Evaporative cooling towers and evaporative condensers represent the highest potential risk for a large-scale Legionnaires' disease outbreak of any water system type. The combination of warm water temperatures, continuous aerosol generation, and atmospheric exposure creates conditions that favour rapid Legionella multiplication if chemical and physical controls are not rigorously maintained. Under the Notification of Cooling Towers and Evaporative Condensers Regulations 1992, all such systems must be registered with the local authority. The technical requirements for managing cooling tower Legionella risk are set out in HSG274 Part 1. An effective cooling tower water treatment programme must address three core control areas. First, microbiological control through a carefully designed biocide programme, typically combining a continuously dosed oxidising biocide such as chlorine or bromine with periodic shock dosing of a non-oxidising biocide to prevent microbial adaptation. Second, corrosion and scale control using a phosphonate or similar corrosion inhibitor combined with a scale inhibitor, to protect heat transfer surfaces and maintain system efficiency. Third, physical cleaning and disinfection at least twice per year, involving system isolation, drainage, mechanical cleaning of fill media and drift eliminators, disinfection with chlorine dioxide or sodium hypochlorite, and documented clearance sampling before return to service. Monitoring must be comprehensive: daily operational checks of dosing equipment, weekly dipslide testing for total viable counts, and monthly UKAS-accredited Legionella culture sampling. Bleed-off management is equally critical — uncontrolled concentration of dissolved solids rapidly compromises biocide efficacy and leads to scaling. All results must be logged and compared against action and trigger levels defined in the written scheme of control.