Water filter marketing often just says "certified" or "filters contaminants" without specifying what that actually means — the real information is in specific certification standard numbers most buyers never check.
NSF certification numbers mean specific things
NSF International tests and certifies water filters against specific standards, each covering different categories of contaminants — a filter isn't just generically "NSF certified," it's certified to specific standard numbers that specify exactly what it's been tested to reduce. Common ones include NSF/ANSI 42 (covers aesthetic effects like chlorine taste and odor, not health-related contaminants), NSF/ANSI 53 (covers specific health-related contaminants like lead and certain chemicals, but only the specific ones the filter was tested against), and NSF/ANSI 401 (covers a broader set of emerging contaminants including some pharmaceuticals and pesticides). A filter certified to NSF 42 alone improves taste and odor but says nothing about removing lead or other health-related contaminants — checking which specific standard, and which specific contaminants under that standard, a filter is certified for matters far more than the word "certified" by itself.
Know what's actually in your water before choosing a filter
Different filter technologies target different contaminants, and there's no single filter type that addresses everything equally well. A water quality report (in the US, utilities are required to provide an annual water quality report, and private well water can be tested independently) tells you what's actually present in your specific water supply, which is the right starting point for choosing a filter that addresses your actual water rather than guessing based on general marketing claims.
Filter types trade off differently
Activated carbon filters (common in pitcher filters and many faucet-mounted systems) are effective against chlorine taste/odor and some organic contaminants, relatively inexpensive, but generally don't remove dissolved minerals, fluoride, or many heavy metals effectively. Reverse osmosis systems remove a much broader range of contaminants, including many the simpler carbon filters miss, but are more expensive, need professional or DIY installation, waste some water in the filtration process, and also remove beneficial minerals along with contaminants. Whole-house systems treat all water entering a home rather than a single tap, appropriate for concerns like sediment or chlorine affecting the whole house, but are a bigger investment and installation project than a single-tap filter.
The one thing people forget
Replace filter cartridges on the manufacturer's recommended schedule, not just when water flow noticeably slows or taste changes — a filter's contaminant-removal capacity typically declines well before flow rate or taste give any obvious warning sign, meaning a filter can be well past its effective lifespan while still looking and functioning normally.