Water Filter Classifications and What They Mean

No federal regulations exist for residential water treatment filters, purifiers, and reverse osmosis systems in the United States. So what do those numbers for water filter classifications mean?

There are no required legal standards, but voluntary national standards and NSF International protocols have been developed that establish minimum requirements for the safety and performance of filtration devices that treat drinking water. There are separate testing procedures for specific chemicals, debris, and elements found in resultant filtered water.water purity standards for home water filter systems

A device’s “passing” of any given test under the specific standards and protocols developed by NSF gives that device a numerical classification such as NSF/ANSI 42 or NSF/ANSI 401. The better water filtration systems have more than one certification.

The standards and protocols are explained in detail below. The numbers in the names reflect the order in which the standard or protocol was developed and are not a ranking or rating system.

You call tell from the classifications below exactly what any specific water filtration system does — or does NOT — filter out of water that flows through it. This list is verbatim from the NSF website (which you can find here), with the one exception noted below.

Bolded emphasis and italics have been added by us, not by NSF. The bolded words in turquoise are words written by the NSF that appear on the NSF page that we simply are highlighting. The italicized words in plum are not NSF words, but our comments.

  • NSF/ANSI 42
    Filters are certified to REDUCE aesthetic impurities such as chlorine and taste/odor. These can be point-of-use (under the sink, water pitcher, etc.) or point-of-entry (whole house) treatment systems. This category is primarily cosmetic. It offers the lowest level of drinking water certification given by NSF. We note here that both the popular Brita water filtration systems and the Japanese-engineered Nakii filtration pitchers are NSF/ANSI 42 certified only.
  • NSF/ANSI 53
    Filters are certified to REDUCE a contaminant with a health effect. Health effects are set in this standard as regulated by the U.S. Environmental Protection Agency (EPA) and Health Canada. Both standards 42 and 53 cover adsorption/filtration which is a process that occurs when liquid, gas or dissolved/suspended matter adheres to the surface of, or in the pores of, an adsorbent media. Carbon filters are an example of this type of product. Two important notes here. (1) The minimum amount of any given contaminant allowed is set by the EPA based on its minimum standards of health risk, so it has greater weight than the NSF/ANSI 42 standard. And (2) Both of NSF/ANSI 42 and NSF/ANSI 53 filtering systems are carbon filtered, so it is important to note that the latter is an upgraded carbon filter that is superior to the other type, but carbon filters are not the only type of household water filter.
  • NSF/ANSI 44
    Water softeners use a cation exchange resin that is regenerated with sodium or potassium chloride. The softener reduces hardness caused by calcium and magnesium ions and replaces them with sodium or potassium ions. This is not specifically a drinking water filter, but usually is some sort of whole house filter largely intended to keep pipes in better condition and to make water “feel” better. They create low-level toxic waste in the filtering by-product.
  • NSF/ANSI 55
    Ultraviolet treatment systems use ultraviolet light to INACTIVATE or KILL bacteria, viruses and cysts in contaminated water (Class A systems) or to reduce the amount of non-disease causing bacteria in disinfected drinking water (Class B). Although it likely is understood without our comment, to inactive or kill is a lot stronger than simply reducing. So NSF/ANSI 55 systems are excellent filtration systems if bacteria, viruses, and cycsts are the sole targets. You will, however, often find this designation in combination with other designations to give a broad spectrum of water purity. It creates almost no negative impact on the environment other than it needs either electricity or some sort of battery-operated power source.
  • NSF/ANSI 58
    Reverse osmosis systems incorporate a process that uses reverse pressure to force water through a semi-permeable membrane. Most reverse osmosis systems incorporate one or more additional filters on either side of the membrane. These systems REDUCE contaminants that are regulated by Health Canada and EPA. Three comments are important here as well. (1) This is a highly similar measurement to NSF/ANSI 53 in terms of the resultant drinking water. It means recognized toxic substances are reduced to at least below recognized levels of toxicity. (2) Unlike what many RO companies advertise, the resultant water is not 100% pure, although it certainly exceeds the NSF/ANSI 42 standard of a bare-bones water filter. And (3) since these standards only focus on the quality of resultant water, they do not take into consideration the toxicity level of waste the filter itself produces. RO system waste is so highly concentrated that it is extremely toxic so disposal of the waste from RO filter systems poses a whole new set of environmental problems.
  • NSF/ANSI 62
    Distillation systems heat water to the boiling point, and then collect the water vapor as it condenses, leaving behind contaminants such as heavy metals. Some contaminants that convert readily into gases, such as volatile organic chemicals, can carry over with the water vapor. The same comments that apply to NSF/ANSI 58 apply here as well.
  • NSF/ANSI 177
    Shower filters attach directly to the pipe just in front of the homeowner’s showerhead and are certified to only reduce free available chlorine. Because our skin is such a huge organ and such a huge intake valve for the human body, many people immediately feel better even with this minimal amount of chlorine reduction in their shower water.
  • NSF/ANSI 244
    The filters covered by this standard are intended for use only on public water supplies that have been treated or that are determined to be microbiologically safe. These filters are only intended for protection against intermittent microbiological contamination of otherwise safe drinking water. For example, prior to the issuance of a boil water advisory, you can be assured that your filtration system is protecting you from intermittent microbiological contamination. The standard also includes material safety and structural integrity, similar to other NSF/ANSI drinking water treatment unit standards. Manufacturers can claim bacteria, viruses and cysts reduction for their filtration system. This comment is vitally important so read it carefully and consider it strongly. Funding for spot inspections of public water systems, including this type of “fail-safe,” has been consistently reduced over the past decade. Most municipalities have some sort of “fail-safe” system, but not all work or work forever. Many areas of the country find their water supplies are contaminated for weeks, possibly months before an inspection finds the problem and puts the community on a boil water advisory. And in some cases, the boil water advisory lasts more than a week.
  • NSF/ANSI 401
    Treatment systems for emerging contaminants include both point-of-use and point-of-entry systems that have been verified to reduce one or more of 15 emerging contaminants from drinking water. These emerging contaminants can be pharmaceuticals or chemicals not yet regulated by the EPA or Health Canada. This is probably the single most important divider between an OK water filter and a really good one as far as we are concerned. The higher-end water filter systems have NSF/ANSI 401 designations.
  • NSF P477
    These point-of-use filters reduce microcystin (toxins produced by blue-green algae) below the health advisory set by the EPA. We also find this an important designation. And again, you will find higher-end filters hold it.
  • NSF P473
    PFOA/PFOS water filters or systems are evaluated on their ability to reduce PFOA and PFOS in drinking water and to meet strict material safety and structural requirements as defined in NSF/ANSI 53. This, too.
  • NSF P231
    Microbiological water purifiers are certified for health and sanitation based on the recommendations of the EPA’s Task Force Report, Guide Standard and Protocol for Testing Microbiological Water Purifiers (1987) (Annex B).
  • NSF/JWPA P72
    Iodine radioisotope point-of-use treatment options are evaluated for reduction of all forms of iodine in drinking water. This protocol was developed in conjunction with the Japan Water Purifier Association (JWPA).

For home water filtration systems that we most highly recommend, see our article here on the Five Best Home Water Filters of 2020. This guide to the classification will help you understand the logic of how we derived the “best.”

You can read more about water safety classifications directly on the NSF website, the Public Health & Safety organization with its worldwide headquarters in Michigan.

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