The NSF certificate has its own testing protocols to verify a product’s claimed performance and quality against NSF standards. The product claims made for water filters must have been tested and verified when the product is certified by NSF. Especially for the claimed lifetime of the water filter, NSF tests up to 115% of the total capacity of the claimed volume, makes sure the filtered water is safe until the last drop.
NSF/ANSI 42 — Reducing chlorine, taste, and odor
NSF/ANSI 53 — Reducing health-related contaminants like particulates, volatile organic chemicals, and asbestos