Not all types of in-home filters are completely effective at removing “forever chemicals” from drinking water, and a few could do more harm than good if not properly maintained, according to a new study released Wednesday by North Carolina researchers.

Water filters in refrigerators, pitcher-style filters, under the sink reverse osmosis systems and whole-house filtration systems can function differently and have vastly different price tags, according to the study, headed by the researchers from Duke University and N.C. State University.

Researchers tested 76 drinking water filtration systems to determine their ability to remove toxic perfluoroalkyl substances, commonly known as PFAS, in homes in Chatham, Orange, Durham and Wake counties in central North Carolina. They also tested in homes in New Hanover and Brunswick counties in the southeastern part of the state.

The conclusion is that “their effectiveness varied widely,” said Heather Stapleton, an associate professor of Environmental Health at Duke’s Nicholas School of the Environment and a lead researcher in the study.