How Water Temperature Affects the Growth of Legionella Bacteria?

Legionnaires’ disease is a severe form of pneumonia which can cause fatal respiratory conditions and in worst cases could lead to death. Under favourable conditions, legionella bacteria can quickly spread in any many-made water system which recirculates water and creates aerosols or tiny water droplets.

It is, therefore, imperative to store, maintain, and distribute water at temperatures that do not encourage the proliferation of legionella bacteria.

What is the effect of temperature on legionella?

As with other living organisms, legionella needs the right conditions to thrive and multiply. Temperatures ranging between 20°C and 45°C provide optimal conditions for rapid legionella growth. If the bacteria are allowed to remain in the water at this temperature range, it can quickly colonize and increase the risk of infection.

These temperatures are to be avoided in any water system but especially in high-risk settings, where vulnerable individuals are present.

Hot water must be stored in hot water cylinders at 60°C or higher to ensure if legionella bacteria were to enter a given water system it would be killed off and not be able to grow and colonise a system. The speed at which this occurs depends on how high the temperature goes. Kill rate increases substantially if the temperature reaches 70°C.

As this is too hot for most domestic uses, water is often stored at this temperature, then delivered to the point of use at a lower temperature. The temperature of the return pipe must also reach no less than 50°C to ensure the entire system temperature will prevent legionella bacteria from growing.

Can Legionella grow in cold water?

At the other end of the scale, water temperatures below 20°C would not kill legionella bacteria. At low temperatures, legionella bacteria enter a low metabolic state which keeps them alive but inhibits growth. At this temperature, biofilms can provide nutrients and protection to legionella to survive.  

However, if water at this temperature is released for use and it is heated during that process, it may increase the risk of legionella growth. A risk assessment should consider whether that is the case.

How to check water temperatures for legionella control?

In non-circulating systems, temperature checks must be done at the sentinel outlets which are nearest and farthest from the cylinder. For circulating systems, the temperature must also be taken at return pipework to ensure the entire system is achieves a temperature of 50°C.

It is critical to check subordinate loops as there can an issue on a long leg where it takes more than a minute for the temperature to rise. We generally recommend recording the temperatures quarterly.

There are specific areas within hot and cold water systems were temperature can be a risk and allow legionella bacteria can grow. These include the bottom of water storage vessels where the incoming cold water enters and mixes with the stored hot water reducing the temperature of the water at the bottom of the vessel where debris and sediment collect.

A comprehensive risk assessment will set out the locations and timeline of high-risk areas where temperature needs to be recorded.

Your water systems need your attention

Monitoring your hot and cold water systems is critical for mitigating any potential risks related to the legionella outbreak.

Legionella testing must be carried out periodically especially in old water systems with lots of pipework and dead legs. Even well-designed systems which are run at low temperatures should be tested once every quarter.  

Our team of water hygiene experts support you in controlling waterborne pathogens in the workplace and helping you comply with health and safety guidelines.

If you have any questions about the safety of your water systems, feel free to get in touch with our specialists.