Legionnaires' Disease and Risk Assessment

What is Legionnaires’ disease?

Legionnaires’ disease is a potentially fatal pneumonia caused by legionella bacteria. Infection is caused by breathing in small droplets of water contaminated by the bacteria. The disease cannot be passed from one person to another.

Where is Legionella found?

Legionella are widespread in the environment in water and may grow in hot and cold water systems. They survive low temperatures and thrive at temperatures between 20oc and 45oc if a supply of nutrients is present. The bacteria can be a risk if they are dispersed in water droplets such as from showers, spa baths and wet based air conditioning systems. In dwellings showers pose a potential risk of legionella, particularly if they are used irregularly.

What are my legal duties?

Under health and safety law you have a duty to consider the risks from legionella that may affect your tenants, employees or members of the public and take suitable precautions to control the risks.

You must:-

  • Identify and assess the source of risks
  • Prepare a course of action for preventing or controlling the risks.
  • Implement the action (and appoint a person to be responsible for implementing the action)
  • It is advisable to keep records of your actions (For employers it is a legal requirement to document your findings and actions if you have five or more employees)

Identification and Assessment of Risk

  • Are conditions present which will encourage bacteria to multiply?
  • Is the water temperature between 20oc and 45oc?
  • Are water droplets produced, e.g. from showers?
  • Can sediment, sludge, scale, organic material or biofilms build up in the water system?
  • Is the full water system used regularly (more than once per week)?
  • Are there any units which are not regularly used (i.e. less than once a week, not including emergency showers)?
  • Are there any areas of redundant pipework which will allow water to stagnate?
  • Has the property been empty for a period of time meaning water may have stagnated in pipework allowing bacteria to multiply?

Preventing or Controlling the Risks

If a risk is identified which cannot be prevented, proper controls must be introduced:

1. Hot Water

Water in the boiler should be kept at a minimum of 60oc and water at each outlet point should be above 50oc within a minute of running the water. (Thermostatic mixing valves for showers can prevent scalding)

2. Cold Water

Water should be less than 20oc and at each outlet point below 20oc within 2 minutes of running the water

3.  Showers

Shower heads and hoses should be dismantled, cleaned and descaled at least quarterly

4. Units not regularly used

Any units not regularly used must be flushed through and purged to drain without release of aerosols, e.g. remove shower heads regularly.

5. Cold Water Tanks

Ensure that the tank is insulated with a closed lid

6. Design of Water Systems

  • Insulate hot water pipes
  • Remove any redundant pipe work
  • Ensure that length of branch pipe work serving equipment and appliances is as short as possible
  • Individual Thermostatic Mixer Valves (TMVs) should be as near the hot water outlet as possible
  • Hot water vessels should have a valve to remove accumulated sludge at the base of the tank, for manual removal of any build up
  • Multiple, linked storage tanks should be avoided because of operational difficulties, due to possible unequal flow rates and possible stagnation
  • Minimise the volume of cold water tanks
  • Avoid fibre washers, hemp, natural rubber, linseed oil-based compound jointing

Monitoring of Controls

Hot and Cold Water Temperatures should be:-


  • Check that the temperature of hot water is at least 50oc within 1 minute of running the water at the first and last tap on the run
  • Check that the temperature of the cold water is below 20oc at the first and last taps from the storage after running the tap for 2 minutes
  • Check the surface temperature of pipe work entering any thermostatic mixer valves is at least 50oc
  • Check the outgoing water from the hot water cylinder is at least 60oc and incoming water is at least 50oc (some tanks have thermometer pockets for this purpose)

6 Monthly

  • Check that the incoming cold water is below 20oc


  • On a rotational basis check that at the taps:-
    • the cold water temperature is below 20oc after running the water for 2 minutes;
    • the hot water temperature is at least 50oc within 1 minute of running the water.

General Monitoring


  • Visually inspect the cold water tank - Ensure the tank is insulated and that there is a closed lid
  • Check the debris and if necessary, clean and disinfect at taps
  • Check that all other control measures remain effective

Record precautions and results of monitoring

Commissioning and Recommissioning

If a hot water system feeding showers is newly installed or has been out of use for over 1 week, the water should be brought up to 60oc for 1 hour before being used. This should kill any legionella bacteria. Alternatively, a chemical biocide can be added for at least 3 hours then the system flushed through before use.

See the Health and Safety Executive (HSE) for further advice and information.

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