Several species of bacteria in water systems can cause infections, including the potentially fatal Legionnaires’ disease pneumonia caused by legionella bacteria. Infection is usually caused by breathing in small droplets of water contaminated by the bacteria. The disease is not usually passed from one person to another. Everyone is potentially susceptible to infection, but some people are at higher risk, for example those over 45 years of age, smokers, heavy drinkers, those suffering from chronic respiratory or kidney disease and people whose immune system is impaired [1].
Legionella bacteria are widespread in the environment and as such can contaminate and grow in water systems such as cooling towers and hot and cold-water services. The bacteria survive low temperatures and can thrive at temperatures between 20°C and 45°C. At temperatures outside their growth range, the growth of legionella bacteria is greatly reduced, and they are killed by high temperatures.
It is a legal requirement to carry out a risk assessment in the workplace to identify potential risks of exposure to legionella and to inform measures to eliminate or control them [1]. These measures are documented within a written control scheme. As no single method will completely eliminate the risk of legionella, a combination of measures is likely to be required. Staff members should also know what action to take if there is concern about legionella contamination in the practice.
Water systems include cold and hot water supplies to sinks and toilets. These are likely to constitute a risk of exposure to legionella that can be mitigated with relatively straightforward control measures (see Control Scheme and Management). In dental practices, dental unit water lines (DUWLs) present a risk that can be mitigated with specific control measures. Showers and air conditioning units also present a risk.
The preferred measure to control legionella is via water temperature control to minimise proliferation of bacteria. In certain systems, addition of disinfectant (biocide) may be recommended. Microbiological testing is usually not necessary. However, testing must be carried out when deficiencies in control measures are identified or there is doubt about their efficacy (e.g. water develops an unpleasant odour or taste, there is a routine failure to meet water temperature targets or there has been a significant legionella breach) [1,2].
The Health and Safety Executive (HSE) provides a summary of the regulatory requirements in Legionnaires’ disease, a brief guide for dutyholders [3] and further details provided in Legionnaires’ disease: the control of legionella bacteria and water systems. (Approved Code of Practice L8, Fourth edition) [1] and HSG 274 Legionnaires’ disease Part 2: The control of legionella bacteria in hot and cold water systems [4]. Specific DUWL guidance is provided by Health Protection Scotland in Literature Review and Recommendations: Management of Dental Unit Waterlines [2].
Sources of information
- Legionnaires’ disease: the control of legionella bacteria and water systems. (Approved Code of Practice L8, Fourth edition, 2013). Health and Safety Executive
- Literature and Recommendations: Management of Dental Unit Waterlines (version 2.0, 2019). Health Protection Scotland
- Legionnaires’ disease, a brief guide for dutyholders (2012) Health and Safety Executive
- HSG 274 Legionnaires’ disease Part 2: The control of legionella bacteria in hot and cold water systems (2014) Health and Safety Executive