Legionella is a rod-shaped bacterium found naturally in freshwater environments, but it can become a human health concern when it colonizes man-made water systems. The bacteria cause a group of illnesses collectively known as legionellosis, the most severe form of which is Legionnaires’ disease, a potentially fatal type of pneumonia. People typically contract the disease by inhaling fine water droplets, or aerosols, contaminated with the bacteria. A Legionella Risk Assessment (LRA) is the process used to evaluate and manage this hazard within a building’s water infrastructure. This assessment involves a detailed review of the water system to identify conditions that could encourage bacterial growth and transmission.
Defining the Scope of a Legionella Risk Assessment
The central purpose of an LRA is to identify potential sources of risk where Legionella bacteria could multiply and be dispersed as aerosols. The bacteria thrive in warm water, specifically within a temperature range of 20°C to 45°C (68°F to 113°F). The assessment focuses on conditions that enable this proliferation, such as water stagnation, the presence of sludge or scale, and biofilm formation, which provides nutrients and a protective habitat for the bacteria.
The scope is facility-specific and covers all water-related assets that could potentially generate aerosols or harbor the bacteria. This includes hot and cold water storage tanks, calorifiers, and the associated pipework. Complex systems like cooling towers and evaporative condensers are particular areas of focus due to their high potential for aerosol generation. Decorative water features, humidifiers, and even infrequently used outlets like showers are also included in the comprehensive review.
Legal Requirements and Duty Holders
Legionella Risk Assessments are mandatory for most commercial, industrial, and residential properties where water systems pose a risk to employees or the public. Regulatory frameworks globally, such as the Control of Substances Hazardous to Health Regulations (COSHH) in the UK, classify Legionella as a hazardous substance, requiring a risk assessment. This legal necessity is rooted in the universal principle of duty of care to ensure the health and safety of individuals who may be exposed to the hazard.
The individual or entity accountable for ensuring the assessment is carried out is known as the “Duty Holder”. This role typically falls to the building owner, employer, or person in control of the premises. While the Duty Holder holds the ultimate legal accountability, they frequently appoint a “Responsible Person” with sufficient authority and knowledge to manage the day-to-day control measures. The assessment must be performed by a “competent person,” meaning an individual with the appropriate training, knowledge, and experience in Legionella control and water systems.
Essential Steps in Conducting the Assessment
A competent assessor follows a structured methodology, beginning with a thorough hazard identification phase. This involves a physical inspection and mapping of the entire water system, including the creation of a schematic diagram if one does not exist. The assessor visually checks for system vulnerabilities such as the presence of “dead legs”—sections of pipework with little or no flow—which promote stagnation and bacterial growth.
During this initial phase, temperature monitoring is a primary investigative tool, as it is a direct indicator of risk. Hot water must be maintained at temperatures above 50°C (122°F) at the outlets and cold water must be consistently below 20°C (68°F), as temperatures outside these ranges significantly restrict Legionella growth. The presence of scale, sediment, and sludge within storage tanks and pipework is also noted, as these deposits nourish the bacteria and shield them from disinfection.
The second phase is risk evaluation, where the assessor analyzes the likelihood of human exposure based on the identified hazards. This involves considering how likely the system is to generate contaminated aerosols and whether vulnerable populations, such as the elderly or those with compromised immune systems, are exposed. A risk rating is assigned to each hazard, allowing the Duty Holder to prioritize the necessary corrective actions. The final step is documentation, resulting in a written report that details all findings, the current state of compliance, and specific, actionable recommendations for control.
Implementing Control Measures and Ongoing Monitoring
Once the risk assessment report is finalized, the Duty Holder is required to implement the recommended control measures to mitigate the identified risks. Common physical interventions include removing or modifying dead legs to eliminate stagnant water and cleaning and descaling water storage tanks and calorifiers. Temperature adjustments are often implemented, ensuring hot water storage temperatures are maintained at or above 60°C and distributed above 50°C. Chemical disinfection, involving the controlled dosing of biocides such as chlorine dioxide or sodium hypochlorite, may be introduced to maintain a disinfectant residual. Regular maintenance procedures, such as weekly flushing of infrequently used outlets, are established to prevent water stagnation.
Ongoing monitoring is a necessary component of the control scheme, requiring routine checks of water temperatures at sentinel points and periodic water sampling for Legionella testing. The entire risk assessment and its associated control measures must be reviewed and updated regularly, especially after any significant change to the water system or building use.