Water

There's no single national number for a private well. In 1999, EPA proposed radon limits for community water systems: 4,000 pCi/L where a state, or the water system itself, runs an indoor air radon program, and 300 pCi/L otherwise. Its archived questions and answers on that proposal are clear on one point: "EPA does not regulate private wells." What well owners have instead is state guidance, and the states don't all draw the line in the same place.

North Carolina's reasoning is worth borrowing. Its program counts radon from soil gas and radon released from water together, with the goal of keeping the combined total "to no more than the Environmental Protection Agency (EPA) recommended level of 4 pCi/L." That's the same idea behind the air table earlier: the water number matters because of what it adds to the air you breathe.

Where the three overlap is useful too. At 10,000 pCi/L and up, all three call for action: Maine recommends a treatment system, North Carolina says to consider treating the water along with an air mitigation system, and Vermont, whose advisory level is 4,000, says to take action to reduce radon in the home. All three also want an air result before you decide on treatment. The middle band is where they part ways, so that's where your air number and your own state's radon office should do the deciding.

Aeration or Carbon: Choosing a Whole-House System

Because the risk comes from water used all through the house, radon treatment goes where water enters the home. EPA calls this point-of-entry treatment, and its consumer guide says it "usually employs either granular activated carbon, or GAC, filters or aeration systems." Here's how its mitigation table and text compare the two.

An aeration unit, as Maine's guide describes it, "mixes your water with air inside a plastic tank and then vents the air and radon outdoors, away from the house." Maine also notes that these systems "work best for higher concentrations," while carbon "works best with moderate, not high levels of radon."

The Catch With Carbon

Carbon doesn't just hold radon and stop there. EPA notes that "radioactive radon by-products can build on carbon," and Vermont's health department "discourages the use of GAC systems to remove radon because the radon collected on the filter could create a radiation hazard." EPA's position is less absolute. It treats carbon as an option at moderate levels and says both technologies have advantages and disadvantages "that should be discussed with your state radon office or a water treatment professional."

So here's how we'd weigh it. If your water result is well above 5,000 pCi/L, aeration is the stronger fit on EPA's own numbers. If it's moderate, your state doesn't discourage carbon, and you've confirmed whether the tank needs shielding and how spent carbon will be handled, carbon can be the lower-cost choice. Before you sign anything, ask the installer:

• What reduction should I expect at my radon level, and will you retest to confirm it?
• Where will the aeration exhaust vent, and how far is it from windows and air intakes?
• If it's carbon, what's the replacement schedule at my radon level, and what's the plan for the spent carbon?
• Does the system handle anything else in my water, or will I need separate treatment for iron, hardness or uranium?

Why a Kitchen Filter Won't Fix It

It's tempting to reach for an under-sink filter, especially if you already own one. EPA explains why that misses the target: point-of-use devices "only treat a small portion of the water you use, such as the water you drink," and they "are not effective in reducing the risk from breathing radon released into the air from all water used in the home." Your shower, washing machine and dishwasher keep releasing radon either way.

A drinking-water system can still earn its place for other contaminants in the same well. If you're weighing one, see which contaminants a membrane handles in our guide to what reverse osmosis removes. For point-of-entry equipment that treats the rest of your water, start with our guide to choosing a whole house water filter system.

After a System Goes In

Retest the water. Vermont's advice is direct: "Re-test your drinking water for radon after any treatment system is installed to make sure levels are below the health advisory level." Collect that sample from a tap served by the system, so you're measuring the water your household actually uses.

Then keep up with maintenance. EPA says aeration needs annual cleaning "to maintain effectiveness and to prevent contamination." For carbon, hold the installer to the replacement and disposal plan you agreed on. And keep testing on a schedule. Maine suggests retesting in two years when a result is 4,000 pCi/L or lower, while Vermont says five years for results under its 4,000 advisory level. For the rest of a private well's testing calendar, see our guide to testing well water.

Frequently Asked Questions About Radon in Well Water

Does EPA's 4 pCi/L level apply to my water result?

No. EPA's 4 pCi/L is its level for radon in indoor air. A water result is a different measurement, and state programs use a rough 10,000-to-1 ratio to estimate what it adds to your air. By that ratio, even 4,000 pCi/L in water adds only about 0.4 pCi/L to the air.

Should I worry about radon if I'm on city water?

It depends on where your utility gets its water. Connecticut's health department notes that when water comes from a lake, river or reservoir, "radon is not a concern as the radon is released into the air before it reaches your home." EPA says a radon-in-water problem is more likely when a public system uses groundwater, and some of those systems treat for it. If you're not sure of your source, ask your water supplier.

Can radon in water raise my lung cancer risk?

That's the main concern with it. EPA's archived guidance says breathing radon released from tap water "increases the risk of lung cancer over the course of your lifetime," even though only about 1 to 2 percent of the radon in indoor air comes from drinking water. An air test measures radon from soil and water together, which is one more reason to start there.