Legionella in the workplace

Introduction

Legionella is a family of pathogenic waterborne bacteria, some of which cause a group of infections called legionellosis. The Legionnaires’ disease is the most severe form of legionellosis.

This article provides an overview of Legionella and explains how growth within constructed water or air conditioning systems leads to exposure. It includes the range of symptoms and the worker groups at high risk. Wherever conditions in a water or air conditioning system are right for Legionella growth and transmission, there is a risk of Legionella infection. Therefore, unless Legionella is controlled in these 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 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]. L. longbeachae is responsible for approximately 5% of Legionella associated pneumonia caused by Legionella species other than L. pneumophila^[3].

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 water system where conditions for growth are favourable.

The route of infection

People normally catch Legionnaires’ disease by inhaling Legionella pneumophila 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^[4].

Pneumonia caused by L. longbeachae has been associated with exposure to contaminated compost in Australia, New Zealand, the USA and Japan^[5][6][7]. In Europe, only a few cases associated with L. longbeachae in compost have been reported in Scotland^[8], the Netherlands, France and Switzerland^[9]. The mode of transmission in this case is likely to be from inhaling contaminated soil particles^[10].

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:

• Reservoir: there must be an environment where the bacteria can survive and multiply to reach a high concentration.
• The effectiveness of transmission: this refers to how the bacteria leaves the reservoir and reaches the host and depends on the likelihood to form aerosols.
• Susceptible host: this is a person (in this case, it also could be an animal) that is vulnerable to infection. The susceptibility to be infected depends on the general health condition of the person, their immune system, age etc.

Infections can be controlled by breaking this chain of events. 

Major human risk groups

Not everyone exposed to Legionella becomes infected and shows symptoms of ill health^[3]. 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^[11]. The groups of people who are at a higher risk of infection and symptoms, include^{[12] [13][14]}:

• People over 50 years of age
• Smokers and those with a history of 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 below.

Symptoms of disease

Legionellosis (diseases caused by Legionella bacteria) lack characteristic symptoms or signs i.e., there is no one typical disease. 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^[15]. Legionnaires’ disease can often be non-specific. The incubation period is usually 2 to 10 days^[16]. It generally involves the onset of an acute and rapid pneumonia with other, often non-specific symptoms^[17][18][19]:

• 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^[3]. In the general population, fatality rates are variable due to differences in susceptibilities and are generally low (lower than 10%) although they 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% thanks to quick recognition of the cause, the timely intervention and the correct use of antibiotics^{[20] [21]}.

Prevalence

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 2021, it can be concluded that legionnaires' disease remains an uncommon and mainly sporadic respiratory infection with an overall notification rate of 2.4 cases per 100 000 inhabitants. However, the annual notification rate increased continuously over the 2017–2021 period from 1.8 per 100 000 in 2017 to 2.4 in 2021. The disease mostly affected men aged 65 years and above^[22].

Conditions leading to growth of Legionella

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

• Water temperatures between 20°C and 45°C, 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 (a type of protozoa) in the natural environment.
• A source of nutrients such as sludge, corrosion and certain plumbing materials.
• The presence of biofilm (thin layers of microorganisms that form slime on surfaces in contact with water).
• 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 environmental conditions described above, could allow the growth of Legionella. The most common sources of Legionella in constructed 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^[3]. However, there are other water systems in which Legionella can grow.

Hot and cold water systems

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. More complex water supply systems can be found in workplaces such as hotels, gyms, spas and healthcare facilities, as well as industrial sites. 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 50°C and the cold water to be below 20°C. 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 the use of biocides should be employed. 

It is also important to minimise areas where water can stagnate by making pipework as short as possible. Unused water outlets and unused pipework (dead legs/dead ends) should be removed. When designing buildings, it should be ensured that the number of water outlets installed is not more than is needed^[23].

Cooling towers (cooling systems)

A cooling tower is a heat exchange device that removes excess heat from HVAC systems or industrial processes by cooling warm water through contact with air. This results in some of the water evaporating into the atmosphere, while the remaining water cools down. The cooled water is then recirculated back into the system to maintain its optimal temperature. Cooling towers are used in wide range of workplaces such as power plants, refineries, food and beverage processing, chemical industry, data centres and hospitals. Cooling towers 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^[3]. 

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 and aerosols can enter it through an air inlet or open window, the workforce may be at risk of exposure to Legionella if the cooling system is contaminated. Research provides evidence that Legionella pneumophila can travel several kilometres from its source by airborne spread. A team of French scientists reviewed the details of an epidemic of Legionnaires' disease in 2003–2004, with 18 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^[24][25]. 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 it 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^[26][27][28]. In 1999, a spa whirlpool installed in an exhibition hall at a Dutch flower show was the cause of 188 cases of Legionnaires’ disease^[29] and in 2012, 21 cases were linked to a poorly maintained demonstration spa pool in a retail store in the United Kingdom^[30]. 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 (new and emerging) water systems at risk of Legionella growth 

Any device or system that has the right conditions for Legionella growth - meaning that it contains water with temperatures between 20°C and 45°C and generates airborne water droplets should be considered at risk of becoming contaminated with Legionella, unless adequate control measures are implemented. Many such systems are unique to a specific workplace or activity. Examples are numerous and 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. 

Studies in Austria and in Scandinavian countries have demonstrated the presence of Legionella in process water in the paper industry. 

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 of ground source heat could encourage Legionella growth in open systems. In order to save water resources, there may be a temptation not to flush through pipework to remove static water. However, resource saving has to be balanced against controlling infection risk.

Overview of Legionella policy in Europe

At EU level, the risk of Legionella falls in the scope of EU directive 2000/54/EC on the protection of workers from risks related to exposure to biological agents at work^[31]. 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 Directive obliges employers to assess the risks of biological agents and take appropriate measures^[32]. In developing a Legionella risk assessment and applying controls, it is important to consider additional risks associated with monitoring and maintenance procedures.

Employers are obliged to inform workers on the risks of Legionella exposure and training them accordingly to prevent exposure.

In 2011, EU-OSHA published an overview of Legionella regulatory framework in the EU Member States and in selected non-EU countries^[32]. 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 and found that, on the whole, guidance was given with respect to Legionella as a public health issue and not an occupational hazard. However, the Member States’ technical guidance related to public health could be equally used to reduce risks to workers from Legionella contaminated systems in the workplace.

Evaluation of the risk

Identification of risk systems

In a first step, it is important to determine whether a water system is at risk of becoming contaminated with Legionella, for example by checking whether any of the following risk factors are present: 

• Does the system produce fine water droplets/aerosols (for example like a shower)?
• Is the water temperature between 20°C and 45°C?
• Is the water stored or stagnant?
• Is the water system used occasionally i.e. as little as once per week?

More information on how to identify systems at risk is available in national guidelines^[32].   

Specific workplaces with heightened risk of exposure and control measures

Nowadays, the majority of 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’^[32]. In general, cases of Legionnaires’ disease are acquired in the community and many are associated with travel or are hospital patients with a weak immunity system. Occupational cases of Legionnaires’ disease, and certainly fatalities following infection in the workplace, are exceptionally rare. However, certain occupations requiring working in close proximity to a water system at risk of Legionella growth are at an increased risk of exposure. Laboratory analysts and researchers of Legionella are also occupations at increased risk. However, in all cases, if adequate control measures and monitoring are implemented to prevent the risk of Legionella growth, the risk of exposure is low.

Water service providers and maintenance workers of 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. Construction workers involved in building, demolition or refurbishment may also be at risk. The risk of exposure can be greatly reduced by disinfecting the water system prior to maintenance or, if this is not feasible, by wearing appropriate, well-fitted respiratory protective equipment should be worn^[23].

Temporary sanitary installations (site welfare units) at construction sites

Several cases of work-related Legionnaires' disease associated with site welfare units have been reported. For example, in France, there was one case of Legionnaires' disease in the Haute-Pyrénées in 2015 associated with the use of such installations, and several cases in Burgundy in 2017, one of which was fatal. In both outbreaks, the cases were linked to the presence of Legionella in the showers of the site welfare units^[33]. Measures should be taken at the design, installation and use stages to prevent the growth of Legionella in temporary welfare units on construction sites. Where possible, mains water should be used instead of stored water. In particular, if the unit is not used for a period of time (e.g. weekends), water systems should be flushed with hot water or disinfectant^[33].

Wastewater treatment plants

There is evidence that Legionella bacteria can survive and multiply in wastewater and cases of legionellosis have been associated with wastewater and wastewater treatment plants^{[34][35][36][37]}. In 2017 in the Netherlands, a wastewater treatment plant at a food processing company was identified as the likely source of contamination for a local increase in pneumonia caused by L. pneumophila. Indeed, the wastewater treatment process produces aerosols that can disperse Legionella into the environment, posing a risk to workers and nearby residents^[38].

Workers in leisure, hotel industry and cruise ships

Workers in close proximity to spa pools at leisure centres and hotels may be at increased risk of exposure due to the extended period of time that they spend working 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 of workers^[39]. Cruise ships require particular consideration in respect to Legionella risk due to need to store water and, in some cases, due to their intermittent use^[3]. 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 the risk of exposure. If it is not feasible to sufficiently reduce the contamination risk, appropriate, well-fitted respiratory protective equipment should be worn^[23].

Laboratory analysts

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

Measures to control risk of exposure in the workplace

A few simple steps can 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 above 45°C 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^[23]. 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^[40]. 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^[41][32].

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[a] or by Adenosine Tri-Phosphate (ATP)[b] analysis. Water samples may need to be collected for analysis of Legionella by a specialist laboratory.

Summary

As nearly all cases of Legionnaires’ disease have been public health related, efforts to control Legionella have centred on public health issues rather than occupational health and safety. This was the conclusion of the EU-OSHA overview of policy across the EU Member States published in 2011^[32]. However, due to the nature of Legionella bacteria and their ability to grow in constructed water systems, the fact that workers may be exposed to these bacteria and may be at risk of infection should not be underestimated. Still, cases of work-related diseases from Legionella are rare and very few occupations are at increased risk.

On a national level, most EU member states have a public health-based policy to combat Legionella and, if a proper risk assessment is carried out and adequate control measures are implemented, the risk to workers should be minimal. Industry- or sector-specific guidance may be valuable to raise awareness of the risks associated with particular activities^[39].