Chlorine (residual disinfectant)

Chlorine (residual disinfectant) is a disinfectant regulated in US drinking water. Added intentionally by water systems to control microbes.

MRDL (Maximum Residual Disinfectant Level) = 4.0 mg/L, enforced as a running annual average — this is a disinfectant limit, not an MCL. MRDLG = 4 mg/L. A detectable residual is required to keep the distribution system safe.

At the residual levels used for disinfection, chlorine can cause eye/nose irritation and stomach discomfort. Its main health relevance is that it forms disinfection byproducts (see TTHM/HAA5).

EPA's enforcement data (SDWIS/ECHO) records 92,396 violations of Chlorine (residual disinfectant)'s rule nationally.

Filter technologies certified to reduce Chlorine (residual disinfectant) include: activated carbon (NSF/ANSI 42). Look for a filter independently certified against the specific NSF/ANSI standard for this contaminant — general "reduces contaminants" marketing claims are not the same as a contaminant-specific certification.