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Legionella is a family of pathogenic waterborne bacteria that include some that cause a group of infections called legionellosis of which Legionnaires’ disease is the most severe. This article provides an overview of Legionella and explains how growth within man-made water systems leads to exposure. It includes the range of symptoms and the groups at high risk. Wherever conditions in a water system are right for Legionella growth and transmission, there is a risk of Legionella infection. Therefore, unless Legionella is controlled in water systems, workers and public alike are at risk. Implementing management strategies can reduce risk. While there is a widespread potential for Legionella exposure associated with man-made water systems, there are very few occupations for which there is a heightened risk.

Legionnaires' disease

Legionella - the organism

Legionnaires’ disease was first identified following a large outbreak of pneumonia among people who attended an American Legion convention in Philadelphia, United States of America (USA) in 1976. A previously unrecognised bacterium was isolated from lung tissue samples, which was subsequently named Legionella pneumophila[1]. This member of the Legionella family of bacteria has 16 serogroups or different cell surface profiles; and causes the vast majority of associated disease. In Europe, approximately 70% of Legionella infections are caused by L. pneumophila serogroup 1, 20-30% are caused by serogroups 2-16 and 5-10% are caused by other Legionella species[2].

Legionella bacteria are common in natural water sources such as rivers, lakes and reservoirs, but usually in low numbers. Their numbers only increase when the water enters a man-made water system where conditions for growth are favourable.

The route of infection

People normally catch Legionnaires’ disease by inhaling Legionella bacteria, in tiny droplets of water suspended in the air (aerosols) or in droplet nuclei (after the water has evaporated). Legionella can also occasionally infect a human via micro-aspiration of contaminated water from respiratory therapy equipment or contaminated ice[3].

L. longbeachae is responsible for approximately 5% of Legionella associated pneumonia caused by Legionella species other than L. pneumophila[4]. It has been associated with exposure to contaminated compost in Australia, New Zealand, the USA, Japan[5][6] and in recent years, Scotland[7]. The mode of transmission in this case is likely to be from inhaling contaminated soil particles[8].

Person-to-person spread of the legionellosis has not been documented. The likelihood that a source of Legionella will cause an infection depends on several factors:

  • the concentration of bacteria,
  • the effectiveness of dissemination,
  • the way in which the bacteria multiply,
  • sources’ ability to form aerosols.

Major human risk groups

Not everyone exposed to Legionella becomes infected and shows symptoms of ill health[4]. In fact, Legionnaires’ disease in healthy humans is unusual. Surveillance data from the UK suggest that 3 out of 4 cases of Legionnaires’ disease cases between 2014 and 2016 had an underlying medical condition [9]. The groups of people who are at a higher risk of infection and symptoms, include:

  • People over 50 years of age
  • Smokers and heavy drinkers
  • People suffering from heart disease, chronic respiratory or kidney disease
  • Anyone with an impaired immune system
  • Men are more susceptible than women.

Specific workplaces with heightened risk of exposure are discussed in chapter 6 of this article.

Symptoms of disease

Legionellosis (diseases caused by Legionella bacteria) lack characteristic symptoms or signs i.e., there is no one typical disease. In fact, it has been suggested that a spectrum of symptoms may occur from a single source[10]. The severity of the disease arising from Legionella bacteria ranges from a mild form called Pontiac fever which is self-limiting with a duration of 2 to 5 days, influenza-like illness with high infection rate (up to 95%) and short incubation period (commonly 24 to 48 hours) to severe Legionnaires’ disease[11]. Legionnaires’ disease can often be non-specific with an incubation period of 2 to 10 days but can be up to 20 days[12]. It generally involves the onset of an acute and rapid pneumonia with other, often non-specific symptoms[13],[14],[15]:

  • Loss of strength
  • High fever
  • Headache
  • Non productive, dry cough
  • Chills
  • Muscle pains
  • Difficulty breathing
  • Central nervous system manifestations e.g. confusion
  • Diarrhoea
  • Renal failure

The duration of Legionnaires’ disease is several weeks but the attack rate is very low, as not everyone exposed to the bacteria will develop symptoms of the disease. Attack rates are 0.1%-5% of the general population and 0.4% to 14% of hospital patients. Case fatality rate can reach 40 to 80% in hospital patients due to their often supressed immune systems and other underlying medical conditions[4]. However, fatality rates in the general population are variable due to differences in susceptibilities but are generally low (lower than 10%) but can also be as high as 30% or even higher.

Treatment of Legionnaires’ disease is by antibiotic therapy. Early recognition of legionellosis is an important factor for patient survival. In the largest recorded outbreak, which occurred in Murcia, Spain in 2001, there were 449 confirmed cases of Legionnaires’ disease but the case fatality was only 1% due to quick recognition of the cause, the timely intervention and the correct use of antibiotics[16],[17].


The European Centre for Disease and Prevention Control (ECDC) monitors the cases of Legionnaires' disease in Europe through the European Legionnaires’ Disease Surveillance Network (ELDSNet). Based on data from 2017, it can be concluded that legionnaires' disease remains an uncommon and mainly sporadic respiratory infection with an overall notification rate of 1.8 per 100 000 inhabitants. However, the annual notification rate increased continuously over the 2013–2017 period from 1.2 per 100 000 in 2013 to 1.8 in 2017. The disease mostly affected men aged 65 years and above [18].

Conditions leading to growth of ''Legionella''

Certain conditions, which occur within man-made water systems, can lead to the growth of Legionella bacteria. These include:

  • Water temperatures between 20oC and 45oC, which is the growth temperature range for Legionella.
  • The presence of protozoa, a type of single cell organism, which act as hosts for Legionella growth. Legionella live within amoebae in the natural environment.
  • The presence of biofilm (thin layers of microorganisms that form slime on surfaces in contact with water).
  • A source of nutrients such as sludge and corrosion or due to certain plumbing materials.
  • Stagnation of water due to storage and / or recirculation as this encourages the formation of biofilm. Occasional or seasonal use of water outlets can also lead to stagnation and biofilm growth. Examples include emergency sprinkler systems and water systems in holiday accommodation.

Water systems at risk of ''Legionella'' growth

Any water system, with the right environmental conditions as described above, could allow the growth of Legionella. The most common sources of Legionella in man-made water systems are hot and cold water systems, cooling towers and spa pools. These systems have been linked to the majority of outbreaks of Legionnaires’ disease[4]. However, there are other water systems in which Legionella can grow.

Hot and cold water system

There are various systems available to supply hot and cold water services to premises and outdoor work facilities. These range in size, scale and complexity and some require the storage of hot water or cold water or both. Water outlets that produce fine droplets or aerosols may expose workers to Legionella bacteria if the water is contaminated. Contaminated showers are most commonly associated with cases of legionellosis but taps and even flushing the toilet could lead to an aerosol and thus infection.

In order to prevent Legionella growth, the design of a hot and cold water system should enable the temperature of the hot water to be greater than 50oC and the cold water to be below 20oC to prevent Legionella growth. Transfer of heat from hot to cold water should be prevented by insulating pipework. If temperature is not used to control Legionella another method such as biocides should be employed. It is also important to minimise areas where water can stagnate by making pipework as short as possible and if removing unused water outlets the pipework should also be removed back to the circulating system. When designing buildings is should be ensured that the number of water outlets installed is not more than is needed[19].

Cooling towers (cooling systems)

A cooling tower is a device that allows removal of waste heat to the atmosphere, which often involves the cooling of water by evaporation. Cooling towers are often associated with industrial processes and can range in size from small units that sit on top of buildings to large structures of up to 200m high and 100m in diameter. However, the vast majority of cooling systems are small including many units installed on or near buildings to discharge heat from air conditioning systems[4]. As heat is reduced by evaporation, aerosols are formed that can travel from the cooling system to the air. If a cooling system is situated close to a workplace so that aerosols can enter through an air inlet or open window, the workforce may be at risk of exposure to Legionella if the cooling system is contaminated. Recent research provides evidence that Legionella pneumophila can travel several km from its source by airborne spread. It was previously believed that transmission was restricted to much shorter distances. A team of French scientists reviewed the details of an epidemic of Legionnaires' disease in 2003–2004, with18 fatalities among 86 confirmed cases. The source of infection was identified as a cooling tower in a petrochemical plant, and an analysis revealed that some infected people lived as far as 6–7 km from the plant. A similar study from Norway concluded that Legionella could have spread more than 10 km. Among 56 case patients, 10 died[20],[21]. This finding has important implications for workers and members of the public, not only in near vicinity of industrial cooling systems but also further away, and underlines the need for control of Legionella growth.

Legionella growth can be controlled, for example, by using biocides and regular maintenance of the cooling system to prevent the build-up of biofilm.

Spa pools

Poorly maintained spa pools have been linked to several outbreaks of Legionnaires’ disease[22],[23],[24]. In 1999, a spa pool at a Dutch flower show was the cause of 188 cases of Legionnaires’ disease[25] and in 2012, 21 cases were linked to a poorly maintained demonstration spa pool in a retail store in the United Kingdom[26]. Spa pools, if not treated with biocides, have by their nature the ideal conditions for Legionella growth. The water is warm and the process of creating bubbles leads to the formation of airborne water droplets that can be easily inhaled by the user or other persons, including workers, in close proximity.

Other water systems including new and emerging hazards

These include any man-made water system, which has the right conditions for Legionella growth and leads to the formation of water droplets. Many such systems are unique to a specific workplace or activity. Examples are numerous but include the use of water sprays to suppress dust in the quarrying industry and vehicle washers. Emergency showers and sprinkler systems also fall into this category as do fresh food humidifiers found in supermarkets and restaurant salad bars. The increasing use of water features in workplaces and shopping malls is also of concern. In fact, any device or system that has the potential to allow water to increase in temperature to above 20oC and forms airborne water droplets should be considered at risk of becoming contaminated with Legionella unless control measures are put in place[19].

Studies in Austria and Scandinavian countries have demonstrated the presence of Legionella in process water in the paper industry. A prevention guide has been issued in Austria[27]. Developments in workplace practices related to resource saving or alternative energy sources may create Legionella hazards. For example, industrial scale use of solar power and ground source heat could encourage Legionella growth in open systems. To save water resources there may be a temptation not to flush through pipework to remove static water but resource saving has to be balanced against controlling infection risk.

Measures to control risk of exposure in the workplace

If a workplace has a water system (as described above) that is at risk of Legionella growth, this can be controlled by undertaking a few simple steps that are applicable to any occupation[19] (also see; Hierarchy of prevention and control measures and Hierarchy of controls applied to dangerous substances articles). Occupational risks from Legionella are covered by the EU Directive 2000/54/EC on the protection of workers from risks related to exposure to biological agents at work[28] and which is transposed into national legislation by the member states. The directive obliges employers to assess the risks of biological agents and take appropriate measures[29]. In developing a Legionella risk assessment and applying controls it is important to consider additional risks associated with monitoring and maintenance procedures.

Identification of risk systems

in a first step, it is important to determine whether the water system is at risk of becoming contaminated with Legionella, for example by answering the following questions:

  • Does it fall into one of the categories described in the section above?
  • Does the system produce fine water droplets e.g. like a shower?
  • Is the water temperature between 20oC and 45oC?
  • Is the water stored or stagnant?
  • Is the water system used occasionally i.e. as little as once per week?

Control of Legionella in water systems

If the water system is at risk of Legionella growth, advice from specialist water engineers or treatment providers must be sought and control measures must be put in place. The control measures are water system dependent but may include raising the water temperature or adding biocides. Regular flushing of hot and cold water systems and cleaning of spa pools can prevent the formation of biofilm required for Legionella growth[19]. Special care is needed in case of shutdown or closure of (parts of buildings) (as was the case during the COVID-19 pandemic) since it can lead to an increased risk for Legionella growth in water systems if not managed adequately [30]. Many governmental agencies, cooling system manufacturers, and trade organisations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in cooling towers[31][29].

Monitoring of water

It is advisable that any water system at risk of Legionella growth is monitored for the growth of total aerobic bacteria and Legionella. The number of aerobic bacteria will indicate the presence of biofilm and can be monitored using a dip slide or by Adenosine Tri-Phosphate (ATP) analysis. A dip slide is an agar coated plastic slide. ATP is the main energy carrier for all living organisms and an excellent indicator of microbial activity in environmental samples. Water samples may need to be collected for analysis of Legionella by a specialist laboratory.

Specific workplaces with heightened risk of exposure and control measures

The majority of modern day workplaces are at risk of exposure to Legionella as almost all have at least a hot and cold water system for the provision of hand hygiene and many have shower facilities and / or air conditioning. Following a review of Legionella policy in European Union (EU) Member States, the European Agency for Safety and Health at Work (EU-OSHA) concluded that ‘many machines and work environments may provide a pool for Legionella but risks from this bacterium are not specific for sectors or jobs. This fact might have influenced the governments’ tendency to face Legionella primarily as a public health issue’[29]. In general, cases of Legionnaires’ disease are acquired in the community with many associated with travel or are hospital patients who have poor defences to infection[4]. Occupational cases and certainly fatalities following infection in the workplace from Legionnaires’ disease are exceptionally rare. However, certain occupations are at an increased risk of exposure as their work activities mean that they are in close proximity to a water system such as those described above that have an increased risk of being contaminated. Laboratory analysts and researchers of Legionella can also be included in those at increased risk. However, in all cases if control measures and monitoring for Legionella is undertaken the risk of exposure is low.

Water service providers and Maintenance workers (water appliances)

Workers required to clean or maintain water systems are at risk of exposure to Legionella due to the potential disruption of Legionella containing biofilm. Most obviously these include workers who clean and maintain cooling towers, air conditioning engineers, plumbers dealing with contaminated showers or pipework and spa pool cleaners. However, recent studies have demonstrated risk from exposure for workers exposed to waste water in treatment plants [32],[33]. ’’Health in the Construction Industry|Construction workers]] involved in building demolition or refurbishment may also be at risk. Risk of exposure can be greatly reduced by disinfecting the water system prior to maintenance or appropriate respiratory protective equipment that has been tested to ensure a good fit should be worn[19].

Workers in leisure, hotel industry and cruise ships

It could be suggested that workers in close proximity to spa pools at leisure centres and hotels could be at increased risk of exposure due to the extended time they are near a water system with an increased risk of Legionella growth. However, good maintenance of the spa pool should prevent growth of Legionella and exposure to workers[34]. Cruise ships require particular consideration in respect to Legionella risk due to need to store water and in some cases their intermittent use[4]. Regular flushing and disinfection of all water outlets including spa pools and water features will reduce risks to workers and clients.

Workers in industries with water spray systems

There are many industrial practices that involve regular or occasional use of a water spray system. Examples include humidification in printing works and textile mills. If these involve re-circulated or stored water for which temperature could sometimes be raised this can lead to Legionella growth. However, good control measures to prevent Legionella growth and monitoring if considered necessary will significantly reduce risk of exposure. If a system is contaminated, appropriate respiratory protective equipment that has been tested to ensure a good fit should be worn[19].

Laboratory analysts

Laboratory workers handling Legionella contaminated samples are at increased risk if precautions to avoid exposure to Legionella aerosols are not taken. These risks can be negated by using a microbiology safety cabinet for analytical and research work.

Overview of Legionella policy in Europe

In 2011, EU-OSHA published an overview of Legionella regulatory framework in the EU Member States and in selected non-EU countries[29]. On EU level, the risk of Legionella is covered in the EU directive 2000/54/EC on the protection of workers from risks related to exposure to biological agents at work. The directive categorises Legionella pneumophila and other Legionella species capable of causing human diseases as a Group 2 pathogen. A Group 2 biological agent is one that can cause human disease and might be a hazard to workers, while it is unlikely to spread to the community and where there is usually an effective prophylaxis or treatment. The report also examined guidance and standards of other EU and international organisations such as the World Health Organization (WHO), the International Standards Organisation (ISO), and the European Committee for Standardisation (CEN). The report summarised codes of practice and guidance from each country but found on the whole advice was given with respect to Legionella as a public health issue and not an occupational hazard. However, the technical guidance of each Member State could be equally used to reduce risks to workers from Legionella contaminated systems in the workplace.


As nearly all cases of Legionnaires’ disease have been public health related, efforts to control Legionella have centred on public health issues and not occupational health and safety[29]. This was the conclusion of the EU-OSHA overview of policy across the EU Member States published in 2011. However, due to the nature of Legionella bacteria and their ability to grow in man-made water systems it should not be underestimated that workers may be exposed to these bacteria and may be at risk of infection if they are susceptible. However, cases of worker ill health are rare with very few occupations at increased risk. On a national level, most EU member states have a public health based policy to combat Legionella and if risk assessment is promoted and control measures implemented the risk to workers should be minimal. Industry- or sector-specific guidance may be valuable to raise the profile of risks associated with particular activities[35][34].


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[7] Pravinkumar, S. J., Edwards, G., Lindsay, D., Redmond, S., Stirling, J., House, R., Kerr, J., Anderson, E., Breen, D., Blatchford, O., McDonald, E. & Brown A. ‘A Cluster of Legionnaires’ disease caused by Legionella longbeachae linked to potting compost in Scotland, 2008-2009’, Eurosurveillance, Vol. 15, Issue 8, 2010. Available at [2]

[8] Government of Western Australia. Code of Practice: Prevention and Control of Legionnaires’ Disease, 2014. Available at [3]

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[12] WHO- World Health Organisation Guidelines for Drinking Water Quality,. Recommendations, Geneva, WHO, 3rd ed. Vol. 1, 2004

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[14] Akbas, E. & Yu, V., ‘Legionnaires’ disease and pneumonia: beware the temptation to underestimate this “exotic” cause of infection’, ''Postgraduate Medicine'', Vol. 109, No 5, 2001, pp. 135–147

[15] Mülazimoglu, L. & Yu, V. L., ‘Can Legionnaires’ disease be diagnosed by clinical criteria? A critical review’, ''Chest'', Vol. 120, No 4, 2001, pp. 1049–1053

[16] García-Fulgueiras, A., Navarro, C., Fenoll, D., García, J., González-Diego, P., Jiménez-Buñuales, T., Rodriguez, M., Lopez, R., Pacheco, F., Ruiz, J., Segovia, M., Balandrón, B. & Pelaz, C. ‘Legionnaires’ disease outbreak in Murcia, Spain’, ''Emerging Infectious Diseases'', Vol. 9, No 8, 2003, pp. 915–921

[17] Roig, J. & Rello, J. ‘Legionnaires’ disease: a rational approach to therapy’. ''Journal of Antimicrobial Chemotherapy'', Vol. 51, 2003, pp. 1119–1129

[18] European Centre for Disease and Prevention Control (ECDC), Legionnaires’ disease, Annual Epidemiological Report for 2017. Available at:

[19] HSE- Health & Safety Executive, Approved Code of Practice and guidance: Legionnaires’ disease, The control of legionella bacteria in water systems (L8). Available at [6]

[20] Nguyen, T.M.N., Ilef, D., Jarraud, S.,Rouil, L., Campese, C., Che, D., Haeghebaert, S., Ganiayre, F., Marcel, F., Etienne, J. & Desencloset, J-C. A community-wide outbreak of legionnaires disease linked to industrial cooling towers—how far can contaminated aerosols spread?", Journal of Infectious Diseases, 193 (1),2006, pp. 102–11. Available at: [7]

[21] Nygård, K., Werner-Johansen, O., Rønsen, S., Caugant, D.A., Simonsen, O., Kanestrøm, A., Ask, E., Ringstad, J., Ødegård, R., Jensen, T., Krogh, T., Høiby, E.A., Ragnhildstveit, E., Aaberge, I.S. & Aavitsland, P., ‘An Outbreak of Legionnaires Disease Caused by Long-Distance Spread from an Industrial Air Scrubber in Sarpsborg’, ''Norway Clinical Infectious Diseases'' , 46(1), 2008, pp. 61-69. Available at:

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[26] Coetzee, N., Duggal, H., Hawker, J., Ibbotson, S., Harrison, T. G., Phin, N., Laza-Stanca, V., Johnston, R., Iqbal, Z., Rehman, Y., Knapper, E., Robinson, S. & Aigbogun, N. ‘An outbreak of Legionnaires’ disease associated with a display spa pool in retail premises, Stoke-on-Trent, United Kingdom, July 2012’, ''Eurosurveillance'', Volume 17, Issue 37, 2012. Available at: [11]

[27] Leifaden Legionellenprävention in der Papierindustrie. Austropapier 2010. Available at: [12]

[28] Directive 2000/54/EC of the European Parliament and of the Council of 18 September 2000 on the protection of workers from risks related to exposure to biological agents at work (seventh individual directive within the meaning of Article 16 of Directive 89/391/EEC). Available at: [13]

[29] EU-OSHA - European Agency of Safety and Health at Work, ''Legionella and Legionnaires’ disease: A policy overview'', 2011. Available at [14]

[30] ESCMID Study Group for Legionella Infections, ESGLI Guidance for managing Legionella in building water systems during the COVID-19 pandemic. Available at: [15]

[31] INRS (2006). Les légionelles en milieu de travail. Available at:[16]

[32] Blatny, J., Ho, J.,Skogan, G.,Fykse, E., Aarskaug, T.,Waagen, V. ‘Airborne ''Legionella'' bacteria from pulp waste treatment plant: aerosol particles characterized as aggregates and their potential hazard’, ''Aerobiologia'', Vol 27, No 2, 2011 , pp. 147-162.

[33] Kusnetsov, J., Neuvonen, L-K., Korpio, T., Uldum, S.A., Mentula, S., Putus T., Tran Minh, N. N., & Martimo, K-P., Two Legionnaires' disease cases associated with industrial waste water treatment plants: a case report. ''BMC Infectious Diseases''. Vol 10, 2010, p 343. Available from: [17]

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[35] HSE, Control of legionella and other infectious agents in spa-pool systems, 2017. Available at

Further reading

EU-OSHA - European Agency of Safety and Health at Work, Legionella and Legionnaires’ disease: European policies and good practices, Factsheet 100. Available at:

EU-OSHA - European Agency of Safety and Health at Work, Legionella and Legionnaires’ disease: a policy overview, Report, Available at:

EU-OSHA - European Agency for Safety and Health at Work, E-fact 53: Risk assessment for biological agents, Available at:

EU-OSHA - European Agency for Safety and Health at Work, Biological agents and work-related diseases: results of a literature review, expert survey and analysis of monitoring systems, Available at:

ESGLI - European Technical Guidelines for the Prevention, Control and Investigation, of Infections Caused by Legionella species. 2017. Available at:

CCOHS – Canadian Centre for Occupational Health and Safety. Legionnaires’ disease. Retrieved 29 January 2013, from:

European Legionnaires’ Disease Surveillance Network (ELDSNet). Home page. Retrieved on 13 May 2020, from:

ISO 11731:2017, Water quality - Enumeration of Legionella.

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Karla Van den Broek

Prevent, Belgium
Ferenc Kudasz

Helena Senior