related to the year’s summer weather. Legionella bacteria are destroyed


Figure 2: Reported European cases of Legionnaires’ disease, defined by their standard types (Source: European Working Group for Legionella Infections (EWGLI))


into the lungs, carrying the legionella that cause infection. Larger particles get trapped in the mucous coatings of the respiratory tract and, in ‘healthy’ people, are normally transported out by the conveyor-like ‘mucociliary clearance’ that carries particles away from the lungs. The bacteria may come from one of any number of reservoirs of water, including (evaporative) cooling towers, pipework and water storage tanks, but the bacteria will also be naturally present in the atmosphere, including in soil, groundwater, seawater and even water on plants. In terms of building services, the principal areas of infection have arisen from cooling towers and hot water systems, including spa pools.


Notifiable disease Since 2010, it has been a notifiable disease in the UK. The instances of Legionnaires’ disease are relatively few, compared to other forms of pneumonia, and tend to peak during the summer months – as indicated in the cases recorded in South East England (Figure 3). The trend probably results from a combination of increased external temperatures and greater use of aerosol- generating devices, such as cooling towers and water sprays, both within and outside the built environment – for example, a recent report in the UK has highlighted vehicle windscreen washers as being potential sources of Legionella infections. The magnitude of annually-reported cases in England and Wales (as shown for 2008 and 2009 in Figure 4) is apparently strongly


Cases of Legionnaires' disease reported in London and the South East, 2008-2009


almost instantly above 70°C; however, at temperatures between 20°C and 46°C, as found in many HVAC water sub-systems, Legionella growth is abundant – the optimum temperature being around 37°C. At temperatures below 20°C, there is little growth in bacteria and at a temperature of 60°C, 90% of the Legionella pneumophila have been shown to be killed in two minutes (compared to several hours at around 50°C). At low temperatures, Legionella becomes dormant but ready to multiply when the opportunity presents itself. Figure 5 relates the range of temperatures to typical applications in building services. Legionella requires nutrients to grow,


and these are normally already available in mains supplied water. Biofilms (of which ‘slime’ is a visible example) created through a combination of nutrients, scale and corrosion in appropriate temperatures, provide an ideal breeding ground for Legionella. Such appropriate conditions are frequently established in stagnant water. Rubber gaskets and seals can provide a source of nutrients for Legionella growth, and the presence of biofilms can make previously ‘safe’ materials such as copper susceptible to the growth of Legionella. Even short lengths of stagnant pipework, connected to otherwise Legionella-‘safe’ systems, can provide a breeding ground for Legionella. In many historic outbreaks of Legionnaires’ disease, it has been difficult to trace the specific principal source of the Legionella that caused the infection, since subsequent treatment by biocides or by pasteurisation reduce the concentration of the bacteria.


Management The successful control of Legionella is as much about management as it is technical detail. Guidance, including the current revisions to ASHRAE Standard 188P[7]


,


focus strongly on the need to establish a robust management and control system. However, there are conditions that will determine whether Legionella has the opportunity to thrive and increase the risk of infection. If systems are maintained consistently


Figure 3: Recorded cases of Legionnaires' disease tend to peak in the warmer months


50 CIBSE Journal July 2012


below 20°C or above 60°C, there is likely to be little risk of Legionella growth. Practically, this is not the case in at least part of many hot water distribution systems that frequently operate between 40°C and 50°C (water higher than 50°C presents increasing scalding risk).


Reported community acquired cases of Legionnaires' disease in England and Wales, 1980-2010


Figure 4: Significant Legionnaires' disease outbreaks are, fortunately, quite rare. The high incidence of individual cases in 2006 coincides with particularly high summer temperatures. This data excludes infections that were attributed to travel or were hospital acquired (Source: www. hpa.org.uk/web/HPAweb&HPAwebStandard/ HPAweb_C/1195733748327)


Cooling towers will typically hold water


at temperatures of between 30°C to 35°C that is ideal for Legionella growth. Drift eliminators are essential to reduce the amount of makeup water carried into the atmosphere; however, some will still escape. Due to the evaporative processes


within a cooling tower, and combined with particulate matter in the outdoor air, a substantial amount of solids can accumulate in a cooling tower. This will provide nutrients for the growth of bacteria and the development of biofilms, and so needs continuous removal and monitored treatment. There are a host of biocide dosing methods including oxidising agents – such as chlorine dioxide and sodium- and calcium-hypochlorite – and those that do not rely on oxidization – such as the surfactant amines, chlorinated phenolics and copper salts. However, this is not a case where more is better, but one where it is important to use appropriate biocides in a planned fashion, based on the monitored conditions of the cooling water. Recent researchA


in test cooling towers


has shown that some non-chemical devices – including magnetic, pulsed electric field, electrostatic, ultrasonic, and hydrodynamic cavitation – that are marketed for treating biological growth, are ineffectual. Pragmatically, although the potential growth of Legionella can be minimized in cooling towers with good design, operation and cleaning procedures, it is unrealistic to expect that Legionella can be eliminated from the environment, and so must be controlled with appropriate procedures and water treatment. Poor maintenance procedures have been linked to outbreaks of Legionnaires’ disease associated with cooling towers, such as the recent Edinburgh outbreak that peaked in June. Particular care must be taken


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