• Lisamarie Lamb

When to test for Legionella bacteria in water systems

There were 254 confirmed cases of Legionnaires' disease in the United Kingdom in 2020, including an outbreak in West Bromwich in September.


While the condition has the potential to be devastating, the good news is that the dangerous bacteria that causes it may be eliminated by properly managing water systems. Furthermore, employers and landlords have a legal obligation to safeguard the health of individuals in their properties.



Photo by Steve Johnson from Pexels


When and where do you find Legionella bacteria?

Legionella bacteria are most usually found in water that fits one or more of the three requirements listed below:


  1. When the water is maintained between 20–45°C

  2. When the water contains nutrients

  3. When the water is stagnant


Legionnaires' disease is caused by inhaling small droplets of water in the air that carry the bacteria. It is a kind of pneumonia that may be lethal, particularly in vulnerable groups such as the elderly, persons with damaged or weakened immune systems, smokers, and alcoholics.


Legionella bacteria are most often seen in:


  • Hot water heaters (calorifiers)

  • Atomisers

  • Cooling towers

  • Wet air conditioning plants

  • Whirlpool or hydrotherapy baths

  • Showers

  • Domestic hot and cold water systems

  • Dead legs or low use areas

What are the first steps?

To treat Legionnaires' disease, the first step is to select a competent individual to analyse the danger of legionella bacteria development in their building's hot and cold water systems. Because each company and facility is unique, the person in charge must understand how their specific systems operate.


This individual must do a legionella risk assessment that takes into account the Legionnaires' disease hazards at their location. If no one in-house is competent, it's best to find an expert such as those at Assured Water to carry out the risk assessment on your behalf.


How often should you test water systems for Legionella?

The Legionella Risk Assessment should identify the threat in your building. However, if there is any dispute about its findings—or if any methods used are ineffective—legionella testing should be considered regularly until the system is brought back under control.


When the findings are clear or the processes are successful, it is safe to minimise or eliminate testing.


Cooling towers are also at danger, hence HSG274 Part 1 requires them to be checked on a regular basis: every quarter unless there are difficulties or the risk assessment suggests differently, in which case testing should be more frequent.

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