What temperature kills Legionella?

What is Legionella, and why is it dangerous?

Legionella is a type of bacteria found naturally in freshwater environments such as rivers, lakes, and reservoirs. In these settings, it rarely causes harm. Problems arise when Legionella bacteria enter building water systems, where conditions can allow them to multiply.

When contaminated water droplets are inhaled – for example, from showers, taps, or cooling towers – Legionella can cause Legionnaires’ disease, a severe form of pneumonia. Symptoms include fever, cough, muscle aches, and breathing difficulties. The illness can be fatal, particularly for older adults, smokers, and people with weakened immune systems.

Because of this risk, Legionella control is a legal requirement for anyone responsible for a building’s water system. UK Legionella guidance makes it clear that you have to manage Legionella in water systems through proper temperature control, monitoring, and regular Legionella testing.

How does Legionella grow in water systems?

Legionella can develop in almost any water system if the conditions are right. Growth is most likely when water is:

• Water sits still for long periods
• Temperatures drift into the risk range
• Scale, rust, or sludge builds up
• Biofilm forms inside pipes or tanks

Biofilm is a common problem. It creates a slimy layer that protects Legionella and makes it harder to remove.

High-risk areas include storage tanks, showers, infrequently used outlets, and long pipe runs. This is why Legionella control measures focus on more than one thing, including temperature, cleaning, flushing, and regular Legionella testing.

What temperature does Legionella grow at?

Legionella grows best in warm water.

The Legionella temperature range for growth sits between 20°C and 45°C. This is where the bacteria can multiply quickly, especially if water isn’t moving much.

Below 20°C, Legionella doesn’t die. It becomes dormant – alive, just inactive. If the water temperature rises again, for example, as cold water passes through warm pipework, the bacteria can start growing again.

This is why Legionella cold water temperature matters just as much as hot water. Cold water should stay cold all the way to the outlet. Even short sections of pipework warming up can increase risk.

What temperature kills Legionella bacteria?

The temperature that kills Legionella bacteria depends on both heat and time.

Does 50°C kill Legionella?

At 50°C, Legionella bacteria start to die, but slowly.

If water is held at this temperature for around two hours, roughly 90% of the bacteria will be destroyed. This is why 50°C is often used as a minimum delivery temperature at outlets, rather than for storage.

Does 60°C kill Legionella?

Yes, and much more effectively.

At 60°C, around 90% of Legionella bacteria are killed in two minutes. This is why Legionella hot water temperature guidance usually requires hot water to be stored at 60°C or above. It makes survival and regrowth far less likely.

Does 70°C kill Legionella instantly?

At 70°C, Legionella is killed almost instantly.

This temperature is commonly used during thermal disinfection, where systems are flushed to destroy bacteria. While effective, this process must be carefully managed due to the risk of scalding.

Legionella water temperature snapshot

• Below 20°C – Legionella survives but doesn’t grow
• 20°C to 45°C – High-risk range for growth
• 50°C – Bacteria starts to die (90% in 2 hours)
• 60°C – Bacteria dies quickly (90% in 2 minutes)
• 70°C – Bacteria is killed instantly

This explains why temperature control is so important. Water that’s genuinely cold or properly hot is far safer than water sitting in the middle. Just remember that maintaining the right temperatures across the whole system – not just at the tank or boiler – is essential for effective Legionella control.

What is HSE guidance on Legionella temperature control?

In the UK, guidance on Legionella comes from the Health and Safety Executive (HSE). Their advice focuses heavily on temperature because it’s one of the most reliable ways to control Legionella bacteria in water systems. The HSE recommends:

• Cold water should be kept below 20°C
• Hot water should be stored at 60°C or above
• Hot water should reach 50°C at outlets within one minute
• In healthcare settings, this increases to 55°C

These temperatures make it difficult for Legionella to grow while still allowing systems to be used safely. Anyone responsible for a building’s water system – often called the dutyholder – must follow this guidance as part of their Legionella risk assessment.

Failing to manage temperatures properly can lead to non-compliance, increased risk, and serious consequences if someone becomes ill.

Why Legionella temperature control alone is not enough

Legionella temperature control is essential, but it doesn’t solve everything.

Legionella can still survive in areas where water doesn’t move much, even if the overall system temperature looks correct. Dead legs, infrequently used outlets, and poor system design can all allow bacteria to sit in pockets of water.

Biofilm makes things worse. It protects Legionella bacteria and reduces the impact of heat and disinfectants. Scale and sludge also provide food and shelter.

That’s why effective Legionella control measures go beyond temperature. They include cleaning, flushing, monitoring, and regular Legionella testing to confirm bacteria levels are under control.

Managing scalding risks while controlling Legionella

Water hot enough to control Legionella can also be dangerous if it reaches users at the wrong temperature. At 60°C and above, scalding can happen very quickly, particularly for children, older people, and anyone with reduced mobility or sensitivity.

This creates a balancing act. On one hand, hot water needs to be stored at high enough temperatures to reduce the risk of Legionella bacteria. On the other, water delivered to taps and showers must be safe to use. Simply turning the temperature down isn’t a solution, as this can increase the risk of Legionella growth within the system.

This is where thermostatic mixing valves (TMVs) come in. TMVs mix hot and cold water to deliver water at a safe, controlled temperature at the outlet, while still allowing hot water to be stored at temperatures that help control Legionella.

TMVs are commonly used in healthcare settings, care homes, schools, and other environments where users may be more vulnerable. Like any safety device, they need regular checks and maintenance to ensure they continue working properly. When installed and managed correctly, TMVs help keep people safe and water systems compliant.

How to test for Legionella in water

Even with good temperature control, Legionella testing is still important.

Testing confirms whether Legionella bacteria are present and whether control measures are working as they should. It’s particularly useful in larger or more complex water systems, or where there’s been a history of issues.

Testing options include:

• Laboratory water sampling
• On-site testing by trained professionals
• Managed or DIY testing kits

Results should always be recorded and reviewed as part of your Legionella risk assessment. Monitoring temperatures, flushing outlets, and keeping accurate records all help demonstrate compliance and reduce long-term risk.

How to prevent Legionella in water

Preventing Legionella in water is about consistency. Key control measures include:

• Keeping water within safe temperature ranges
• Flushing unused outlets regularly
• Cleaning and inspecting tanks and pipework
• Reducing dead legs where possible
• Carrying out regular monitoring and testing

No single action works on its own. A combination of good system design, temperature control, and routine checks offers the best protection against Legionella in water systems.

When do you need a Legionella risk assessment?

A Legionella risk assessment is required in most non-domestic buildings. It should also be reviewed when things change. For example:

• A new building or change of use
• Alterations to the water system
• Prolonged periods of low occupancy
• A positive Legionella test result

Legionella temperature FAQs

What temperature kills Legionella instantly?

Legionella is killed almost instantly at 70°C. This temperature is typically used during thermal disinfection rather than normal system operation.

Can Legionella survive in cold water?

Yes. Legionella can survive in cold water below 20°C, but it doesn’t grow. If the water warms up, the bacteria can become active again.

Does boiling water kill Legionella?

Boiling water will kill Legionella bacteria. However, boiling is not a practical or safe way to control Legionella in building water systems.

Can Legionella grow in hot water tanks?

Yes, if hot water tanks are stored below 60°C. This is why maintaining the correct Legionella hot water temperature is so important.

How long does Legionella survive at 50°C?

At 50°C, Legionella starts to die, but slowly. It can take up to two hours to kill off around 90% of the bacteria.

Is Legionella killed in showers?

Legionella is not killed at the showerhead. Control depends on the temperature and condition of the entire water system, not just the outlet.