Radon in Drinking Water
EPA limits, health effects, and what to do if your water is affected.
🩨 Health Effects
Radon is a naturally occurring radioactive gas that can dissolve into groundwater. When released from tap water during showering or other household uses, it becomes an inhalation hazard. Radon in air is the second leading cause of lung cancer in the US after smoking. Ingested radon (from drinking) is a smaller risk but may contribute to stomach cancer.
📍 Sources in Water
Radon comes from the natural breakdown of uranium in soil and rock. It is most common in groundwater drawn from granite and certain other rock formations. States with elevated radon include New England, Appalachia, Montana, Idaho, and parts of the Southeast. Surface water typically has very low radon levels.
✅ What To Do
The EPA has proposed but not finalized an MCL for radon in drinking water. Aeration (running water over air) is very effective at removing radon from tap water. Activated carbon filters also work but concentrate the radon in the filter. If you are concerned about radon in your home's air (the bigger risk), contact your state radon office for testing resources.
📜 Regulation History
The EPA proposed a radon MCL of 300 pCi/L in 1999 (with an alternative MCL of 4,000 pCi/L for systems with multimedia mitigation programs), but the rule was never finalized. Radon remains unregulated in drinking water at the federal level. The WHO suggests a reference level of 100 Bq/L (approximately 2,700 pCi/L) for radon in drinking water. Several states including Maine and Connecticut have set guidelines.
🔬 How To Test Your Water
Radon-in-water test kits cost $20-$40 and are available from state radon offices or online. Certified lab tests cost $25-$50. Water samples must be collected carefully to avoid letting radon escape (no aerating the sample). Test groundwater sources, not surface water, as radon dissipates quickly from open water.
💧 Which Filters Remove Radon?
Aeration systems (point-of-entry) are the most effective treatment, removing 95-99% of radon from water. Granular activated carbon (GAC) filters also work but concentrate radioactive radon decay products in the filter, requiring special disposal. For the primary radon risk (indoor air), address soil gas entry with sub-slab depressurization before treating water.
🔗 Related Contaminants
Check your tap water for Radon
Search your ZIP code to see if your water system has had Radon violations, plus lead testing results and an overall safety grade.
Search your ZIP code →Data from the EPA's Safe Drinking Water Information System (SDWIS). MCLs reflect minimum federal standards; some contaminants may pose health risks below these thresholds.