Legionella bacteria occur in most fresh water, usually at such low levels that they are harmless. Once inside a water system, however, if conditions favour growth, they can multiply to levels that pose risk of infection, but this level does not equate directly to risk as there are other factors, including the degree of exposure and the susceptibility of those exposed, so there is no sharp divide between a ‘safe’ and ‘unsafe’ level. Legionnaires’ disease is always serious and is fatal in approximately one case in eight. It is preventable. The mechanism of infection is generally accepted as by inhaling minute droplets of water contaminated with legionella in a spray or mist known as an aerosol, such as is generated by a shower, cooling tower, hot tub or any of many other installations.

Health and safety legislation requires the risk of legionnaires’ disease to be controlled and applies to employers, the self-employed or anyone in control of premises (the duty-holder). In the UK, the Control of Substances Hazardous to Health Regulations 2002 (known as COSHH) require risk assessments to be carried out, and for suitable control measures to be put in place if the risk is deemed significant. Formal guidance specifies in detail what is suitable and sufficient to constitute all reasonably practicable precautions to control the risk. To date, not one case of legionnaires’ disease has been attributed to any water system that was designed, installed, maintained and operated correctly; in every case one or more faults, be they physical, operational or human, have been the underlying cause.