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If you breathe in water that contains Legionella, you can develop an infection known as Legionnaires' disease. This infection is a serious, and sometimes fatal pneumonia.

Infection with legionella bacteria can be fatal in approximately 12% of reported cases.

Legionnaires' disease is a serious illness that may require hospital treatment. And although most people make a full recovery, it can be life-threatening.

Find out more about the 12 symptoms of Legionnaires disease .

If you are responsible for a water system, it's important to prevent the growth of Legionella bacteria. Larger systems (like those in offices or hotels) are more at risk, particularly if parts of the system are not in regular use.

You can get Legionnaires' disease if you breathe in tiny droplets of water containing bacteria that causes the infection.

It's usually caught in places like hotels, hospitals or offices where the bacteria have got into the water supply. It's less common to catch it at home.

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

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

Through temperature control, you can prevent Legionella bacteria growth within your water systems. Specifically, avoiding the temperatures that allow the bacteria to multiply and thrive.

At what temperature does Legionella grow?

The temperatures to avoid where possible within your water system are 20°C - 45°C . Legionella loves the water at this temperature and it will encourage the bacteria to multiply. Fast.

How long does Legionella take to develop in water?

Not long. In ideal conditions, in warm and stagnant water, Legionella can multiply quickly and become a problem within days .

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.

Most water systems are designed and controlled to prevent these perfect conditions, and stop Legionella.

What water temperature kills Legionella?

Hot water can kill Legionella bacteria.

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 .

But 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 cold 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 of the hot tap is, well, too hot!

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

Does cold water kill Legionella?

Cold water doesn't kill Legionella. But if the water is cold enough it can make the bacteria dormant, so water systems can also be safe from Legionella by storing water at a cooler temperature

Legionella bacteria are dormant in water temperatures below 20°C .

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

While the bacteria 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.

What are safe water temperatures to prevent Legionella?

Safe temperatures are below 20°C or above 60°C .

As we have discussed, in water temperatures 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.

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

Cold water should be cold stored and distributed below 20°C. Hot water should be hot. Storage of hot water should be above 60°C to kill any Legionella bacteria.

The 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 dangerous temperatures for Legionella growth from staying within the system.

Are water temperatures the only way to stop Legionella?

Avoiding the danger water temperatures (20°C - 45°C) is just one way of controlling the risk of Legionnaires' disease. But temperature control is the primary Legionella control in most standard water systems.

Other conditions support Legionella growth. Like slow and stagnant water, or the presence of sludge, scale, rust, algae and other organic matter that 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.

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Keeping hot water hot, and cold water cold is the best way to stay safe from Legionnaires' 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.