When you shop for a water filter or reverse osmosis system, you’ll often see “NSF certified” or references to NSF/ANSI 42, 58, or 53. These certifications indicate that a product has been tested by an independent body to meet specific standards for material safety and contaminant reduction. Here’s what the main ones mean and how to use them when choosing a filter.
What NSF Certification Is
NSF International (now NSF) is a nonprofit that develops standards and certifies products for food, water, and consumer goods. NSF/ANSI standards for drinking water treatment are developed with the American National Standards Institute (ANSI) and other stakeholders. Products that earn certification have been tested in NSF-accredited labs to verify they meet the standard’s requirements. So “NSF certified” isn’t a single thing—it always refers to a specific standard (and sometimes a specific claim, like lead or PFOA reduction).
Key Standards for Home Water Filters
NSF/ANSI 42 – Aesthetic effects
- Focus: Reduction of chlorine and particles that affect taste, odor, and appearance.
- Typical products: Faucet filters, many pitcher and under-sink carbon filters.
- Use: Good for improving taste and odor of municipal water; does not address health-related contaminants by itself.
NSF/ANSI 53 – Health effects
- Focus: Reduction of contaminants that can affect health (e.g., lead, cryptosporidium, certain chemicals).
- Products are tested for specific contaminants; the product listing will say which contaminants it’s certified to reduce (e.g., lead, PFOA/PFOS).
- Use: When you want certified reduction of particular contaminants; often used with carbon filters.
NSF/ANSI 58 – Reverse osmosis
- Focus: Reverse osmosis systems—material safety, structural integrity, and reduction of a long list of dissolved contaminants (TDS, lead, arsenic, nitrate, sodium, PFOA/PFOS when claimed, etc.).
- Use: The main standard for under-sink RO systems. Check the product’s official listing for which contaminants it’s certified to reduce.
NSF/ANSI 401 – Emerging contaminants
- Focus: Reduction of certain emerging compounds (e.g., some pharmaceuticals, pesticides, PFAS beyond PFOA/PFOS).
- Not all filters are certified to 401; it’s often an optional claim.
- Use: When you’re specifically concerned about a compound listed under 401; verify the product’s scope.
Other standards (e.g., 55 for UV, 62 for distillation) apply to specific technologies. For most homeowners, 42, 53, and 58 are the ones they’ll see.
How to Use This When Shopping
- Match the standard to your goal. Want better taste? Look for NSF 42. Want reduction of lead, PFAS, or other health-related contaminants? Look for NSF 53 and/or 58 with the right claims.
- Check the exact claims. A product might be “NSF 58 certified” but only for some contaminants. NSF’s online listing or the product packaging will state which contaminants are certified.
- Replace on schedule. Certification assumes proper installation and maintenance. Using non-certified replacement filters or skipping changes can void performance claims.
- Avoid vague wording. “Tested to NSF standards” or “meets NSF 58” without official certification is not the same as “NSF certified.” Look for the NSF mark and verify on nsf.org if needed.
Why It Matters
Uncertified or poorly maintained filters may not reduce contaminants as claimed. Certification gives you an independent check that the product has been tested for material safety and, for 53 and 58, for specific contaminant reduction. When you know your water quality concerns (from a utility report or lab test), you can choose a filter that is certified for those concerns and maintain it so it keeps performing as intended.