Guide
Water filter certifications: what NSF and WRAS marks mean and how to check one
Filter pages are full of logos and standard numbers: “NSF certified”, “WRAS approved”, “tested to NSF 53”. They sound alike and cover very different things. A certification is a statement about one product and one claim, so what matters is whether the exact model you are considering is listed for the claim you care about.
What the common marks cover
NSF certification means NSF has independently tested the product, and lists it against a named standard for the specific claims in its listing. It applies to the listed model and those claims, and to nothing else about the product.
NSF is an independent standards and certification organisation. Each of its drinking-water treatment standards has its own number. The numbers identify different standards; they are not grades, and a higher number isn’t better (NSF numbers them in the order they were developed). NSF explains the main ones in its own article on water treatment standards.
| Name | What it covers | What it does not tell you |
|---|---|---|
| NSF/ANSI 42 | Aesthetic effects: things like taste, smell and appearance of the water. | Nothing about health-related contaminants. |
| NSF/ANSI 53 | Health-effect contaminants. Certification is for the specific contaminants a product is listed for. | That it reduces anything beyond its listed claims. A product certified for one contaminant isn’t certified for the rest. |
| NSF/ANSI 58 | Reverse osmosis drinking-water treatment systems. | A product certified to 58 only has the claims listed in its entry. It isn’t a blanket result. |
| WRAS approval | A voluntary way for a manufacturer to evidence that a fitting or material is of an appropriate quality and standard under the water fittings regulations (regulation 4(1)(a)), with conditions of use listed in the WRAS approvals directory. | That your installation complies, or that the product is suitable for your circumstances: the installer checks where and how it is used. It says nothing about what a filter removes. |
| Maker’s own claim or “tested to” | What the manufacturer says it has done or found. | Whether anyone independent has confirmed it for this model. |
Checking a claim, step by step
- Write down the exact model and variant, plus the cartridge code. A product family name isn’t enough.
- Find the entry in the certifier’s own listings, not the maker’s page. For NSF use its certified products and systems search; for WRAS, the approvals directory.
- Check the entry names your model. A near match, a sibling product or a different cartridge doesn’t count.
- Read the claim line. Which standard, and which contaminants or effects, and at what conditions? That line is the certification.
- Compare it with what the product page says. Which advertised claims are covered by the entry, and which aren’t?
- Check the date and status, and whether the entry applies to the cartridge you will actually buy and replace.
If you can’t find the entry, you can’t say the claim is independently confirmed. It might be, but ask the seller for the certificate and check it against the listing.
Wording to be careful with
- “NSF certified” with no standard. Certified to what, and for which claims?
- “Tested to” or “meets” a standard. That can describe the maker’s own testing. Look for a certification listing.
- A standard number used as a mark of quality. 53 is not better than 42; they are different standards covering different things.
- A certificate for a different product in the range, or for the old version of the cartridge.
- “WRAS approved” presented as a performance claim, or as proof an installation is compliant. It is a statement about the product’s materials and quality, with conditions of use.
What a certificate can’t tell you
A certificate says nothing about your own water. If you have a specific worry, your supplier publishes water-quality information for your area, and the Drinking Water Inspectorate’s guidance for England and Wales is the place to read about home treatment in general: domestic water filters and softeners. A certified claim also only holds while the cartridge is within its rated life, so it ties back to how often you change it. For the labels we use on Tap Sorted, see How we check.
Standards are revised and listings change; always check the current entry for the exact model.