Monochloramine disinfection of municipal water sup-plies is associated with decreased risk for Legionnaires'disease. We conducted a 2-year, prospective, environmen-tal study to evaluate whether converting from chlorine tomonochloramine for water disinfection would decreaseLegionella colonization of hot water systems. Water andbiofilm samples from 53 buildings were collected forLegionella culture during 6 intervals. Prevalence ratios(PRs) comparing Legionella colonization before and aftermonochloramine disinfection were adjusted for water sys-tem characteristics. Legionella colonized 60% of the hotwater systems before monochloramine versus 4% afterconversion (PR 0.07, 95% confidence interval 0.03–0.16).The median number of colonized sites per buildingdecreased with monochloramine disinfection. Increasedprevalence of Legionella colonization was associated withwater heater temperatures <50°C, buildings taller than 10stories, and interruptions in water service. Increasing use ofmonochloramine in water supplies throughout the UnitedStates may reduce Legionella transmission and incidenceof Legionnaires' disease
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