EWG's Tap Water Database — 2019 UPDATE



Oak Park Water Service

Radon is a radioactive gas that comes from soil and groundwater, and causes lung cancer. Highest exposures come from radon entering a house through its basement or crawl spaces, or from it volatilizing in the water. Read More.

Radon is the most common source of radiation exposure in the home. Radon in tap water is a smaller source of exposure compared to radon in indoor air. Water utilities report radon in picocuries per liter (pCi/L), which is a measure of radioactivity in water. The federal government has not set a Maximum Contaminant Level (MCL) for radon in tap water. In 1991, the EPA proposed a drinking water standard of 300 picocuries per liter for radon, which was not finalized. In 1999, the EPA proposed a combined approach for reducing radon risk in indoor air and limiting radon in drinking water. This approach recommended a maximum of 4,000 picocuries of radon per liter of drinking water in states with programs to address radon air risks. Where there was no state program for radon mitigation, a water radon limit of 300 picocuries per liter was to apply. This proposal has not yet become a federal law.

Click here to read more on radiological contaminants.

The draft federal limits for radon do not sufficiently protect public health. The EPA's analysis on radon cancer risk suggests that for a water limit of 300 picocuries per liter, 200 in every 1 million people drinking water with this much radon could develop cancer over their lifetimes. For a water limit of 4000 picocuries per liter, over 2,000 people in every 1 million could develop cancer over their lifetimes. The EPA estimates that radon in drinking water causes about 168 cancer deaths per year - nearly 90 percent of which come from lung cancer caused by breathing radon released from water during showering or other water use. The remainder of cancer deaths are primarily from stomach cancer caused by consuming radon in water.





Samples exceeding legal limit (MCL)


Samples exceeding
health guidelines

Testing results - average by year

YearAverage resultSamples takenDetectionsRange of results
2012457.00 pCi/L1111276.00 pCi/L - 621.00 pCi/L
2013361.95 pCi/L1919233.00 pCi/L - 579.00 pCi/L
2014439.63 pCi/L1918ND - 729.00 pCi/L
2015382.47 pCi/L1917ND - 796.00 pCi/L
2016400.35 pCi/L2018ND - 622.00 pCi/L
2017277.91 pCi/L2015ND - 1,030.00 pCi/L

pCi/L = picocuries per liter

State and national drinking water standards and health guidelines

EWG Health Guideline 1.5 pCi/L

The EWG Health Guideline of 1.5 pCi/L for radon was defined by the Environmental Protection Agency as a one-in-a-million lifetime risk of cancer. Values greater than one-in-a-million cancer risk level can result in increased cancer cases above one in a million people.

pCi/L = picocuries per liter

All test results

Date Lab ID Result