Legionella control for building water systems
Legionella bacteria can pose potential health risks when building water systems are not effectively managed. Organizations responsible for complex water systems may face regulatory, operational and reputational impacts if potential Legionella risks are not identified and addressed.
Legionella control focuses on understanding system conditions, recognizing factors that may contribute to bacterial growth and applying appropriate management practices to help reduce risk. Effective control typically requires a structured, risk-based approach tailored to the specific characteristics and use of a building’s water systems.
UL Solutions supports organizations through a range of Legionella control services designed to help customers evaluate risk, align with applicable guidance and inform water safety management strategies. These services are intended to support informed decision-making related to water system design, operation and maintenance.
Protecting against Legionnaires’ disease
Why Legionella control matters
Legionnaires’ disease is a potentially serious form of pneumonia associated with exposure to Legionella bacteria. These bacteria can grow in building water systems — such as hot and cold water networks, cooling towers and other engineered systems — This can present serious health risks when conditions support growth and aerosolization.
Understanding where risks may arise within water systems is a critical step in managing potential exposure and supporting occupant health.
Comprehensive services for water system safety
Legionella control may involve multiple coordinated activities, including risk assessment, system review, sampling strategies and management planning. UL Solutions offers advisory services to help organizations identify potential risk factors and support the development of water safety and control approaches aligned with recognized guidance and standards.
Our services include:
- Legionella risk assessments
- Water system safety management guidance
- Sampling coordination and interpretation support
- Guidance on corrective actions and system management considerations
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Who is responsible for managing Legionella risk?
Organizations with responsibility for building water systems — such as employers, landlords, facilities managers and property owners — may have obligations related to Legionella management depending on jurisdiction, building type and occupancy.
Understanding duty holder responsibilities and how Legionella guidance applies to a given operation can help organizations assess risk and determine appropriate next steps for managing water systems responsibly.
Legionella control in North America
Standards and guidance in the U.S. and Canada
In North America, Legionella management commonly aligns with guidance from organizations such as the Centers for Disease Control and Prevention (CDC), American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE), and Centers for Medicare & Medicaid Services (CMS). These frameworks outline expectations related to water system evaluation, control measures and documentation, particularly for healthcare and other highrisk environments.
Legionella risk assessment and prevention
Legionella risk assessments typically involve a structured review of building water system components, operating conditions, and existing controls to identify areas where bacterial growth may occur.
Prevention strategies often focus on system operation, maintenance practices, and control measures intended to address identified risks.
Consulting and sampling services
Consulting and sampling services can provide valuable insight into water system conditions and support informed decisionmaking. Legionella sampling may help organizations better understand microbial presence, while consulting services help place sampling results in the context of system operation and overall risk management.
Monitoring and corrective actions
Ongoing monitoring and periodic review of water system conditions can support early identification of changes that may increase risk. Corrective actions may be recommended when monitoring data or assessment findings suggest that adjustments to system management practices could be appropriate.
Legionella control in the U.K. and EU
Regulatory requirements for Legionella control
In the U.K. and EU, Legionella management is governed by health and safety regulations that assign specific responsibilities to duty holders. These frameworks outline expectations related to risk assessment, documentation and ongoing water system management.
Approved Code of Practice (ACoP) L8
ACoP L8 provides guidance on identifying and managing Legionella risks in workplace and public water systems, setting expectations for consistent and documented control approaches.
Risk assessment requirements
Risk assessments focus on identifying hazards, evaluating system conditions and determining where Legionella risks may arise.
Control schemes and the responsible person role
Control schemes describe how water systems are managed in practice and often designate a responsible person to oversee implementation, reporting and review.
Recordkeeping and review
Maintaining documentation and regularly reviewing control measures supports accountability and consistency in Legionella management activities.
HSG274: Technical guidance supporting ACoP L8
HSG274 provides practical guidance on managing different types of water systems, including:
- Evaporative cooling systems (Part 1)
- Hot and cold water systems (Part 2)
- Other risk systems, such as spa pools and decorative water features (Part 3)
The guidance addresses system operation, monitoring approaches, maintenance considerations and supporting water quality parameters.
European technical guidelines for Legionella
European technical guidelines provide a shared reference across EU jurisdictions and address:
- Purpose and scope of Legionella management
- Risk assessment protocols
- Sampling and laboratory methods
- Remedial actions and monitoring plans
- Outbreak investigation procedures
Take the next step in strengthening your Legionella risk management strategy
Frequently asked questions about Legionella control
- What are the health risks associated with Legionella bacteria?
Legionella bacteria can cause Legionnaires’ disease, a potentially serious form of pneumonia, as well as Pontiac fever, a milder flu‑like illness. Infection may occur when individuals inhale aerosolized water containing Legionella bacteria. Severity of illness can vary depending on individual health factors, with older adults, smokers and people with weakened immune systems generally considered at higher risk.
- What are the common sources of Legionella contamination?
Legionella bacteria may develop in building water systems where conditions support growth and aerosolization. Common sources include hot and cold water systems, cooling towers, evaporative condensers, decorative water features, spa pools and other engineered water systems. Stagnant water, inadequate temperature control and insufficient disinfection can increase risk.
- How do I know if my building water system is at risk for Legionella?
A building water system may be at risk if it includes complex distribution networks, areas of low water flow, warm water temperatures, significant nutrients for bacterial growth or lack of chemical or thermal control. Legionella risk assessments typically evaluate system design, operating conditions, maintenance practices and existing controls to identify areas where bacterial growth may occur.
- What industries are most vulnerable to Legionella outbreaks?
Industries that manage large or complex water systems or serve vulnerable populations may face elevated Legionella risk. This can include healthcare facilities, senior housing, hotels and hospitality venues, commercial buildings, educational institutions, and industrial or manufacturing sites with cooling towers or process water systems.
- What is the difference between Legionella testing and Legionella sampling?
Legionella sampling refers to the collection of water samples from building systems, while Legionella testing involves laboratory analysis of those samples to detect and quantify Legionella bacteria. Sampling provides data points, while testing results are typically interpreted within the broader context of system conditions and risk management practices.
- How often should Legionella sampling be performed for compliance?
Sampling frequency may vary based on jurisdiction, building type, system complexity, and risk profile. Some guidance recommends routine or risk‑based sampling, particularly for high‑risk systems or environments. Organizations often determine sampling frequency as part of a broader water safety or risk management program aligned with applicable guidance.
- What happens if I fail to comply with ACoP L8 requirements in the U.K.?
Failure to demonstrate adequate Legionella risk control can result in enforcement action — including improvement notices, prohibition notices or other legal consequences. Beyond regulatory risk, inadequate control measures can increase health risks and damage an organization’s reputation. Many organizations use ACoP L8 as a practical benchmark for showing they are exercising reasonable and proportionate control of Legionella risk.
- Can alternative methods be used instead of the methods outlined in ACoP L8?
Yes. In the U.K., duty holders may use alternative approaches to those outlined in ACoP L8 if they can demonstrate that those methods provide an equivalent or higher level of risk control. Any alternative approach should be supported by appropriate risk assessment, documentation and technical justification.
- What were the key updates in the European technical guidelines for Legionella in 2025?
Recent updates to European technical guidance emphasize strengthened risk assessment protocols, enhanced sampling and laboratory methods, clarified remedial action thresholds and expanded guidance for outbreak investigation and monitoring. The updates support more consistent Legionella management practices across EU member states.
- What corrective actions are recommended if Legionella is detected in a water system?
Corrective actions may vary depending on detection levels, system type and regulatory expectations. Actions can include operational adjustments, system cleaning or disinfection, temperature or flow changes, review of existing controls, and follow‑up monitoring. Guidance typically recommends basing actions on a combination of testing and risk assessment rather than test results alone.
- Do cooling towers and Air Scrubbers require special Legionella control measures?
Yes. Cooling towers, evaporative cooling systems and air scrubbers are commonly considered higher‑risk for Legionella because they can generate and disperse aerosolized water. There have been many high profile Legionella outbreaks linked to these types of systems. Control measures often include regular inspection, cleaning and disinfection, water treatment programs and routine monitoring consistent with applicable guidance and regulations.
- What role does water temperature play in Legionella growth?
Water temperature is a key factor in Legionella growth. The bacteria commonly thrive in warm water environments, typically between approximately 20 degrees C and 45 degrees C (68 degrees F to 113 degrees F). Managing water temperatures outside this range is often considered an important element of Legionella risk control strategies.
- How can UL Solutions help with outbreak investigations and emergency response?
intended to support informed decision‑making and align response activities with applicable public health and regulatory expectations.
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