The June 2012 outbreak of Legionnaires’ disease in Edinburgh resulted in the deaths of three people, with more than 100 people also being treated as confirmed or suspected cases. Professor Hugh Pennington, a microbiologist at the University of Aberdeen, believes that Legionnaires’ disease is one of the nastiest and most lethal of infectious diseases in the UK, but is “utterly preventable". Yet 5000 people suffer from the disease each year and it is responsible for 600 deaths. In this article, Gordon Tranter looks at recent developments in the control of the Legionnaires’ disease.

Conditions for growth

Legionellosis is the collective name given to the pneumonia-like illness caused by legionella bacteria. Of these, the most serious is Legionnaires’ disease, a potentially fatal form of pneumonia caused by the bacteria Legionella pneumophila. Legionnaires’ disease can be caught by inhaling water aerosols containing the bacteria. The bacteria can live in all types of water, including both natural sources such as rivers and streams, and artificial water sources such as water towers associated with cooling systems, hot and cold water systems, and spa pools. They only become a risk to health when the temperature allows the legionellae to grow rapidly.

Legionella pneumophila can grow at temperatures between 20ºC and 40ºC, optimal growth occurs at approximately 36ºC; below 20ºC the bacteria is dormant though still viable and above 45ºC the bacteria will begin to perish and will not survive above 60°C. Areas where there are high concentrations of rust, algae and organic particles provide the nutrients necessary for growth.

Control of legionella bacteria in water systems

Advice on controlling the risk from exposure to legionella bacteria is provided in in the Health and Safety Executive publication Legionnaires’ Disease: The Control of Legionella Bacteria in Water Systems: Approved Code of Practice and Guidance (L8). Currently, as part of the review of its Approved Codes of Practice (ACOPs), as recommended by the Löfstedt review, the HSE is proposing to revise L8. The proposal is to revise Part 1 (the ACOP) and remove Part 2 (the guidance), which will be made separately available as revised technical guidance.

Assess the risk

Both the Management of Health and Safety at Work Regulations 1999 and the Control of Substances Hazardous to Health Regulations 2002 require a suitable and sufficient risk assessment to identify and assess the risk of exposure to legionella bacteria from work activities and water systems on the premises, and to identify any necessary precautionary measures. The basic procedures for the risk assessment are well documented. Specific requirements for assessing the risk exposure to legionella bacteria are dealt with in L8 and in more detail in BS 8580: 2010Water Quality. Risk Assessments for Legionella Control. Code of Practice.

Appoint a person to manage and supervise

If the assessment shows that there is a reasonably foreseeable risk, a person(s) should be appointed to take managerial responsibility and to provide supervision for the implementation of precautions. If the appointed person’s expertise is not sufficient, it may be necessary to use support from outside the organisation. In such circumstances, this person should take all reasonable steps to ensure the competence of those carrying out work not under their direct control, and to establish responsibilities and lines of communication.

Guidance on the levels of service, which should be expected from service providers, is available in The Control of Legionellosis: A Recommended Code of Conduct for Service Providers, developed by the Water Management Society and the British Association for Chemical Specialities. It is essential that the appointed person ensures that contracts with water treatment companies clearly specify the work to be carried out.

Not sufficient to rely on an apparently competent contractor

If contractors are used, the employer must check that the work is carried out to the required standard. Delegating the treatment of water to a third party does not automatically absolve the employer of responsibility. In 1999, Ceri Davies, the Managing Director of GTS (Fabrication) Ltd, was prosecuted for failing to take all reasonably practicable precautions to minimise the risk of legionellosis to the company’s employees during the operations of its water cooling tower. The managing director of the company had appointed one of his workforce to be responsible for health and safety. The employee engaged a specialist contractor to institute a cleaning regime for the control towers, but this regime was ineffective and dangerous levels of bacteria built up in the towers. The managing director claimed that he had relied on the expertise of an apparently competent contractor. The judge ruled that it is not adequate for an employer to seek to transfer responsibility for all matters to an expert third party.

Controlling the risk

Measures to control legionella bacteria should include the following.

  • Controlling the release of water spray to reduce exposure to water droplets and aerosols.

  • Avoiding conditions that allow proliferation in the system by:

    • avoiding water stagnation

    • avoidance of water temperatures between 20°C and 45°C that favour the proliferation of legionella bacteria and other micro-organisms

    • avoiding the use of materials that harbour bacteria and other micro-organisms, or provide nutrients for microbial growth

    • maintaining the cleanliness of the system and the water in it.

  • Using water treatment techniques.

Water treatment techniques

A variety of techniques are used to control legionella bacteria in water systems. These include:

  • temperature regimes

  • biocidal chemicals

  • ultra-violet sterilisation.

The growth of legionella bacteria can be prevented by storing hot water above 60°C and distributing it at above 50°C, and keeping cold water below 20°C. Storing hot water above 60°C can be costly in terms of manpower and energy, and there is the risk of scalds. Scalding can be avoided by the use of thermostatic valves, which must be installed close to the point of use to avoid dead legs in which legionella bacteria can multiply.

The biocidal chemicals used to control legionella bacteria include the oxidising biocides: chlorine (which is much less effective above pH7); chlorine dioxide; and chloroisocyanurates, hypochlorite and ozone. Copper-silver ionisation has been widely used to treat water to prevent the growth of legionella bacteria. However, as a consequence of action taken at European Union level under the Biocidal Products Directive (98/8/EC), since 1 February 2013 it has been illegal to sell or use water treatment systems that use elemental copper in order to add copper ions to water as a biocide. This was the consequence of no manufacturer supporting the use of elemental copper for use as biocides in these systems during a review period that ended in September 2011. The HSE has submitted an “essential use derogation” for use of copper in legionella control systems in the UK. Further information is available on the HSE website.

A public inquiry?

The seriousness of the Edinburgh outbreak led the Scottish Labour Party to call for a public inquiry into the outbreak of the disease. This call was supported by Professor Pennington, who said: “Something went badly wrong in Edinburgh. A public inquiry would be the most effective way to establish the facts and to prevent yet more outbreaks.”


There are real fears that the HSE and local authorities’ inspection regimes are inadequate and are failing to provide protection against these fatal outbreaks. Professor Pennington said that he had seen information from more than one source that suggests the inspection frequency of cooling towers is really very low indeed. He said some premises with cooling towers could be “falling through the gaps” in the inspection regime and that it would be "optimistic" to assume inspections are held even once a decade at the hundreds of cooling towers across the UK.

Last reviewed 14 May 2013