Filters
How to choose a water filter: certifications decoded
The label 'removes 99% of contaminants' means nothing without the right certification. Here's what each NSF/ANSI standard covers and how to verify a filter in two minutes.
Filter marketing is full of big percentages and vague claims. The only reliable way to know what a filter removes is independent certification to an ANSI/NSF standard — and, crucially, checking that the certification lists the specific contaminant you care about.
The standards that matter
| Standard | What it covers | Typical use |
|---|---|---|
| NSF/ANSI 42 | Aesthetic effects: chlorine, taste, odour, particulates | Pitchers, fridge filters, whole-house carbon |
| NSF/ANSI 53 | Health effects: lead, VOCs, TTHMs, cysts, and — where listed — PFOA/PFOS | Pitchers, faucet mounts, under-sink carbon block |
| NSF/ANSI 58 | Reverse osmosis systems: TDS plus listed contaminants like arsenic, nitrate, fluoride, PFAS, lead | Under-sink and countertop RO |
| NSF/ANSI 401 | Emerging compounds: certain pharmaceuticals, pesticides, BPA and microplastics | Often paired with 42/53 |
| NSF/ANSI 55 | Ultraviolet (UV) disinfection — Class A for microbiologically unsafe water | Wells, cabins, boil-water-prone areas |
| NSF/ANSI 44 | Water softeners (cation exchange) | Whole-house softeners |
| NSF/ANSI 62 | Distillation systems | Countertop distillers |
| NSF/ANSI 61 | Materials safety for components that touch drinking water | Pipes, valves, filter housings |
These standards are written through the American National Standards Institute process. Products can be certified by several accredited bodies: NSF International, the Water Quality Association (WQA Gold Seal), IAPMO, CSA Group and UL. Any of them is fine — what matters is the listing.
“Tested to” vs “certified to”
- Certified to NSF/ANSI 53: an accredited body tested the product, audits the factory, and lists it publicly. This is what you want.
- “Tested to NSF/ANSI 53 standards” or “meets NSF standards”: the company may have paid a lab for a one-time test. Not the same thing.
Verify a filter in two minutes
- Find the model number on the box or listing.
- Search the certifier’s public database (NSF, WQA, IAPMO, CSA or UL listings).
- Check that the listing includes your contaminant by name — e.g. “Lead,” “PFOA/PFOS,” “Arsenic V,” “Nitrate.”
- Note the rated capacity (gallons or litres). A filter certified for lead is only certified up to that capacity.
Matching filters to problems
| Your concern | Minimum you need |
|---|---|
| Chlorine taste or smell | NSF/ANSI 42 |
| Lead | NSF/ANSI 53 (lead) or 58 |
| PFAS (PFOA/PFOS) | NSF/ANSI 53 or 58 with PFOA/PFOS listed |
| Disinfection byproducts (TTHMs) | NSF/ANSI 53 (VOC/TTHM) |
| Nitrate, arsenic, fluoride, uranium | NSF/ANSI 58 (RO) with that contaminant listed |
| Bacteria, viruses | NSF/ANSI 55 Class A UV (or boil during notices) |
| Microplastics | NSF/ANSI 401 (microplastics) or a fine carbon block / RO |
| Hardness | NSF/ANSI 44 softener |
Run the Filter Finder to combine several concerns into one setup.
Maintenance is half the job
A neglected filter can perform worse than no filter. Saturated carbon stops capturing contaminants, and wet media can grow bacteria. Replace cartridges at the rated capacity or time, whichever comes first; replace RO membranes per the maker’s schedule; and sanitise housings when you change cartridges.
Bottom line
Ignore the percentages on the box. Pick the standard that matches your problem, confirm the listing names your contaminant, and set a calendar reminder for cartridge changes.
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Get my free water check →Sources and methodology: see our editorial policy. This guide is general information, not medical or legal advice.