Do You Really Need a Water Filter?

A ll sorts of nasties—lead, mercury, radium, nitrates, norovirus, agricultural runoff, PFAS , and more—can lurk in your water. No surprise, then, that about 40% of U.S. homes filter theirs.

But the options are endless, from simple $20 charcoal-filter pitchers to reverse osmosis, point-of-entry, or ultraviolet filters. Depending on what you choose, a water filter can cost you tens of dollars, or thousands.

Here’s what to know if you’re considering a water filter—and how to tell if you need one.

How do you know if you need a water filter?

Plenty of public systems produce impressively pristine water. New York City, for example, puts out a billion clean gallons of water a day, says environmental health scientist David Nadler of the New York Institute of Technology. But the water that leaves the reservoir and treatment plant is not necessarily the same water that comes out of your tap.

In cities, water is typically pumped up to rooftop tanks, which feed the flow down to spigots via gravity. If the tank is not kept clean, the water will suffer too. Older buildings may still have lead pipes or asbestos in the walls, which can contaminate water as well.

“Your supply company could send you the purest water in the world. But if your pipes are old and have lead in them, there's really no protecting you,” says Nadler. “You’re at the mercy of your plumbing.”

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In cities, suburbs, and rural areas, periodic flooding can contaminate otherwise-clean water with industrial or agricultural runoff or sewer overflow. Algae and fungi, which bloom in the spring and summer, can pose a danger too. “There can be differences caused by the seasons and by weather, in addition to those caused by plumbing,” says registered dietitian Mindy Haar, assistant dean at the New York Institute of Technology’s School of Health Professions. The Safe Water Drinking Act , signed into law in 1974, provides certain nationwide minimum standards for water cleanliness, but, says Haar, “there can be variety from place to place.”

Everybody is susceptible to contaminants in water, but older people and those with compromised immune systems are especially at risk. Knowing how clean your water supply is can take some doing. A provision of the Safe Water Drinking Act requires water suppliers to release an annual water quality report to the community. This is typically mailed to households, but can also be accessed online, says Nadler. People who live off the water grid—such as those with their own wells—do not have the benefit of an annual report and must test their water on their own. And again, even when an annual report is released, it only reveals the state of the water when it left the supplier, not when you consume it or bathe in it.

“As a homeowner or a renter, the only way to know unequivocally [the state of your water] is to have it tested by a professional,” says Haar. That, in turn, is the only way to know what kind of filter—with what level of protection—you should buy.

What kind of water filters are out there?

Water filters come in all makes and models. Pitcher versions (like a Brita) filter water by running it through a block of charcoal, a piece of mesh, or both as you fill the reservoir. Most pitchers use physical barriers like these and are effective at screening out chemicals like benzene and chlorine, metals like lead and mercury, and particulates, including asbestos. They do not, however, filter out viruses and bacteria.

Other filters need a plumber or a home handyperson to install. They sit either on the countertop or under the sink, at just a single faucet (so-called “point-of-use” systems), or outside the home, filtering all water as it enters the house (“point-of-entry” systems). These provide much higher quality—and more expensive—filtration, and they typically use one of several technologies, according to the U.S. Centers for Disease Control and Prevention (CDC). Depending on the type of filter, the barrier provides either microfiltration, ultrafiltration, or nanofiltration. Microfiltration has an average pore size of 0.1 microns—or millionths of a meter. (The period at the end of this sentence measures about 500 microns.) Ultrafiltration has a pore size of about 0.01 microns. Nanofiltration weighs in at just 0.001 microns.

The smaller the filter pores, the smaller the pathogens and toxins that can be strained out of your water. Microfiltration (found in some higher-end pitchers ), is highly effective at eliminating protozoa, moderately effective at screening out smaller bacteria, and not effective against viruses or chemicals, according to the U.S. Centers for Disease Control and Prevention (CDC). Nanofiltration—also found in some pricier pitchers , as well as in reverse osmosis and distillation systems—is highly effective against protozoa, bacteria, and viruses and moderately effective against chemicals.

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Reverse osmosis filters, which are installed under the sink or on the countertop, use a different technology from that of pitchers, forcing water across a membrane that strains out impurities. Distillation systems, which are also both countertop and undersink devices, work by yet another technology, heating water to the boiling point and collecting and condensing the vapor, leaving contaminants behind. Reverse osmosis and distillation systems are more effective and costly than charcoal and mesh filters.

Finally, there are ultraviolet systems, which kill pathogens using UV light. They are highly effective against microorganisms, but not against chemicals, and are available in both point-of-use and point-of-entry configurations.

Consumer Reports recommends a handful of under-sink models that use a range of different types of filters and are certified to remove PFAS, but advises checking the brand you’re considering with the National Sanitation Foundation ( NSF ), the Water Quality Association ( WQA ), or the International Association of Plumbing & Mechanical Officials ( IAPMO ). The system you buy should depend on just which impurities are in your water.

“You can buy a filter and include a lot of different add-ons,” says Nadler. “But there is no magic box that does everything.”

Like Consumer Reports, the CDC recommends turning to the NSF to check the make and model of any filter you’re considering buying to determine its effectiveness against various contaminants.

How often should you change your filter?

A dirty filter does not do you a fraction of the good a clean filter does. The CDC recommends checking the manufacturer’s recommendation for changing filters and following that guidance. Brita calls for changing pitcher and faucet charcoal filters every two to six months, depending on make and model and how much water is used. Culligan recommends changing its charcoal filters every two to six months; reverse osmosis filters every three to five years ; and whole-home systems every several years.

When it comes to ultraviolet systems, Nadler recommends changing the lights every year or so. “If they’re running 24/7, you get maybe 9,000 to 10,000 hours out of them, “ he says. For any filter, Nadler urges consumers to know the manufacturer’s guidelines and comply with them. “If you don’t change the filters, eventually they’re going to fail,” he adds.

Apart from the danger posed by contaminants, Haar recommends using filtration systems if only to make water taste better and smell better—and to wean people away from sodas and other sugar-sweetened beverages which, as a dietitian, she sees as a scourge.

“If somebody’s got a Brita pitcher on the table,” she says, “and it’s perceived that the water might taste better and be more pure, people would likely drink more. That’s going to be a good idea.”

Write to Jeffrey Kluger at jeffrey.kluger@time.com .