Drinking water consumed by millions of Americans from hundreds of communities spread across the United States is contaminated with dangerous levels of toxic chemicals, according to testing data released on Thursday by the Environmental Protection Agency (EPA).

The data shows that drinking water systems serving small towns to large cities – from tiny Collegeville, Pennsylvania, to Fresno, California – contain measurable levels of so-called “forever chemicals”, a family of durable compounds long used in a variety of commercial products but that are now known to be harmful.

The water of as many as 26 million Americans is contaminated, according to an analysis of the new EPA data performed by the Environmental Working Group (EWG), a Washington DC-based non-profit.

Studies have linked the chemicals to cancers, immunodeficiencies, reproductive harms and developmental effects in children.

Scientists and environmental advocates have increasingly warned about the harms of chemicals like perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) in recent decades, leading to an agreement between the the EPA and chemical manufacturers such as DuPont and 3M to phase out PFOA by 2015.

However, lasting pollution of the environment and human bodies with forever chemicals continues. Studies show nearly all Americans have some level of PFOA, PFOS, and similar chemicals, scientifically known as per- and polyfluoroalkyl substances (PFAS), circulating in their bodies. Additional analyses calculate that hundreds of millions of Americans are probably exposed through drinking water contamination.

But, the EPA’s testing program, part of a 27-year-old effort to sample the nation’s drinking water for unregulated chemicals, offers the most robust look into exactly which communities are polluted. The data released on Thursday is the first round of a program that will test most US water systems serving more than 3,300 Americans for 29 different forever chemicals, along with the metal lithium, over the next three years.

This first batch, which analyzed data from about 2,000 systems across the country, already spells trouble.

According to the data, 220 water systems found some level of PFOA, PFOS, or both chemicals in their drinking water. That means about one in 10 drinking water systems contain the two most notoriously dangerous forever chemicals.