Know your responsibilities: a guide to Legionella control

The bacterium Legionella pneumophila and related bacteria are common in natural water sources such as rivers, lakes and reservoirs, but usually in low numbers. They may also be found in purpose-built water systems such as cooling towers, evaporative condensers, hot and cold water systems and spa pools. Domestic water systems differ from natural systems in that they pose additional risks, which is why a specialised Legionellarisk assessment is necessary. Such a risk assessment should be an integral part of any water safety programme and is mandatory for organisations with five or more employees, often forming the basis for future monitoring, preventive and reactive measures. It must be carried out by a competent person.

So, what is a Legionellarisk assessment and why is it important to have one? Similar to a traditional health and safety risk assessment, a Legionella risk assessment is a systematic examination of a particular environment and/or system, usually within a work setting, commercial property or somewhere that members of the general public have access to. 

There are five core aspects that have to be considered as part of any established assessment: contamination; amplification; transmission; exposure; and host susceptibility. While each assessment will vary depending on the water system itself and the nature of the building it serves, all should cover a few basic points (see Box 1). Levels of risk associated with the assessment will vary depending on who may be impacted by the water system. For example, if people who are immunocompromised use the property or facility, then additional measures must be taken as a precaution.