Data source: U.S. EPA, CDC, National Cancer Institute

What Is Radon?

Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock. It is colorless, odorless, and tasteless — completely undetectable without testing. Radon moves through soil and can enter buildings through foundation cracks, construction joints, and openings around pipes. It also dissolves into groundwater and can be released when water is used indoors.

Health Effects

Radon is the second leading cause of lung cancer in the United States, responsible for approximately 21,000 deaths annually according to the EPA:

• Lung cancer — radon decay products attach to airborne particles and are inhaled; they emit alpha particles that damage lung tissue over time; smokers face dramatically elevated risk
• Stomach cancer — ingesting radon-contaminated water contributes a smaller but real risk of stomach cancer from internal radiation exposure
• Cumulative exposure — radon risk is dose-dependent; long-term exposure in a high-radon home is significantly more harmful than short-term exposure

There is no known safe level of radon. Risk increases proportionally with concentration and duration of exposure.

How Radon Gets Into Homes and Water

Radon enters buildings primarily through the path of least resistance from soil:

• Foundation entry — cracks in concrete slabs, basement floors, block walls, and openings around utility penetrations allow soil gas to enter
• Well water — radon dissolved in groundwater is released into indoor air when water is used for showering, washing, or boiling; this pathway is most significant for homes served by private wells
• Building materials — some granite countertops and concrete products emit low levels of radon, though this is rarely the dominant source
• Geology — homes built on granite, shale, or uranium-rich soils face the highest entry rates

EPA Standards and Radon Zones

There is currently no federal MCL for radon in drinking water. The EPA proposed an MCL of 300 pCi/L (with a 4,000 pCi/L alternative if states implement multimedia mitigation programs) but never finalized the rule.

For indoor air, the EPA action level is 4 pCi/L. The EPA recommends considering mitigation at 2–4 pCi/L and strongly recommends action above 4 pCi/L.

The EPA divides U.S. counties into three radon potential zones:

Zone	Average Screening Level	Action
Zone 1	Greater than 4 pCi/L	Highest priority for testing
Zone 2	2–4 pCi/L	Moderate priority
Zone 3	Less than 2 pCi/L	Lower priority — but testing is still recommended

Zone designation is based on county-level geology. A Zone 3 home can still test high — the only way to know your radon level is to test your home.

How to Reduce Radon

Homes (Air)

• Sub-slab depressurization (SSD) — the most effective and widely used technique; a vent pipe and fan draw radon from beneath the foundation and exhaust it outside before it can enter; typically reduces levels by 50–99%
• Sealing cracks and openings — reduces entry points but is not sufficient alone
• Increased ventilation — can dilute radon but is not a standalone solution

Certified radon mitigation contractors are available in most states. Test after mitigation to verify results.

Water (Radon in Groundwater)

• Aeration systems — spray, packed tower, or bubble aeration removes radon from water before distribution; very effective (up to 99% removal)
• Granular activated carbon (GAC) — point-of-use or point-of-entry filters can capture radon; NSF 61 certified systems are available; requires periodic filter replacement and safe disposal of spent media (which becomes radioactive)

Radon Data in ZipCheckup

Unlike other contaminants on this site, radon has no federal MCL for drinking water and is not tracked through standard CCR reports or SDWIS enforcement actions. Radon risk is assessed through geological survey data rather than water system monitoring.

ZipCheckup uses EPA Radon Zone designations (Zones 1–3) to indicate geological risk for each ZIP code. This data comes from county-level geology surveys, not from water utility testing.

Is Radon a Problem in Your Area?

Radon risk is driven by local geology. High-risk areas include Pennsylvania, Ohio, Iowa, Montana, Idaho, Colorado, Kentucky, and New England states. However, elevated radon has been detected in every state. EPA Radon Zone 1 counties have the highest geological potential.

ZipCheckup displays EPA Radon Zone data for each ZIP code in water quality reports, giving you a starting-point assessment of your area's geological risk.