Legionella risk assessment

The risk assessment should identify and assess the risk of exposure to legionella bacteria from work activities and water systems and specify any precautionary measures needed [1]. In a dental practice, a particular focus of risk assessment will be the dental unit water lines (DUWLs). Specific DUWL guidance is provided by Health Protection Scotland in Literature and Recommendations: Management of Dental Unit Waterlines [2].

The dutyholder is a named individual (e.g. employer, those in control of premises, those with health and safety responsibilities for others) who is responsible for ensuring a legionella risk assessment is carried out and that suitable precautions are in place [3]. The dutyholder must ensure that the person who carries out the risk assessment and provides advice on control measures is competent to do so, with sufficient understanding of the practice’s water systems and associated equipment [1]. It is likely to be necessary to appoint someone who has the skills required to conduct the risk assessment, which should meet British Standard BS 8580-1:2019 [4]. The Legionella Control Association provides a voluntary Code of Conduct for Service Providers and a standard for the delivery of legionella risk assessment services that details competencies for legionella risk assessment [5,6]. For dental settings, the individual who conducts the risk assessment must understand dental chair units and dental unit water lines.

Health Protection Scotland advise that the following criteria need to be considered when assessing whether there is a foreseeable risk of legionella infection from a water system [2]:

Is water stored or recirculated as part of the system?
Is the water temperature in all or part of the system between 20°C and 45°C?*
Are there deposits that support bacterial growth visible (e.g. biofilm)?
Can water droplets be produced and dispersed (e.g. aerosols)?

Risk identified from meeting any of criteria 1, 2 or 3 together with droplet transmission in criterion 4 highlights that there could be a foreseeable risk of legionella infection [2], which is heightened if any employees or patients are more susceptible to infection.

*Temperature control is the traditional strategy for reducing the risk of legionella in water systems. 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. The Health and Safety Executive (HSE) recommends hot water be stored at, at least 60°C and distributed so it reaches a temperature of 50°C (55°C in health care settings) within one minute at the outlets [7].

The risk assessment should include [3]:

Management responsibilities, including the name of the responsible person (see Control Scheme and Management)
A description of the water system, including pipework, pumps, valves, cold water supply/storage tanks, hot water generation/storage tanks, sinks, toilets, showers, air-conditioning units, dental unit water lines
Potential risk sources (see criteria listed above)
Any controls currently in place to control the risk (e.g. measures to avoid stagnation, flushing regimes, use of biocide, cleaning and draining procedures)
Monitoring, inspection and maintenance procedures
Records of the monitoring results, inspection and checks carried out
Date for scheduled review of the risk assessment and conditions that would trigger an unscheduled review of it

A risk assessment may show that the risks are being properly managed and that no additional control measures are necessary. If it is not possible to access all parts of the water system (e.g. due to the fabric of the building) this would be documented in the risk assessment and suitable additional control measures might be necessary. Once the initial risk assessment has been carried out, it must be reviewed regularly and specifically when there is any reason to believe that it may no longer be valid (e.g. a change to the water system, checks indicate control measures are no longer effective or a significant legionella breach arises). [1,2].

Sources of information