How to choose a water filter by contaminant — and what NSF certifications mean

PurityRadar Editorial

8 min read

[ Choosing a water filter ]

There is no single "best" water filter — only the best filter for *your* water. A filter that's excellent at improving taste may do nothing for lead, and a system that removes PFAS may be overkill if your only issue is chlorine. The process is simple: find out what's in your water, then match the technology and certification to it.

Step 1: know your contaminant

Start with your water report, your utility's annual report, and — for lead or PFAS — a certified test. Buying a filter before you know your contaminant is guessing.

Step 2: match the technology

Activated carbon (most pitchers, faucet, fridge filters): great for chlorine, taste, odor, some VOCs, and — if certified — lead. Limited against dissolved salts and nitrate.
Reverse osmosis (RO): the broadest reduction — lead, PFAS, arsenic, nitrate, and more — at the cost of slower flow and some water waste. Usually installed under the sink.
Ion exchange / water softeners: target hardness (calcium, magnesium) and some metals; softeners don't make water "safe," just softer.
Distillation and UV: distillation removes most contaminants; UV disinfects microbes but doesn't remove chemicals.

Step 3: insist on NSF/ANSI certification

A filter's marketing claims mean little without independent certification. NSF/ANSI standards (certified by NSF and others) tell you exactly what a product is proven to reduce:

NSF/ANSI 42 — aesthetic effects: chlorine, taste, and odor.
NSF/ANSI 53 — health effects: lead, certain VOCs, cysts like Cryptosporidium.
NSF/ANSI 58 — reverse osmosis systems (covers contaminants like arsenic, nitrate).
NSF/ANSI 401 — "emerging" contaminants such as some pharmaceuticals.
NSF/ANSI 53 or 58 for PFOA/PFOS — the certifications to look for to reduce PFAS.

Step 4: match certification to your contaminant

This is the step most people skip. If your concern is lead, a filter certified only to NSF/ANSI 42 (taste) won't help — you need 53. For PFAS, look specifically for PFOA/PFOS reduction. Always confirm the certification covers *your* contaminant, not just "water filtration" in general.

The bottom line

Test first, then buy a filter independently certified to reduce the specific contaminant you have — and maintain it on schedule, since an expired cartridge can be worse than none. We keep neutral, non-affiliate filter guidance in our Filter Guide.

Frequently asked

What water filter removes the most contaminants?

Reverse osmosis (RO) systems reduce the broadest range of contaminants, including lead, PFAS, arsenic, and nitrate, especially when certified to NSF/ANSI 58. The trade-offs are slower flow, some water waste, and higher cost than carbon filters. Match the system to a certified test of your water.

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

They identify which independent standard a filter is certified against: 42 for aesthetics (chlorine, taste), 53 for health effects (lead, VOCs, cysts), 58 for reverse osmosis, and 401 for emerging contaminants like pharmaceuticals. Always confirm the certification covers your specific contaminant.

Do I need a whole-house filter or just one for drinking water?

For most health contaminants you drink (lead, PFAS, nitrate), a certified point-of-use filter at the kitchen tap is sufficient and far cheaper. Whole-house systems make sense mainly for issues affecting all water use, like sediment, hardness, or system-wide chlorine.

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