Legionella bacteria are common and can be found naturally in water sources such as rivers, ponds and reservoirs, usually in low numbers. Outbreaks occur when the bacteria colonise manufactured water systems where temperatures are suitable to encouraging bacterial growth.
Legionella bacteria can thrive in man-made water systems given the correct water temperatures and a supply of nutrients. They can survive at temperatures ranging from 6°C to 60°C. They can remain dormant at low temperatures and multiply readily at temperatures between 20°C and 45°C. They are at their most virulent at 37°C.
The bacteria also require a source of nutrients in order to multiply. These are derived from a wide variety of sources, including algae, sediment, sludge, scale, corrosion by-products, biofilms and other bacteria.
Therefore, anywhere where water is stored or used between the temperatures of 20°C and 50°C, that also offers a supply of nutrients, is a potential breeding ground for legionella bacteria. Where infected water is formed into a spray or mist, the risk of inhalation and infection is significant.
Areas of particular concern are:
- Water systems involving a cooling tower or evaporative condenser;
- Hot and cold water systems;
- Spa baths, Jacuzzis and other pools in which water is agitated and re-circulated;
- Other systems where water is stored between the temperatures of 20-50°C and which produce an aerosol or spray.
The control of legionella in the UK is governed by strict government guidelines and statutory responsibilities directed by law. Our guide to Legionella Legislation identifies the legal duties imposed on employers and those responsible for the control of premises where water is used or stored.