The warnings came in the mail this spring to 47 community water systems serving more than 400,000 Illinois residents: Elevated levels of harmful PFAS, better known as “forever chemicals,” had been found in their drinking water.

The notice , mailed by the Illinois Environmental Protection Agency in April, was triggered by a tighter federal PFAS standard adopted by the state’s pollution control board that month. The state told the water systems to immediately notify their customers by mail, email, post card, text message or telephone.

In some cases, other communities would tap into these water systems in an emergency, so the Illinois EPA sent another notice in May to warn those other systems.

PFAS — perfluoroalkyl and polyfluoroalkyl substances — are found in products ranging from clothin

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