Water Filter Ratings and Certifications: How to Read a Filter Box

By GoldBlock WorkshopPublished Updated

The short answer

A water filter’s box is only as good as the proof behind it: a certification means an accredited body tested that exact model and keeps checking it, while “tested to” or “meets” a standard may mean only a one-time lab test the maker arranged, with no public listing or follow-up (NSF and WQA describe certification that way; see below). Match the standard to your worry (germs, lead, PFAS or taste), then look the model up in the certifier’s own online directory. If you cannot find it listed for the claim you care about, the claim has no certification behind it.

Key numbers

  • 99.9999%of bacteria, 99.99% of viruses and 99.9% of Cryptosporidium or Giardia: what a "purifier" claims to remove (6, 4 and 3 log)Source: CDC Yellow Book
  • 0.3 micronabsolute pore size or smaller to remove bacteria and parasites; not virusesSource: CDC
  • 5ANSI-accredited certifiers named by the EPA: CSA Group, IAPMO R&T, NSF, UL and WQASource: EPA
  • No datarequired by federal pesticide law before a water filter sold as a device goes on saleSource: EPA

What should I look for, based on what I am worried about?

Start from the problem, then find the rating that covers it. The table pairs common worries with what the CDC, EPA and NSF say to look for.

Your worry Look for Notes
Germs in untreated water (creek, lake, flooded area) A purifier claim (bacteria, viruses and parasites), ideally certified to NSF P231; or filter and then disinfect Most portable filters do not remove viruses (CDC)
Parasites only (Giardia, Cryptosporidium) Certified to NSF/ANSI 53 or 58 for cyst reduction, or an absolute pore size of 1 micron or smaller CDC’s wording for parasite removal
Bacteria and parasites Absolute pore size of 0.3 micron or smaller Still not viruses (CDC)
Lead in tap water Certified to NSF/ANSI 53 with a claim of lead reduction to 5 parts per billion or less, plus, the EPA recommends, NSF/ANSI 42 for particulate reduction (Class I) EPA lead consumer tool
PFAS (PFOA and PFOS) Certified to NSF/ANSI 53 or 58 for PFAS reduction, confirmed in the certifier’s directory. The EPA’s fact sheet adds that “the current certification standards for PFAS filters (as of April 2024) do not yet indicate that a filter will remove PFAS down to the levels EPA has now set for a drinking water standard,” so check the model’s current certified reduction figure, not only the mark EPA PFAS fact sheet (April 2024)
Chlorine taste and odor NSF/ANSI 42 Aesthetic only; says nothing about germs

Sources for the table: CDC: water treatment when hiking, camping or traveling; CDC Yellow Book: water disinfection; EPA: filters certified to reduce lead; EPA: reducing PFAS with a home filter; NSF: standards for water treatment systems.

What is the difference between a water filter and a water purifier?

A purifier claims to remove viruses as well as bacteria and parasites; a filter does not. In U.S. usage, “purifier” points to a test benchmark that came from the EPA, and the CDC sets out what it means: makers claiming the EPA “purifier” designation “must conduct their own testing to demonstrate their filters can remove at least 99.9999% of bacteria, 99.99% of viruses,” and 99.9% of Cryptosporidium oocysts or Giardia cysts.

Two details matter:

  • The maker runs the test. The CDC adds: “The EPA does not independently test the validity of these claims.” A purifier claim on its own is the maker’s word.
  • Certification exists for it. NSF certifies microbiological water purifiers to NSF P231, which NSF says is based on the EPA’s 1987 task force report. A model listed under P231 has had the claim checked by a certifier, not only by the maker.

Most portable filters are not purifiers. The CDC: “Most portable water filters will remove parasites, but not viruses or bacteria,” and “Many filters do not reliably remove viruses.” That is why the CDC advises filtering first and then disinfecting when viruses are possible.

What does “log reduction” mean on a water filter?

Each “log” is a factor of ten: a 1 log reduction removes 90% of the germs, and every extra log adds another 9 to the percentage. It is a short way to write very high percentages.

Log reduction Percent removed Left from 1,000,000 germs
1 log 90% 100,000
2 log 99% 10,000
3 log 99.9% 1,000
4 log 99.99% 100
5 log 99.999% 10
6 log 99.9999% 1

So the CDC’s purifier figures are 6 log for bacteria, 4 log for viruses and 3 log for cysts. A claim like “removes 99.9% of bacteria” is only 3 log, and lets through a thousand times more germs than 6 log, even though both look like “almost all” (1,000,000 × 0.001 = 1,000 left at 3 log; 1 left at 6 log).

What do the NSF/ANSI numbers on a water filter mean?

Each number is a different standard with a different job, and only some of them say anything about germs or health. A filter certified to one standard has proved nothing about the others.

Standard What it covers (NSF’s description) What it does not tell you
NSF/ANSI 42 Aesthetic impurities “such as chlorine and taste/odor” Nothing about germs, lead or other health contaminants
NSF/ANSI 53 Reduction of “a contaminant with a health effect”; the CDC and EPA point to it for cysts, lead, and PFOA and PFOS, each a separate claim Only the contaminants listed for that model
NSF/ANSI 58 Reverse osmosis systems that reduce contaminants regulated by the EPA and Health Canada Only the listed claims
NSF/ANSI 55 Ultraviolet systems. Class A: inactivate or kill bacteria, viruses and cysts in contaminated water. Class B: reduce non-disease-causing bacteria in water that is already disinfected Chemicals; Class B is not for untreated water; the CDC notes UV works poorly in cloudy water
NSF/ANSI 62 Distillation systems Some volatile chemicals can carry over with the vapor (NSF)
NSF/ANSI 401 One or more of 15 “emerging contaminants” listed by NSF Germs
NSF/ANSI 244 Filters “only intended for protection against intermittent microbiological contamination” of water already treated as safe Not for untreated water
NSF P231 Microbiological water purifiers (bacteria, viruses, cysts) Chemicals
NSF/ANSI 61 What the product’s materials add to water “Does not establish performance”: not a removal claim
NSF/ANSI/CAN 372 Lead content of the materials (“lead content only”) Not a removal claim of any kind

Sources: NSF: standards for water treatment systems; NSF/ANSI 61; NSF/ANSI/CAN 372; EPA PFAS fact sheet.

You may also see NSF P248, “NSF Protocol P248 Emergency Military Operations Microbiological Water Purifiers,” which “applies to handheld IWP devices, and was written primarily for military end use”; NSF staff wrote that “The U.S. Army Center for Health Promotion and Preventative Medicine, in conjunction with NSF International, developed a protocol that addressed the specific needs of the military” (WaterWorld, October 5, 2006, by Bruce Bartley, Nicole Beetsch and Michael Blumenstein of NSF International, read October 19, 2026). Like P231, it is a test method; what matters is whether the result was certified and for which organisms.

You may see a standard printed as “NSF/ANSI/CAN 372” rather than “NSF/ANSI 372”; NSF’s own pages use both forms, and the number is what identifies the standard. A materials standard such as 61 or 372 on a box is not a performance claim. It says the parts are safe to touch drinking water, not that the filter removes anything (NSF).

What is the difference between “certified” and “tested to” a standard?

“Certified” means an accredited certifier tested that model, lists it publicly, inspects the factory and retests over time; “tested to” or “meets” may mean only a one-time lab test the maker arranged, with no public listing or follow-up. NSF describes certification as running from “extensive product testing and material analyses to unannounced plant inspections,” with “regular re-testing of products,” and says it acts when a product stops complying: “product recall, public notification or de-certification.” WQA describes its Gold Seal the same way: certified companies “submit to annual facility inspections” and “must retest their certified products regularly.”

A one-time lab report is evidence of what it tested: which lab, which standard, which contaminants, at how many gallons, and whether the test used the full rated life of the filter or only a small part of it. Where a box is certified for some contaminants and only “tested” for others, the certified claims are the only ones a certifier checked.

How do I check whether a filter is really certified?

Look the exact model up in the certifier’s own online directory; never rely on the box or the seller’s page alone. The EPA’s steps for lead filters work for any claim:

  1. Find the certifier’s mark on the filter, the packaging or the smallest container it comes in. The EPA notes the mark is usually paired with a statement such as “Tested and Certified by (certification body) against NSF/ANSI Standards 42 and 53.”
  2. Note which certifier it names. The EPA names five ANSI-accredited certifiers with product directories: CSA Group, IAPMO R&T, NSF, UL and WQA.
  3. Search that certifier’s directory for the brand and the exact model number: NSF certified products, WQA certified products, or the directory of the certifier named on the box.
  4. Check the claim, not just the name. A listing shows which contaminants that model is certified to reduce. The EPA says to confirm the specific claim (for example PFOA and PFOS, or lead) in the directory, and that claims can typically be found on the performance data sheet inside the packaging. That sheet is where the certified claims are listed.
  5. Check the replacement cartridge. A listing names the models it covers; a replacement element that does not appear in the listing is not covered by it.

NSF also publishes public notices on products and manufacturers “in direct violation of certification/registration policies,” including “Unauthorized claims of NSF certification on website” (for example its May 29, 2019 notice on Grayl): NSF public notices. Brands are compared one by one, with their listings checked, in Emergency Water Filters Compared.

Is a water filter approved by the EPA?

No. The EPA does not approve or test ordinary water filters. Under federal pesticide law (FIFRA), the EPA treats water filters, “except those containing substances,” as devices, and its own manual says FIFRA “does not require device producers to submit any data concerning either safety or efficacy” before they are sold; the producer or seller is responsible for the claims.

What you may see on a label, and what it means:

  • “EPA Est. No.” An establishment number identifies the factory. The EPA says establishment numbers “are not the same as EPA pesticide registration numbers.” It is not an approval.
  • “EPA Reg. No.” A registration number appears on products the EPA registers as pesticides. The EPA explains that when a product incorporates a substance that controls germs, “the entire product is considered to be a pesticide and is subject to registration.”
  • Filters with silver or another germ-killing substance that make germ-killing claims fall under that registration rule; the EPA’s own orders against the Black Berkey elements applied it that way. How it applies to specific brands is covered in Emergency Water Filters Compared.

There is no EPA approval for filter devices, so “EPA approved” on a filter has no meaning.

What do micron ratings tell me about a filter?

A micron rating describes the size of the holes in the filter, and only an “absolute” rating promises that no hole is larger. The Minnesota Department of Health defines the two terms: absolute means “all pores in the filter are smaller than the reported value”; nominal means “a majority of the pores are equal in size or smaller than the reported value.”

The CDC’s guidance uses absolute pore sizes:

  • 1 micron absolute or smaller: removes parasites, but not viruses or all bacteria.
  • 0.3 micron absolute or smaller: removes bacteria and parasites, “but not viruses.”
  • Ultrafilters with a pore size of 0.01 micron “should be effective for removing viruses, bacteria, and parasites” (CDC Yellow Book), which also says “Filter pore size is the primary determinant of a filter’s effectiveness.”

A pore size printed on a box is the maker’s claim unless a certifier lists it.

A nominal rating, or a size with no word in front of it, does not meet the CDC’s description, which specifies absolute pore size. Pore size also says nothing about chemicals, which depend on the filter media and its certification.

What can a rating or certification not tell me?

A certification proves a model met a test under set conditions; it does not prove your filter is working today. Things it cannot cover:

  • Contaminants not listed. NSF: certification “does not mean that a filter, purifier or treatment system will reduce all possible contaminants.”
  • Use past the rated capacity. The rating is for a stated number of gallons; past that, the claim no longer applies.
  • Missed maintenance. The EPA: “Filters are only effective if they are maintained according to the manufacturer’s instructions,” and skipping replacements can raise exposure.
  • Damage and counterfeits. A cracked, frozen or fake unit is not the unit that was certified. Care, freezing and spare cartridges are covered in Emergency Water Filters Compared.
  • Your water. A rating built on test water says nothing about what is in your source. For a private well, the CDC says the owner is responsible for testing it; see Making Water Safe to Drink.

What marketing claims are red flags on a water filter?

A claim that cannot be matched to a named standard, a named certifier and a listing has no verified test behind it. The patterns below follow from the facts above.

  • “NSF certified” with no listing. If the model is not in the certifier’s directory, the claim is not certified. NSF publishes notices about companies making unauthorized certification claims or using its mark without authorization.
  • The wrong standard for the claim. A germ or PFAS claim backed only by NSF/ANSI 42, 61 or 372 is not backed at all: 42 is taste and odor, and 61 and 372 are about materials.
  • “Tested to,” “meets,” “exceeds” or “lab-verified” with no lab, no standard number or no report you can read.
  • “Removes 99.9% of everything.” Percentages without the organism, the log value or the test standard. A 3 log figure next to a germ claim is far below a purifier’s 6 log for bacteria (the CDC’s figure).
  • “EPA approved.” The EPA does not approve filter devices.
  • “Military grade.” Not a certification. A real military test has a name (for example NSF P248) and a report.
  • Pressure tactics. The FTC’s 2022 dark-patterns report names “countdown timers designed to make consumers believe they only have a limited time to purchase a product or service” among practices that mislead consumers (FTC press release, September 15, 2022).
  • Capacity figures that disagree between the box, the listing and the seller’s page.

Are there rules for water filter claims in California?

Yes, for home systems. California requires any water treatment device “that makes a health or safety claim of contaminant reduction in drinking water” to be registered with the State Water Resources Control Board, and it must first be certified by an ANSI-accredited third-party certifier (State Water Board FAQ, revised January 2026). The program covers devices designed for residential use; portable, commercial and industrial devices are outside it, and devices that claim only taste, odor, hardness or color need no registration.

Common misunderstandings about water filter ratings

Each of these leads people to trust a filter for something it was never shown to do.

  • “NSF on the box means it removes germs.” Only some standards cover germs; 42, 61 and 372 do not (NSF).
  • “The EPA tested it.” The EPA does not test or approve filter devices, and it does not verify “purifier” claims (EPA; CDC).
  • “A filter and a purifier are the same thing.” A purifier claims viruses too; most portable filters do not remove viruses (CDC).
  • “Certified once, certified forever.” Certification carries ongoing inspection and retesting, and certifiers can de-certify (NSF; WQA). A one-time lab test has none of that.
  • “Smaller micron number, safer water.” Only an absolute rating means every pore is that small, and no pore size removes chemicals (Minnesota Department of Health; CDC).
  • “A certified filter works forever.” Ratings are for a stated capacity and correct maintenance (EPA).

Common questions

What does NSF 53 mean on a water filter?

NSF/ANSI 53 certifies that a filter reduces a contaminant with a health effect, such as lead, cysts or PFOA and PFOS. Each contaminant is a separate claim, so check which ones are listed for that exact model in the certifier’s directory.

NSF 53 or NSF 58: which is better?

They cover different systems: 53 is for filters that reduce health contaminants, and 58 is for reverse osmosis systems. Neither is better in general; what matters is whether the model is certified for the contaminant you care about. The EPA names both for PFAS reduction.

Is NSF 42 enough for drinking water safety?

No. NSF/ANSI 42 covers aesthetic impurities such as chlorine taste and odor. It says nothing about germs or health contaminants.

Does a water filter remove viruses?

Most portable filters do not, according to the CDC. A purifier claims to; certification to NSF P231 is the standard written for that claim. The CDC says reverse osmosis removes viruses and that ultrafilters of about 0.01 micron “should be effective” against them.

What does 99.9999% mean on a water filter?

It is a 6 log reduction: about 1 germ left from every million (1,000,000 × 0.000001 = 1). The CDC uses 99.9999% for bacteria in its description of a purifier.

Is WQA certification as good as NSF certification?

The EPA lists WQA alongside NSF, CSA Group, IAPMO R&T and UL as ANSI-accredited certifiers that evaluate filters against the NSF/ANSI standards. What matters is the listing for your model and your claim, not which of the five issued it.

Where to go next

Sources