Construction micro-organisms: Legionella from water systems

Work involving water or old / poorly maintained water systems might expose construction workers to Legionella. This page tells you how to control this risk and why. You also need to be aware of the general information on micro-organisms.  

What you must do

Follow the Assess, Control and Review model. Pay particular attention to the following things:


Identify and assess: The risk of being exposed to Legionella is mainly during refurbishment and demolition work. The risks are linked to:

  • water stored or stagnating and re-circulated at temperatures between 20–45 °C – eg redundant parts of water systems or water storage tanks where the water is not frequently replaced
  • sources of nutrients such as rust, sludge, scale and organic matter
  • production of contaminated water droplets / aerosol spray
  • workers, visitors etc who are more susceptible to infection due to age, illness, a weakened immune system etc


Prevent: Think about limiting the risks before work starts:

  • drain water systems during demolition or refurbishment where possible. This prevents water stagnation
  • water outlets (taps, shower heads, etc) which are still needed but not regularly used should be flushed through regularly
  • treat water to either kill Legionella (and other microorganisms) or limit their ability to grow where appropriate - consider suitable biocides and other water treatment processes
  • ensure stored water is maintained either below 20°C or, if part of a hot water system, above 60°C - replace standing / stored water in tanks, pressurised water bottles etc regularly during warmer weather where it is likely to reach temperatures higher than 20°C

Control: Where there is still reason to suspect a Legionella risk, control this by:

  • Water – keep any system and the water in it clean. Ensure the water cannot stagnate - properly control the release of any water spray.


Supervise: Be aware of anyone suffering from the symptoms of Legionnaires' disease.

What you should know

The infectious bacterium, Legionella pneumophilia, occurs naturally in water and also in water systems where the temperature high enough to encourage growth; eg hot water systems or water storage tanks in the summer months.

These bacteria can cause illnesses collectively called Legionellosis. This includes the most serious illness, Legionnaires' disease, as well as the similar but less serious conditions of Pontiac fever. Legionnaires' disease is a potentially fatal form of pneumonia.

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Updated 2021-08-06