Your Water Filter Might Not Catch All Toxic Chemicals

Your Water Filter Might Not Catch All Toxic Chemicals

Water filters may not remove all of the drinking water contaminants you’re most concerned about, researchers report.

While using any water filter is better than using none—many household filters are only partially effective at removing toxic perfluoroalkyl substances, commonly known as PFAS, from drinking water, according to a new study. And a few, if not properly maintained, can even make the situation worse.

“We tested 76 point-of-use filters and 13 point-of-entry or whole-house systems and found their effectiveness varied widely,” says Heather Stapleton, associate professor of environmental health at Duke University’s Nicholas School of the Environment.

“All of the under-sink reverse osmosis and two-stage filters achieved near-complete removal of the PFAS chemicals we were testing for,” Stapleton says.

“In contrast, the effectiveness of activated-carbon filters used in many pitcher, countertop, refrigerator, and faucet-mounted styles was inconsistent and unpredictable. The whole-house systems were also widely variable and in some cases actually increased PFAS levels in the water.”

“Home filters are really only a stopgap,” says Detlef Knappe, professor of civil, construction, and environmental engineering at North Carolina State University. “The real goal should be control of PFAS contaminants at their source.”

PFAS have come under scrutiny in recent years due to their potential health impacts and widespread presence in the environment, especially drinking water.


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Exposure to the chemicals, used widely in fire-fighting foams and stain- and water-repellants, is associated with various cancers, low birth weight in babies, thyroid disease, impaired immune function, and other health disorders. Mothers and young children may be most vulnerable to the chemicals, which can affect reproductive and developmental health.

Some scientists call PFAS “forever chemicals” because they persist in the environment indefinitely and accumulate in the human body. They are now nearly ubiquitous in human blood serum samples, Stapleton notes.

For the study, researchers analyzed filtered water samples from homes in Chatham, Orange, Durham, and Wake counties in central North Carolina and New Hanover and Brunswick counties in the southeastern part of the state.

They tested samples for a suite of PFAS contaminants, including three perfluoroalkal sulfonic acids (PFSAs), seven perfluoroalkyl carboxylic acids (PFCAs), and six per- and poly-fluoroalkyl ether acids (PFEAs). GenX, which has been found in high levels in water in the Wilmington area of southeastern North Carolina, was among the PFEAs for which researchers tested.

Key takeaways include:

  • Reverse osmosis filters and two-stage filters reduced PFAS levels, including GenX, by 94% or more in water, though the small number of two-stage filters tested necessitates further testing to determine why they performed so well.
  • Activated-carbon filters removed 73% of PFAS contaminants, on average, but results varied greatly. In some cases, the filters completely removed chemicals; in other cases they didn’t reduce them at all. Researchers saw no clear trends between removal efficiency and filter brand, age, or source water chemical levels. Changing out filters regularly is probably a very good idea, nonetheless, researchers say.
  • The PFAS-removal efficiency of whole-house systems using activated carbon filters varied widely. In four of the six systems tested, PFSA and PFCA levels actually increased after filtration. Because the systems remove disinfectants used in city water treatment, they can also leave home pipes susceptible to bacterial growth.

“The under-sink reverse osmosis filter is the most efficient system for removing both the PFAS contaminants prevalent in central [North Carolina] and the PFEAs, including GenX, found in Wilmington,” Knappe says.

“Unfortunately, they also cost much more than other point-of-use filters. This raises concerns about environmental justice, since PFAS pollution affects more households that struggle financially than those that do not struggle.”

The paper appears in Environmental Science & Technology Letters.

Additional coauthors are from NC State and Duke. The NC Policy Collaboratory through the NC PFAS Testing Network and the Wallace Genetic Foundation funded the work.

Original Study

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