Evaluating Legionella Risk Reduction Strategies in a Large Healthcare Setting: An Integrated Forward and Reverse QMRA Approach

Abstract

Legionella pneumophila is a leading cause of drinking water–associated outbreaks in the United States and presents a significant risk in healthcare facilities where complex plumbing systems and immunocompromised patient populations increase susceptibility to infection. Although water management programs and secondary disinfection systems are widely implemented, translating environmental monitoring data into meaningful health risk estimates remains challenging. This study evaluates Legionella risks at the University of Washington Medical Center (UWMC) Montlake Campus using an integrated Quantitative Microbial Risk Assessment (QMRA) framework. Environmental surveillance data collected between 2018 and 2021 from two major hospital buildings—Cascade Tower and Pacific Tower—were analyzed to characterize the occurrence and distribution of Legionella within the facility’s water systems. Forward QMRA models were applied to estimate infection risks associated with exposure to aerosolized water from showers and handwashing sinks, incorporating measured Legionella concentrations, exposure scenarios, and established dose–response relationships. In addition, a reverse QMRA approach was used to assess fixture-specific infection risks and determine the microbial concentrations corresponding to established health risk benchmarks. This analysis enabled comparison of potential infection risks associated with different fixtures and helped identify areas where targeted interventions may be most effective. The integrated QMRA framework provides a practical approach for translating environmental monitoring data into quantitative health risk estimates and supports evidence-based decision-making for Legionella management in complex healthcare water systems.