Legionnaires' disease is a type of pneumonia. It was named after an outbreak of severe pneumonia that affected a meeting of the American Legion in 1976. It is an uncommon but serious disease.
Legionnaires' disease occurs more frequently in men than women. It usually affects middle-aged or elderly people, and it more commonly affects smokers or people with other chest problems.
About half the cases of Legionnaires' disease are caught abroad - useful advice on travel can be obtained from the European Working Group for Legionella Infections. The other half are the result of infections acquired in the UK.
How people get it
The agent that causes Legionnaires' disease is a bacterium called Legionella pneumophilia. People catch Legionnaires' disease by inhaling small droplets of water suspended in the air, which contain the bacteria.
Certain conditions increase the risk from legionella:
- a suitable temperature for growth, 20 to 45oC
- a source of nutrients for the organism, eg sludge, scale, rust, algae, and other organic matter;and
- a way of creating and spreading breathable droplets, eg the aerosol created by a cooling tower or spa pool
However, remember that most people exposed to legionella do not become ill, and Legionnaires' disease does not spread from person to person.
What the symptoms are
The symptoms of Legionnaires' disease are similar to those of flu:
- high temperature, fever and chills
- muscle pains
In a bad case there may also be pneumonia, and occasionally diarrhoea and signs of mental confusion.
Where it comes from
Legionella bacteria are widespread in nature, mainly living in natural water systems,for example rivers and ponds. However, the conditions are rarely right for people to catch the disease from these sources.
Outbreaks of the illness occur from exposure to legionella growing in purpose-built systems where the water is maintained at a temperature high enough to encourage growth, eg cooling towers, evaporative condensers, spa pools, and hot water systems used in all sorts of premises (work and domestic).
Most community outbreaks in the UK have been linked to installations such as cooling towers, which can spread droplets of water over a wide area. These are found as part of air-conditioning and industrial cooling systems.
Cooling Towers and Evaporative Condensers
Fatal cases of Legionnaires' disease have also been associated with spa pool demonstrations.
Measures to control legionella
To prevent exposure to the legionella bacteria, you as a duty holder must comply with legislation that requires you to manage, maintain and treat water systems in your premises properly. This will include, but not be limited to, appropriate water treatment and cleaning regimes.
Remember, legionella can grow in any workplace if the conditions are right - you do not have to work with microbiological agents, eg in a laboratory, for exposure to occur. If you are responsible for any of the water systems described in HSE's Approved Code of Practice (ACoP) and Guidance "Legionnaires' disease: The control of legionella bacteria in water systems" (L8) you will need to assess the risk of employees and others in the workplace contracting Legionnaires' disease.
Copies of L8 can be purchased from HSE Books. HSE also publishes several free leaflets and a video explaining legal duties and the control of legionella in cooling systems and hot/cold water systems available from HSE Books.
Further information about Legionnaires' disease can be found on the Health and Safety Executive (HSE) website.
Page last reviewed: 11 October 2023
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