How activated carbon works

Carbon removes organic molecules by adsorption onto an enormous internal pore surface. Capacity is finite, and an exhausted filter looks exactly like a fresh one.

Certification is specific: A carbon filter certified to NSF/ANSI 53 for lead is certified for lead. It is not thereby certified for PFAS, or for anything else.
Why replacement discipline matters: Performance declines gradually and invisibly. A long-overdue cartridge can pass through everything it was fitted to remove.
Biological growth: A damp, nutrient-rich carbon bed supports bacterial growth. Household filtered water should be refrigerated and used within a day or two.

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Water containing chlorine, pesticides and taste compounds enters a bed of activated carbon granules. Each granule has an internal pore network with a surface area around a thousand square metres per gram. Organic molecules stick to those pore surfaces and are removed from the water. Dissolved salts, nitrate and metals in solution pass through unaffected. Once the pore surfaces are occupied, the filter stops working, with no visible change.