How to read water-filter certification labels

Choose a filter for the problem you want to address, then check the exact product and its reduction claim. A box's certification numbers need to be read alongside the claim you need.

Read the standard and its claims

NSF explains that standard numbers follow the order in which standards were developed; they are not a quality ranking. NSF/ANSI 42 concerns aesthetic reduction claims, such as chlorine, taste and odour. NSF/ANSI 53 concerns reduction of contaminants with health effects. Neither number means that a filter reduces every possible contaminant. NSF's consumer standards guide.

The standard tells you which kind of claim to look for. For a specific contaminant, find its name in the listing. A taste-reduction claim does not answer a question about a different contaminant.

NSF/ANSI 58 covers point-of-use reverse-osmosis systems. TDS reduction is a required claim, while claims such as lead, fluoride and nitrate/nitrite reduction are optional. Check the individual claims as well as "58" on the label. NSF's technical overview of Standard 58.

Check the exact model

NSF's drinking-water treatment directory has fields for manufacturer, brand or model, product standard and reduction claims. Its claim search can return products certified for any selected claim, or for all selected claims when that option is checked. Choose the search that matches your question and read the claims in each result. NSF's certified-product directory.

WQA's Gold Seal directory also provides manufacturer, brand or model, contaminant and product-standard search fields. If the package names WQA as its certifier, check its documentation there. WQA's certified-product search.

Before ordering, keep a record:

• Copy the system's model designation from its label or manual.
• Record the replacement cartridge or component you intend to buy.
• Save the matching certification entry and the reduction claim you need.
• Read the product's performance data sheet and operating instructions alongside the entry.

Directory formats need not be identical. If a designation does not match, ask the certifier or manufacturer to resolve it. A similar-looking model or a familiar brand name is insufficient.

Certification and a laboratory test

WQA describes regular product retesting and annual manufacturing-facility inspections as parts of its certification program. In our reading, a statement about one laboratory test should not be treated as equivalent to a current certification listing. Ask who issued the certification and where you can check the documented claim. WQA's certification controls.

Check whether you can follow the installation, use and replacement instructions. Keep the paperwork for later cartridge purchases. In a rental, ask the host for the model and instructions before relying on the filter on the counter.

If the property has a water notice, begin with its instructions. Our water-notice guide helps you identify what the issuer asks you to do. A product choice needs its own evidence, separate from the response to a current notice.