PROVIDENCE — Rhode Island has begun checking water systems for chemicals that have contaminated water at sites nationwide.

The chemicals, called perfluorinated chemicals, have been linked to cancer and other illnesses but currently aren't federally regulated in drinking water. Water has been contaminated near sites of industrial facilities and U.S. military bases.

The state Department of Health and Brown University are partnering to look for perfluorinated chemicals in more than 30 water systems across Rhode Island. The sampling began Wednesday and will continue through at least September.

Health officials said that there are a lot of old manufacturing sites in Rhode Island and factories have been connected to contamination in other states.

"The issue here is that we do not definitively know what sites, if any, are sources of contamination," said Joseph Wendelken, a health department spokesman. "That is one of the reasons why we are sampling so broadly."

Public water systems serving over 10,000 people were tested for perfluorinated chemicals nationwide between 2013 and 2015, under the direction of the U.S. Environmental Protection Agency. The agency issued stricter guidelines last year regarding human exposure to perfluorooctane sulfonate and perfluorooctanoic acid, or PFOS and PFOA.

Most of Rhode Island's larger water systems didn't have measureable levels. One system did, but the measurement was below the EPA's current advisory level of 70 parts per trillion. A second system measured above that level, but the measurement decreased during follow-up testing.

These new tests are focused on water systems serving fewer than 10,000 people, which weren't sampled previously. Brown is collecting the samples and the state will oversee the analysis.

Jennifer Guelfo, a postdoctoral researcher at Brown University, has conducted research on these compounds for eight years. She collected the first water sample Wednesday.

Guelfo said that to put it in perspective, the amount of the chemicals that would meet the advisory level is roughly equivalent to three and a half drops in the average, Olympic-sized swimming pool.

"I know it raises alarm when people hear we're looking for water contamination, but I think it's better to be proactive," Guelfo said.

If any of the results are over the EPA's level, resampling will be done immediately, Wendelken said. If the second sample is still high, the Department of Health would require the water system to submit a plan of action outlining how it will lower the level or connect people to a safe drinking water source.