Do Home Water Filters Work? What Experts Say

Lead, bacteria and PFAS are among the contaminants cropping up in drinking water.

Credit...Thea Traff for The New York Times

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By Dana G. Smith

Over the past few years, water safety crises have cropped up in several cities, including Baltimore, Flint, Mich., Jackson, Miss., and Newark, N.J., where lead or bacteria have leached into tap water, forcing people to rely on bottled water or on boiling their tap water to rid it of pathogens.

In Wilmington, N.C., high levels of perfluoroalkyl and polyfluoroalkyl substances, chemicals commonly known as PFAS, were detected in the local watershed. PFAS have been linked to a host of health issues, including cancer, liver damage and problems with fertility. The Environmental Protection Agency proposed new regulations in March that would crack down on drinking water levels of six types of PFAS, substantially lowering the allowable detectable amounts. (Drinking water is not the only source of exposure to PFAS, which show up in food wrappers, cooking pans and waterproof clothing, among other places, but reducing contact wherever possible is advisable.)

These events raise questions about just how safe municipal water supplies in the United States are, and whether additional filtration steps are required even outside of areas experiencing an acute crisis. And if that's the case, are there home water filters that will help?

Water sanitation is often listed as one of the greatest health advancements of the 20th century, helping to significantly reduce the death rate from infectious diseases. Water safety standards were enshrined in the Safe Drinking Water Act of 1974, which gives the E.P.A. authority to restrict the amounts of many metals, bacteria, pesticides and other harmful contaminants that can be detected in water. State agencies monitor water treatment plants to ensure they are adhering to the law, and if any violations emerge, they are required to notify consumers within 24 hours. (Owners of private wells are responsible for ensuring that their water is free of contaminants.)

Since the passage of the Safe Drinking Water Act, though, other water-monitoring issues have arisen. For example, most water treatment plants are not set up to remove more modern contaminants, such as PFAS, pharmaceutical drugs and endocrine disrupting chemicals, said Detlef Knappe, a professor of civil, construction and environmental engineering at North Carolina State University, who was one of the first to publish on the Wilmington PFAS problem.

Another concern is whether we are "setting standards at a pace that is reflective of what we know about the science of our water," said David Cwiertny, a professor of civil and environmental engineering at the University of Iowa. He gave the example of nitrate, an agricultural pollutant that's present in the water supply in Des Moines. While the local water treatment plant takes steps to remove the contaminant, there are questions about whether the allowable levels could still cause health harms.

Aging infrastructure is also a problem. In several of the recent crises, contamination occurred when lead leached into the water as it traveled through the distribution pipes. National regulations about the amount of lead permitted in pipes have been strengthened over the years, but many old water distribution systems have not been updated and contain unsafe levels.

"Often, things go wrong because of just underinvestment into this type of infrastructure," Dr. Knappe said. "The rate at which we’re replacing the distribution system pipes in the network is not keeping up with the rate at which the system really needs to be maintained."

Finally, experts say water treatment plants are not equipped for the extreme weather events that have become more common with climate change. That was part of the problem in Jackson, where flooding caused by heavy rains overpowered one of the city's treatment plants, resulting in untreated, bacteria-laden water traveling to people's homes.

The crises in Flint, Baltimore, Jackson and Newark are currently exceptional cases — public water supplies in the U.S. are generally safe, said Thanh Nguyen, a professor of civil and environmental engineering at the University of Illinois, Urbana-Champaign. But "the number of exceptions may increase with time if we don't" update the infrastructure, she said.

If there is a known crisis in your area, local officials will provide recommendations for how best to keep yourself safe. If you’re generally concerned about potential contaminants, at-home water filters can help with some issues.

Most filters contain activated carbon to capture contaminants, which can be used in pitchers, refrigerator dispensers, faucet attachments or systems installed under the sink. Activated carbon is good at removing many chemicals and metals but not all (it doesn't capture nitrate, for example), and it cannot filter out most bacteria.

The American National Standards Institute and NSF — two independent groups that evaluate product performance — have established standards for water filters. Companies aren't required to make products that meet NSF/ANSI standards, but because "there is no federally regulated requirement," certification can help to "ensure that the product isn't a counterfeit or it's actually effective," said Kyle Postmus, senior manager of the Global Water Division at NSF.

NSF/ANSI Standard 42 is for aesthetics, such as taste, smell and appearance (people often want to filter out traces of the disinfectant chlorine). Standard 53 focuses on safety, ensuring levels of lead or mercury, as well as some pesticides and industrial chemicals, are below the accepted limit. The certifications are for individual contaminants, and the product should specify all the contaminants it is approved to reduce.

Home filters appear to work decently well for PFAS and can now be NSF/ANSI Standard 53 certified for some of those chemicals, too. In a study published in 2020, Dr. Knappe and his co-authors found that, on average, pitcher and refrigerator filters that use activated carbon reduced PFAS levels by about 50 percent. More advanced filtration systems that use a process known as reverse osmosis were over 90 percent effective, but they are much more expensive and waste a significant amount of water.

Sometimes filters can cause more harm than good. Dr. Nguyen's research revealed that if water sits in a faucet or under-sink filter for a long period of time, such as overnight, it can actually pick up more contaminants, including lead and bacteria. That's because the water is essentially bathing in high concentrations of the contaminants that were trapped by the activated carbon. When the faucet is turned on again, the contaminant-infused water comes rushing out. As a result, Dr. Nguyen said that it is important to flush your water filter for at least 10 seconds before drinking from it. Also, be sure to change your filter regularly.

Most of the experts interviewed for this article said that they used an at-home filter, but none said it was essential. Some used filters for taste issues, while others said it was a precautionary measure. "Not everybody needs them, but I can think of a lot of reasons why people might," Dr. Cwiertny said. "What I would encourage is that people make informed decisions and know why they’re purchasing a device" — for example, for a specific taste concern or for filtering out a known contaminant.

If you’re worried about the quality of your water and want to know whether you should invest in a filter, you can use a home test kit. You can also request a Consumer Confidence Report from the E.P.A. that will detail what's in the water when it leaves your local treatment plant, although the report does not account for what the water might encounter as it travels through the distribution pipe network.

The experts cautioned that if your region has a known issue with lead or another contaminant, a filter is a bandage on a wound that needs surgery — the larger problem with the pipes or water supply still must be addressed.

An earlier version of this article incorrectly described activated carbon as capturing contaminant particles in drinking water. Activated carbon captures dissolved substances, not particles.

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Dana G. Smith is a reporter for the Well section, where she has written about everything from psychedelic therapy to exercise trends to Covid-19.

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