Legionella and Temperature Control: A Complete Guide.

Legionella bacteria are naturally occurring microorganisms commonly found in freshwater environments such as rivers and lakes. However, when these bacteria multiply in man-made water systems, such as those in buildings, they can cause Legionnaires' disease, a severe, often fatal form of pneumonia.

The first recognised outbreak of Legionnaires' disease occurred at the American Legion convention in Philadelphia in 1976, and since then, controlling Legionella in water systems has become a critical part of public health safety.

One of the key strategies for managing Legionella risk is temperature control. Temperature control can be highly effective in preventing Legionella growth, but it should always be part of a comprehensive Legionella management plan. This plan should also include system design, routine maintenance, regular monitoring, and disinfection procedures. To ensure compliance with UK health and safety law, a Legionella risk assessment should always be carried out by a qualified and competent person.

The primary regulatory guidance for managing Legionella in water systems is provided by HSG 274 Part 2: Legionnaires' disease – The control of Legionella bacteria in hot and cold water systems, along with other resources from the Health and Safety Executive (HSE).

How Legionella Behaves at Different Temperatures

Legionella bacteria respond to water temperature in specific ways that influence their growth and survival. Understanding these temperature ranges is key to controlling Legionella in water systems.

• Below 20°C: At temperatures lower than 20°C, Legionella bacteria remain dormant. In this state, the bacteria grow very slowly, if at all.

• 20–45°C: This is the optimal growth range for Legionella. Bacteria can multiply rapidly in water within this temperature range, especially if the water is stagnant or has the presence of biofilm or scale.

• 50–60°C: At this range, bacterial growth is inhibited. Legionella bacteria begin to die off when exposed to water temperatures of 50°C or higher. To prevent growth, hot water systems should maintain a temperature of at least 60°C in storage tanks and around 50–55°C at water outlets, such as taps and showers.

• Above 65–70°C: At these higher temperatures, Legionella bacteria die rapidly. Thermal disinfection or pasteurisation is often used to kill bacteria if contamination is detected in the system.

  
  

Given these varying temperature effects, optimal temperature control for Legionella management requires balancing the need to keep hot water systems sufficiently hot to prevent bacterial growth, while also ensuring that cold water systems stay below 20°C to keep bacteria dormant.

What Temperature Strategies can be used for Controlling Legionella

There are two primary temperature management strategies for controlling Legionella: prevention and remediation.

Prevention Strategies

1. Hot Water Systems: Store hot water at temperatures of at least 60°C and ensure that water distributed to outlets reaches 50°C within one minute (in healthcare settings, it should reach 55°C within one minute).

2. Cold Water Systems: Keep cold water as cool as possible, ideally below 20°C, and minimise stagnation by ensuring water flows regularly through the system. Dead legs (sections of pipework that do not receive regular flow) should be removed or isolated to prevent water from stagnating.

3. Circulation Loops: It is essential to monitor all circulation loops, including principal, subordinate, and tertiary loops, to ensure that no section falls into the temperature range where Legionella can grow. Regular monitoring of these loops helps prevent Legionella proliferation.

   
   

Remediation Strategies

1. Pasteurisation / Thermal Disinfection: If Legionella is detected in the system, raise the water temperature temporarily to greater than 70°C to kill the bacteria. After thermal disinfection, all outlets should be flushed according to the recommended procedures to ensure the bacteria are completely eradicated.

2. Maintenance: Routine cleaning and de-scaling of water tanks, pipes, showers, and taps are important to remove scale, sediment, and biofilm, which provide an ideal breeding ground for bacteria. This should be part of a regular maintenance schedule to prevent the regrowth of Legionella.

   
   

Instrumentation and Monitoring

To ensure that temperature control is effectively managed and that the water system remains safe, proper monitoring and instrumentation are required.

1. Thermometers: Use accurate thermometers that are suitable for the specific type of water system. For instance, immersible thermometers are often used for tank monitoring, while surface thermometers or pocket thermometers are used for individual outlets.

2. Calibration: Regularly calibrate all thermometers and other instruments to maintain accuracy. This is essential for ensuring that your temperature readings are reliable and can be used as evidence of compliance.

3. Temperature Profiling and Logging: Implement a temperature profiling system, which involves monitoring the temperature at sentinel outlets, return legs, and all sections of the water system. Maintaining detailed records of temperature readings will help demonstrate that appropriate controls are in place and that the system is in compliance with HSE guidance.

   
   

Temperature Requirements for Different Systems

HSG 274 Part 2 outlines specific temperature requirements for different water systems. These guidelines help landlords, building managers, and healthcare providers manage the risk of Legionella effectively:

• Cold Water: Maintain cold water temperatures below 20°C wherever possible.

• Hot Water Storage: Hot water should be stored at 60°C or higher, and water distributed to outlets should reach 50°C within one minute (for NHS and care settings, water should reach 55°C).

• Low Volume Water Heaters: Ensure that small water heaters reach the recommended temperatures to prevent bacterial growth in areas with limited hot water demand.

• Water Loops: Different loops in the system—principal, subordinate, and tertiary—should be regularly monitored to prevent stagnation or a drop in temperature that might allow Legionella to thrive. This is especially important in healthcare settings, care homes, and large multi-unit buildings where water systems are complex.

  
  

Why a Legionella Risk Assessment is Essential

While temperature control is an important part of the strategy for managing Legionella, it is not sufficient by itself. A Legionella risk assessment is essential to ensure that all potential hazards in the water system are identified and addressed. A comprehensive risk assessment will:

• Identify high-risk areas in the water system where Legionella is most likely to proliferate.

• Recommend control measures beyond temperature, such as chemical treatments, regular flushing schedules, and preventative maintenance.

• Ensure that a robust written scheme of control is in place, outlining all the measures taken to reduce Legionella risk.

• Demonstrate compliance with UK health and safety legislation, protecting both tenants and employees from Legionnaires' disease.

  
  

A competent, qualified assessor is needed to carry out the risk assessment, ensuring that appropriate measures are implemented in a legally compliant manner. This reduces the risk of Legionella outbreaks and helps protect the health of building occupants.

Temperature control is a critical element in the management of Legionella bacteria in water systems. By maintaining cold water systems below 20°C, hot water systems above 50–60°C, and regularly monitoring all water loops and outlets, you can significantly reduce the risk of bacterial growth and proliferation.

If Legionella contamination is detected, thermal disinfection (pasteurisation) can be applied to kill the bacteria.

Temperature control alone is not enough to guarantee safety. A full Legionella risk assessment, carried out by a qualified professional, is essential to identify hazards, implement effective controls, and ensure ongoing compliance with UK health and safety laws. By combining risk assessments, comprehensive control measures, regular monitoring, and effective temperature management, businesses, landlords, and healthcare providers can safeguard their water systems and protect the health of those who use them.