Making Sense of Water Filter Certification


Only a few states require National Sanitation Foundation (NSF) certification for water filters, and the requirement applies only to a small number of original equipment manufactured (OEM) products.

Performance is tested using real-world volumes and real-world water, containing sediment, cysts, bacteria, and viruses (particle substitutes are permitted for some tests). Filter removal and flow rates are measured at the beginning, middle, and at the end of the test.

It is important to understand which NSF rating applies and to what components. An NSF-certified filter might be rated only for structure and leaching (NSF-42); in this case, all other ratings (removal of chlorine, bacteria, viruses, etc.) are based on manufacturer testing. Here are the most common filter standards.

NSF-42 Aesthetic Affects

This standard covers materials of construction, strength, and filtration efficiency. It usually applies only to the filter materials, confirming the filters structural integrity and that it won't leach lead and certain plasticizers. The standard also covers chlorine reduction and filtration efficiency, but there are multiple tiers of approval; a certified faucet-end filter might remove as little as 25 percent of the chlorine and particles greater than 50 microns, while a carbon-block filter will typically remove more than 99 percent of the chlorine and particles greater than 0.5 microns. To know which applies, check the certified specs. You can search the NSF certification database by manufacturer, part number, or product type at the NSF website:

Protocol P-231, Microbiological Purifiers

Based on the latest EPA research, this procedure has not been formally accepted by the NSF or the American National Standards Institute (ANSI). The filtration device must reject bacteria and viruses at all stages of a 10.5-day test, using water that mimics U.S. municipal water for six days and domestic sewage (high TOC, high TDS, high turbidity, high pH, low temp) for four-plus days. Reverse-osmosis systems generally fail this test, since the membranes commonly have some minor leaks. Double O-ring seals are standard. While there are systems approved that use combinations of reverse osmosis, ultraviolet sterilization, and chemicals, we have reviewed only cartridge filter systems. In our opinion, any P231-certified device is certainly safe.

NSF-53 Health Effects

This standard requires substantial removal of a wide range of chemical pollutants spelled out in the Environmental Protection Agencys drinking water standard. Additionally, it requires removal of 99.95 percent of cryptosporidium cysts. Relatively few filters are certified to NSF-53.

NSF-55 UV Sterilizers

Ultraviolet purifiers are certified in two categories: Class A is for water of unknown microbiological content and requires 40 milliwatts per centimeter squared of UV in the 250- to 300-nanometer range, while Class B is intended for water that is microbiologically safe and requires only 16 milliwatts per centimeter squared. The unit must demonstrate efficacy against bacteria and viruses.

NSF Filters
Darrell Nicholson, editor of Practical Sailor, grew up boating on Miami’s Biscayne Bay on everything from prams to Morgan ketches. Two years out of Emory University, after a brief stint as a sportswriter, he set out from Miami aboard a 60-year-old wooden William Atkin ketch named Tosca. For 10 years, he and writer-photographer Theresa Gibbons explored the Caribbean, crossed the Pacific, and cruised Southeast Asia aboard Tosca, working along the way as journalists and documenting their adventures for various travel and sailing publications, including Cruising World, Sail, Sailing, Cruising Helmsman, and Sailing World. Upon his return to land life, Darrell became the associate editor, then senior editor at Cruising World magazine, where he worked for five years. Before taking on the editor’s position at Practical Sailor, Darrell was the editor of Offshore magazine, a boating-lifestyle magazine serving the New England area. Darrell has won multiple awards from Boating Writer’s International, including the Monk Farnham award for editorial excellence. He holds a U.S. Coast Guard 100-ton Master license and has worked as a harbor pilot and skippered a variety of commercial charter boats.


Please enter your comment!
Please enter your name here