Legionellosis is a collective term for diseases caused by legionella bacteria including the most serious: Legionnaires’ disease, as well as the similar but less serious conditions of Pontiac fever and Lochgoilhead fever. Legionnaires’ disease is a potentially fatal form of pneumonia and everyone is susceptible to infection.

The risk increases with age, but some people are at higher risk, e.g.

• people over 45
• smokers and heavy drinkers
• people suffering from chronic respiratory or kidney disease
• diabetes, lung and heart disease
• anyone with an impaired immune system

Legionella bacteria are widespread in natural water systems, e.g. rivers and ponds. However, the conditions are rarely conducive for people to catch the disease from these sources. Outbreaks of the illness occur from exposure to legionella growing in purpose-built systems where water is maintained at a temperature high enough to encourage growth, e.g. cooling towers, evaporative condensers, hot and cold water systems and spa pools used in all sorts of premises (work and domestic).

Legionnaires’ disease is normally contracted by inhaling small droplets of water (aerosols), suspended in the air, containing the bacteria. Certain conditions increase the risk from legionella if:

• The water temperature in all or some parts of the system may be between 20–45 °C, which is suitable for growth
• It is possible for water droplets to be produced and if so, they can be dispersed
• Water is stored and/or re-circulated
• There are deposits that can support bacterial growth, such as rust, sludge, scale, organic matter and biofilms

It is important to control the risks by introducing measures which do not allow proliferation of the organisms in the water systems and reduce, so far as is reasonably practicable, exposure to water droplets and aerosol. This will reduce the possibility of creating conditions in which the risk from exposure to legionella bacteria is increased.