A Legionella risk assessment is a statutory requirement for any dutyholder responsible for water systems in premises accessible by others. Under the Health and Safety at Work Act 1974 and the Control of Substances Hazardous to Health Regulations 2002, dutyholders must identify and assess the risk of Legionella bacteria in their water systems. The HSE's Approved Code of Practice L8 and the accompanying HSG274 technical guidance set out exactly how this must be done. Legionella pneumophila is a bacterium found naturally in water sources that can multiply to dangerous levels in artificial water systems where temperatures are between 20°C and 45°C and where conditions favour biofilm growth. Inhalation of contaminated aerosols causes Legionnaires' disease, a potentially fatal form of pneumonia. A compliant risk assessment involves a thorough physical inspection of all water assets including cold water storage tanks, calorifiers, showers, cooling towers, and distribution pipework. The assessor measures water temperatures, identifies dead legs and stagnant areas, reviews system schematics, checks tank conditions, and assesses the overall risk level. The assessment produces a written report identifying all risks, a schematic drawing of the water system, and a written scheme of control detailing the control measures required, the monitoring frequency, and the responsible persons. ACOP L8 requires the assessment to be carried out by a competent person. LCA-registered contractors demonstrate the necessary competence and provide the documentary evidence that dutyholders need to satisfy regulatory inspection. Assessments should be reviewed at least every two years or whenever there is reason to believe the original assessment is no longer valid.