Legionnaires' disease

Legionellosis is the term for infections caused by Legionalla Pnuemophila with the best known illness being Legionnaires' disease. This is a type of pneumonia that principally affects people that are susceptible due to age, illness, immuno-supression, smoking and can be fatal. The bacteria can also cause less serious illnesses which are not fatal or permanently debilitating, but which can affect all people.

The risk

Legionella can enter water systems and have no affect on people working or living in a building. A number are required to create the risk of Legionnaires' Disease. There are:

  • Conditions suitable for multiplication of the organisms to high numbers, such as temperatures of 20oC to 45oC
  • A source of nutrients from sludge, scale, rust, algae and other organic matter
  • A means of creating and spreading breathable droplets, such as the aerosol generated by a cooling tower or shower
  • Susceptible individuals


Typical symptoms include fever, headache, muscle pains, difficulty in breathing, cough, diarrhoea, vomiting and mental confusion.

Premises are at risk include:

  • water systems incorporating a cooling tower
  • water systems incorporating an evaporative condenser
  • hot water services where the volume of hot water and the system does not exceed 300 litres
  • hot and cold water services irrespective of size in premises where occupants are particularly susceptible,
  • humidifiers and air washers which create a spray of water droplets and in which the water temperature is likely to exceed 20oC
  • spa pools and baths in which warm water is deliberately agitated and recirculated
  • other systems containing water which is likely to exceed 20oC and that can release a spray or aerosol during normal operation or when being maintained may also present a risk for example showers

Legal requirements

The Approved of Code of Practice and Guidance - "The control of Legionella Bacteria in Water Systems" sets out further statutory requirements for dealing with this risk. It applies whenever water is stored and used in a way which may create a reasonably foreseeable risk of Legionellosis and in particular to the plant and systems whenever the Health and Safety at Work Etc Act 1974 applies. The Code of Practice places responsibility on employers and persons responsible for buildings and processes to:

The risk assessment should take account of:

  • The potential for droplet formation
  • The water temperature
  • The likely risk to those who will inhale the droplets
  • The means of preventing or controlling the risk
  • Prepare a scheme for preventing or controlling the risk
  • The primary objective should be to avoid conditions which permit Legionella to proliferate and to avoid creating a spray or aerosol

This can be achieved by:

  • Avoiding water temperatures between 20oC and 45oC. Water temperature is a particularly important factor in the control of the risk
  • Avoiding water stagnation. Stagnation may encourage the growth of biofilm that can harbour Legionella and provide local conditions that encourage its growth. If water is allowed to stand for long periods in warm buildings or in hot weather its temperature is more likely to rise above 20oC
  • Keeping the system clean so as to avoid the build-up of sediments, etc. which may harbour bacteria or provide a nutrient source for them
  • An adequate water treatment regime
  • Ensuring that the system operates safety and correctly and is well maintained
  • Keeping records of the precautions implemented

In addition to prevent or minimise the multiplication and dissemination of Legionella you should:

  • Control the release of water spray
  • Avoid water temperatures and conditions that favour the growth of Legionella and other micro-organisms
  • Avoid water stagnation, e.g. by keeping pipe lengths as short as possible
  • Avoid the use of materials that harbour Legionella and other micro-organisms, or provide nutrients for their growth
  • Maintain the cleanliness of the system and the water in it
  • Use water treatment techniques which either kill Legionella (and other micro-organisms) or limit their ability to grow
  • Take action to ensure the correct and safe operation and maintenance of the water system and plant

Cooling tower notification

All premises where cooling towers and evaporative condensers are situated must register with the local authority under The Notification of Cooling Towers and Evaporative Condensers Regulations 1992. The prime purpose of this is to identify potential areas that could give rise to spread of infectious disease, e.g. Legionella and to ensure preventative measures are taken to eliminate the risk of such infection arising to employees and the public.

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