It’s boom time for water filters, ever since the EPA announced that as many as 10 percent of US water systems may have unsafe levels of PFAS, also known as “forever chemicals,” in the water. Lead pipes are still all over the place, even if they’re no longer in Flint, Michigan, as of July 2025. Heck, there are maybe heavy metals in the groundwater, depending where you are, and bottled water risks excess microplastics.

I don’t say this to panic you. Your drinking water is probably in compliance with federal standards, and only around 4 percent of water systems end up with unsafe contaminant levels, according to an EPA study in 2020. This said, local water systems won’t have to comply with the new federal PFAS and PFOA maximums for drinking water until 2031.

In response to worries about the water supply, a whole new generation of water filters are making wondrous promises about reductions in contaminants: chlorine, PFAS, heavy metals, stray pharmaceuticals like acetaminophen, fluoride if that’s a worry for you. Some of these are whole-home or under-sink reverse osmosis systems. Others are simple countertop systems, gravity-fed through mesh or carbon or other filters.

So how do you sort out which water claims are credible? I’ve been testing water filters since 2024, often using chemical indicators and equipment, and can attest that it isn’t always easy. Here’s a quick guide to checking up on that water filter that caught your eye.

What Are NSF/ANSI Water Filter Standards?

The first main sources of comfort for me when assessing water filters are certifications and independent testing. Vague claims of 99 percent reduction in chlorine or PFAS or arsenic are harder to trust if the testing is internal to the company itself. To be credible, any claims of third-party testing should also clearly identify the lab that conducted the testing.

The most reliable, standardized form of certification is testing against NSF/ANSI standards. The NSF, formerly called the National Sanitation Foundation, is a Michigan-based, nongovernmental nonprofit founded in 1944 that develops and verifies adherence to a number of different testing standards—including those for water filters. ANSI, or the American National Standards Institute, is an independent nonprofit that accredits accreditation and testing organizations like the NSF.

There are three main NSF/ANSI standards that apply to filters for home drinking water.

• NSF-ANSI 42: Drinking Water Treatment Units—Aesthetic Effects.
  This certifies removal of substances that aren’t considered harmful but may affect cosmetics or taste of the water. This includes the chlorine and chloramine used by cities to disinfect drinking water.
• NSF/ANSI 53: Drinking Water Treatment Units—Health Effects
  This certifies removal of any number of compounds thought to be harmful, including lead, arsenic, PFAS, and others. Each compound is tested individually, among more than 50 possible contaminants.
• NSF/ANSI 401: Emerging Compounds/Incidental Contaminants
  This certifies filtering of compounds that are “emerging” as potential concerns, including pharmaceuticals, herbicides, and pesticides.

If you have the item in hand, these may be marked on the box. But don’t assume certification is the norm. Among shower water filters in particular, only one filter received certification from NSF for free chlorine removal: the Weddell Duo.