Many of them are classified as per- and polyfluoroalkyl substances, or PFAS for short.
In Southern California, an ABC7 analysis shows PFAS were found in more than 200 water systems, servicing more than 18 million people at some point over the last 10 years.
"We all have these chemicals in our bodies, but we want less of them than we have," said Scott Bartell, a professor of environmental occupational health at UC Irvine.
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That's because these substances can cause serious health issues like kidney and liver problems, cancer and decreased immune-system functions. They've also been nicknamed "forever chemicals" because of their inability to break down.
"And so there's a good reason to want to remove those from not just all those other products I mentioned, but also specifically from water supplies," Bartell said.
In March, the EPA proposed new legal limits on two of the main PFAS chemicals - PFOS and PFOA - at four parts per trillion each in drinking water. That's much lower than a previous 2016 non-legal health advisory which set levels at 70 parts per trillion.
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But research is still needed to fully understand things like how much exposure to these chemicals could lead to specific problems, as well as which of these thousands of chemicals cause which diseases.
Bartell is leading the California piece of a CDC-funded cross-country study looking at the effects of PFAS on the health of both adults and children in Orange County.
Bartell and his team are still recruiting participants.
"Doing these kinds of studies is just to help us better understand to what extent these chemicals are a problem or not," said Bartell.
"There's some health effects, like the neurobehavioral effects in kids, where I think there's a lot of debate going on in the scientific literature right now about whether that's really a problem in terms of PFAS causing those effects," he continued.
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The study is also focusing on a wider list of PFAS chemicals than previous studies.
In the meantime, Bartell says NSF-certified water filters do a good job of removing at least those two PFAS chemicals most studied: PFOS and PFOA.
Those interested in the study can go to UCI's PFAS health study website to find out more.
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