Cysts (Cryptosporidium and Giardia)

What it is

Cysts are the dormant, shelled form of two protozoan parasites, Cryptosporidium and Giardia. They enter drinking water through fecal contamination from humans or animals, often when surface water carrying runoff, sewage, or agricultural waste reaches a water supply. The cyst's tough outer wall lets the organism survive in water for long periods and resist standard chlorine disinfection.

Why it matters

EPA regulates Cryptosporidium and Giardia under its Surface Water Treatment Rules using a treatment-technique requirement rather than a numeric maximum contaminant level, meaning utilities must filter and disinfect to specified standards instead of meeting a single concentration limit. Because these cysts resist chlorine, they have driven boil-water advisories when treatment is disrupted. We score whether a filter is certified to reduce this contaminant, not any health outcome.

What removes it

Cyst reduction is a mechanical claim under NSF/ANSI 53, typically met with a roughly 1-micron absolute filter that physically strains the cysts out, or with reverse osmosis certified to NSF/ANSI 58. The trap is that an ordinary carbon pitcher is not built to an absolute pore size and is not certified for cysts, so it should not be assumed to remove them. Certification specifically for cyst reduction is what separates a filter that is verified to handle these parasites from one that only claims general filtration.

Reference: EPA Surface Water Treatment Rules (treatment-technique requirement, Cryptosporidium and Giardia); NSF/ANSI 53 (cyst reduction) and NSF/ANSI 58 (reverse osmosis).