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5th December, 2018

The Water Temperatures That Kill Legionella Bacteria

Legionella is a type of bacteria that can grow in water, usually warm and stagnant water. Water contaminated with the bacteria can spread Legionnaires’ disease to people who come into contact with it. This infection is a serious, and sometimes fatal pneumonia.

To stop Legionnaires disease, we can kill the Legionella bacteria.

There are few ways to kill Legionella bacteria. One of the most common ways to control the risk is through managing water temperatures.

Legionnaires’ disease is caused by breathing in small droplets of water contaminated by the bacteria. We recently wrote a blog post about the 12 symptoms of Legionnaires disease, and how they can be fatal.

Through temperature control, we can prevent Legionella bacteria growth within water systems. Specifically, avoiding danger temperatures. These are temperatures that allow the bacteria to multiply and thrive.

Hot = Kill

To kill Legionella bacteria, you need to make sure that the water is too hot for them to live. You don't need to boil them, but you do need to get them above 60°C.

Water at that temperature wouldn't be good for you either - if you were to have a 60°C bath, you would burn. Did you ever run a bath and forget to add hot water, or put your hands under a tap that's too hot to touch? Why would water be this hot out the tap if we can't use it at that temperature?

Sometimes the reason water is stored in systems at such high temperatures is so it kills off any Legionella bacteria. That's why you might find the water coming out the hot tap is, well, too hot!

Above 60°C, the Legionella bacteria cannot survive. Above 50°C, the bacteria will start to die off.

Cold = Dormant

Water systems can also be safe from Legionella by storing water at a cooler temperature. Below 20°C, the Legionella bacteria are dormant.

It is important to remember that while the bacteria are dormant below 20°C, it doesn't kill them off.

While they might not be growing and multiplying, they are in a state of hibernation. They are still present. If the temperature rises, they can become a threat again. If cold water is stored, it needs to stay cold.

Danger Temperatures

So what are the danger temperatures?

The temperatures to avoid where possible within your water system are 20°C - 45°C. Safe temperatures are below 20°C or above 60°C.

As we have discussed, below 20°C, the Legionella bacteria are dormant, and above 60°C the bacteria are killed.

45°C - 60°C is not an optimum temperature for the bacteria to thrive, but they can survive.

The water temperatures that Legionella really love are 20°C - 45°C. Water at this temperature will encourage bacteria to multiply. Fast.

Bacteria when in ideal conditions will grow, if you put legionella on an agar plate and incubate it for 7 – 10 days with a temperature range of between 20˚C and 50˚C, a colony will form of millions of bacteria that you will be able to see.

Debbie Green IOSH London Metropolitan Branch Meeting 08/01/2013

Safe Temperatures

Your water system should be designed and maintained to avoid the Legionella danger water temperatures.

Cold water should be cold – stored and distributed below the 20°C threshold. Hot water should be hot. Storage of hot water should be above 60°C so that any Legionella can’t survive.

Distribution of hot water should be at 50°C or higher. Where thermostatic mixer valves are fitted, these should be as close to the outlet as possible. This helps to prevent water at the danger temperature staying within the system.

Other Conditions

Avoiding the danger water temperatures is just one way of controlling the risk of Legionnaires’ disease. However, this is the primary control in most standard water systems.

There are other conditions that support Legionella growth. Like slow and stagnant water, and the presence of sludge, scale, rust, algae and other organic matter acts as a food source.

In some water systems you may not be able to avoid the danger temperatures and other controls, such as water treatment, may be required.


Keeping hot water hot, and cold water cold is the best way to stay safe from Legionnaire’s disease.

It is important to assess the risks from Legionella within your water systems, and take appropriate action to control the risks. Especially in larger buildings, or complex systems.

Download our Legionella risk assessment template to carry out a comprehensive risk assessment within your premises.

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This article was written by Emma at HASpod. Emma has over 10 years experience in health and safety and BSc (Hons) Construction Management. She is NEBOSH qualified and Tech IOSH.

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