What is Legionella

Legionella is a group of water-borne bacteria that thrive in warm, stagnant water systems and can cause a serious lung infection known as Legionnaires’ disease when inhaled as fine droplets. These bacteria occur naturally in lakes and rivers but become hazardous when they multiply in man-made systems where water is stored, circulated, or sprayed into the air.

If you’re a facilities manager, you’ll recognise that any building with stored or recirculated water can present a risk - from hot and cold-water systems to cooling towers and spa pools. The bacteria themselves are not visible, but the right conditions can allow rapid growth and the formation of biofilm, a slimy layer inside pipes and tanks that provides shelter and nutrients.

How Legionella Grows And Where The Risk Lies

Legionella bacteria grow best in warm water and poorly managed systems. The combination of temperature, stagnation, and nutrients can quickly turn a low-risk system into a serious hazard.

Key growth conditions and temperature control

Growth is most likely when:

• water temperature sits between 20 °C and 45 °C,

• water remains stagnant in pipework or tanks,

• scale, rust, or organic matter form a biofilm, and

• the system produces aerosols (tiny droplets) that can be inhaled.

Maintaining effective temperature control is one of the simplest and most reliable prevention methods. Cold-water should be kept below 20 °C, and hot-water storage above 60 °C, reaching outlets at 50 °C or more within a minute of running.

Typical systems to monitor for water hygiene management

Legionella can colonise almost any system where water is stored or recirculated. Common examples include:

• Water storage tanks and calorifiers

• Showers, taps, and spa or hydrotherapy pools

• Cooling towers and evaporative condensers

• Pipework dead legs or infrequently used outlets

• Thermostatic mixer valves (TMVs), which can maintain water in the ideal growth range if not managed correctly

Each of these should be part of a formal monitoring programme that includes temperature checks, flushing routines, and regular cleaning or disinfection.

Legal responsibilities and control measures

As someone responsible for managing buildings, you are a duty-holder under UK health and safety law. This means you must assess and control the risk from Legionella bacteria.

Your legal duties and guidance

The Health and Safety Executive (HSE) requires that all employers and those in control of premises carry out a Legionella risk assessment and act on its findings. The main guidance document is the Approved Code of Practice L8 (ACOP L8) – Legionnaires’ disease: the control of Legionella bacteria in water systems.

In summary, you must:

• Identify and assess potential sources of risk.

• Implement control measures, such as temperature management and regular cleaning.

• Monitor water systems and record key findings.

• Review the risk assessment whenever systems or usage patterns change.

The UK Health Security Agency (UKHSA) also provides public health advice for cases of Legionnaires’ disease and outbreak control.

Practical steps for maintaining safe water systems

To maintain control over Legionella risk:

• Keep hot-water storage at 60°C or above and distribute so outlets reach 50°C quickly.

• Maintain cold-water storage below 20°C.

• Flush unused outlets weekly to prevent stagnation and dead-leg formation.

• Clean and descale shower heads quarterly.

• Inspect and clean tanks at least annually.

• Sample and monitor open systems such as cooling towers using a UKAS-accredited laboratory.

• Ensure all work is recorded and reviewed by a competent person as part of your written water hygiene management plan.

You can find detailed guidance on implementing these controls in the HSE’s Legionnaires’ disease overview.

Conclusion

Understanding what Legionella is goes hand in hand with understanding your duty to control it. Facilities managers play a crucial role in maintaining safe water systems, reducing the risk of Legionnaires’ disease, and ensuring compliance with health and safety law.

By applying a consistent temperature-control strategy, conducting regular flushing and cleaning, and reviewing your risk assessment, you can significantly lower the risk of Legionella growth in your premises.

For more practical guidance on Legionella control and compliance, visit our Learning Centre, or for detailed support on safe water management, visit our water hygiene services pages.

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