Most people first hear about radon in the context of basements — a test kit on a shelf, a fan and a pipe if the number comes back high. Far fewer know that if you are on a private well in a radon region, your water may be delivering radon into the house every time someone takes a shower.
The counterintuitive part is where the risk actually sits. It is not primarily what you drink. It is what you breathe.
How Water Becomes an Air Problem
Radon is a radioactive gas produced by the decay of uranium in bedrock. Groundwater moving through granite, shale, and certain sandstones picks it up and holds it in solution under pressure underground.
The moment that water is agitated at atmospheric pressure, the gas comes out of solution:
- Showering — the single largest contributor, warm water plus maximum surface area in a small room
- Running the dishwasher or washing machine
- Filling a bath, running taps
The radon enters household air and is inhaled. The main documented health risk from radon is lung cancer, and it is the second leading cause of lung cancer after smoking. A smaller stomach-cancer risk is associated with ingesting radon in water directly.
This is why wells in radon country deserve attention even from households that filter their drinking water. A drinking-water filter does nothing about the shower.
The Numbers, and Their Odd Status
There is no enforceable federal limit for radon in private well water. What exists:
- 4,000 pCi/L — the EPA's proposed alternative maximum contaminant level, applicable where a state operates an indoor-air radon reduction programme.
- 300 pCi/L — the proposed level where no such programme exists.
Many state health departments issue their own guidance in that range. Because the rule was never finalised for public systems, and private wells were never covered anyway, the practical answer is that you are choosing your own action level.
The bridge between water and air is the useful number:
Roughly 10,000 pCi/L in water contributes about 1 pCi/L to indoor air. The EPA action level for indoor air is 4 pCi/L. So water at 40,000 pCi/L would, on its own, account for the entire indoor-air action level.
That ratio is how to think about your own result. A water result of 5,000 pCi/L is adding about 0.5 pCi/L to your air — real, but small next to a soil-gas problem. A result of 50,000 pCi/L is a different conversation.
Testing: Not the Kit From the Hardware Store
An indoor-air radon kit does not test water, and a standard well panel does not include radon. You need a water radon test from a lab that offers it, and the sampling is unusually strict:
- The vial must be filled with no headspace and no bubbles — radon escapes into any air gap and your result reads low.
- Do not aerate the water. Fill gently, from a tap without an aerator screen where possible.
- Follow the lab's flush instructions, then cap under the stream.
- Get it to the lab fast. Radon has a short half-life, and labs correct for decay based on the sample time you record. A vial that sits in a truck for a week is not a result.
Test the air as well, in the lowest occupied level of the house. If your indoor air is elevated, you need to know how much of it is coming from soil gas under the slab versus your water, because the fixes are entirely different and soil gas is far more often the dominant source.
Treatment: Aeration First
Aeration is the preferred method. Water enters a sealed vessel where it is sprayed, bubbled, or cascaded to drive the radon out of solution, and the gas is vented outside above the roofline. It removes a very high proportion of the radon, does not accumulate radioactivity, and treats the whole house — which matters, because the shower is the exposure point.
Aeration systems need power, space, a vent run, and periodic cleaning, and hard or iron-bearing water will scale the internals over time.
Granular activated carbon works at lower concentrations and is simpler and cheaper to install. It has a real drawback: the radon and its decay products accumulate in the filter bed, so the vessel itself becomes a low-level radiation source that needs shielding, siting away from occupied space, and careful disposal at end of life. For high-radon water this is why aeration wins.
Neither reverse osmosis nor a water softener does anything useful here.
Where This Sits in Your Priorities
Radon is a long-term risk, not an emergency. It belongs after the tests that can hurt you this month:
- Bacteria and nitrate, annually, always first.
- Ask your state geological survey whether you are in a documented radon area — this is the cheapest information available and it is usually mapped.
- Test indoor air if you never have. It is inexpensive and tells you the total exposure.
- Test the water for radon if you are in a radon region, especially if your air result is elevated and a soil-gas mitigation system has not solved it.
Log the water and air results together with their dates. If you later install aeration or a soil-gas system, the before-and-after pair is the only evidence that it worked — and radon is invisible enough that evidence is the only thing you will ever have.
Frequently Asked Questions
Is radon in well water dangerous to drink? The larger risk is not from drinking it. Radon escapes from water into household air whenever water is agitated — showering, dishwashing, laundry — and the main documented health risk from radon is lung cancer from inhalation. There is a smaller stomach-cancer risk associated with ingestion.
What is a safe radon level in well water? There is no enforceable federal limit for radon in private well water. The EPA has proposed 4,000 picocuries per litre in water where a state runs an indoor-air radon programme, and 300 pCi/L where it does not. Many state health departments recommend acting somewhere in that range.
How does radon in water compare to radon in air? A commonly used rule of thumb is that roughly 10,000 pCi/L of radon in water contributes about 1 pCi/L to indoor air. Since the EPA action level for indoor air is 4 pCi/L, water at 40,000 pCi/L would alone account for the entire air action level.
Do I need a special test for radon in water? Yes. It is a different test from an indoor-air radon kit, and it must be sampled without agitating or aerating the water, in a sealed vial filled with no headspace, then delivered to the lab quickly because radon decays with a short half-life. Follow the lab timing instructions exactly.
What removes radon from well water? Aeration is the preferred treatment: the water is agitated in a sealed vessel and the radon is vented outside above the roofline. Granular activated carbon also works at lower concentrations but accumulates radioactivity in the filter bed, which becomes a handling and disposal issue.