<i>Legionella</i> Occurrence in Buildings: Implications of Public Water System Disinfectant and Residual

ABSTRACT Legionella pneumophila is an opportunistic drinking water microorganism of serious health concern. Anticipated revisions to U.S. EPA's Microbial and Disinfection Byproduct Rules may propose a numeric minimum disinfectant residual—rather than the current “detectable”—in part to better control Legionella in drinking water distribution systems and buildings served. Data to inform distribution system disinfectant residual concentration impacts on L. pneumophila concerns in buildings are scarce. In 2024, the General Services Administration tested water quality, including Legionella , at ~8250 federally owned and leased buildings. This study analyzed summary data from ~5600 buildings, detailed results from 40 buildings, and corresponding distribution system disinfectant residuals to assess links between residual levels and Legionella occurrence. Utilizing this largest and most nationally representative dataset available to date, findings varied by building and water system, with no clear numeric distribution system residual threshold consistently linked to lower Legionella occurrence in buildings.