Legionnaires’ disease is a severe respiratory illness caused by the bacterium Legionella pneumophila and closely related species. It is frequently associated with cooling towers, which are large mechanical systems used to remove heat from buildings and industrial facilities. Cooling towers can emit fine mists and aerosols that may contain bacteria. Once released, these aerosols can travel through the air and may expose nearby populations. However, it is not fully known how environmental conditions and system design influence infection risks. This project will combine field data, laboratory experiments, and modeling to identify and quantify the biological and environmental parameters that determine risks of disease transmission. Results from the project will help improve the management of cooling tower water quality, treatment, and operating conditions to reduce health risks. In addition, the project will provide training opportunities for students and early-career researchers in environmental engineering and public health, helping to strengthen the science and engineering workforce needed to address environmental health challenges. This project will develop a mechanistic and predictive framework to understand how environmental and operational conditions influence the persistence, aerosolization, transport, and risk of Legionella pneumophila bacteria from cooling tower systems. The research will integrate laboratory experiments, field measurements, and modeling to quantify interactions among Legionella, free-living amoeba hosts, and cooling tower water chemistry under realistic operating conditions. Controlled experiments will evaluate how factors such as nutrient levels, disinfectant residuals, pH, and desiccation influence pathogen–host dynamics in both pure cultures and pilot-scale cooling tower reactors designed to replicate operational environments. Field campaigns at full-scale cooling towers will measure aerosol generation rates and droplet size distributi