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Legionella, an Overview and Guide to Prevention

Published on 12/22/2015

Legionnaires disease is a somewhat rare but fatal type of pneumonia caused by the bacterium legionella commonly found throughout the natural environment. In order to catch legionella or the lesser Pontiac Fever inhalation of water droplets (aerosol) contaminated with the legionella bacterium must first take place.


Since aerosol can be created from almost any open water system such as showers, spray taps and spa pools there is a significant foreseeable risk of legionella where open water systems are present in industrial, municipal and domestic environments.


Symptoms of legionella include a high fever, dry cough and chills that can go on to become fatal if not treated promptly. Many cases are missed due to under-reporting or misdiagnosis by clinicians; it is critical that the appropriate antibiotics are administered to prevent the disease progressing further or causing permanent damage to the lungs resulting in lifelong disability.


Many cases are acquired abroad due to poor building health and safety practice and the lack of legislation and knowledge surrounding preventative measures.


History and Legislation



Following a convention of the American Legion in Philadelphia, 1976 a large proportion of the delegates became ill with pneumonia that eventually resulted in 29 deaths. A subsequent investigation into the cause of the deaths revealed the presence of a previously unidentified bacteria, subsequently named Legionella Pneumophilia, of which around 90% of cases of legionella today can be attributed to.


Cases of Legionnaires disease are rare in Western Europe and North America due to better health and safety awareness and water hygiene within buildings. When an outbreak of Legionella occurs however it tends to attract significant media attention and be devastating for the business deemed responsible with corporate manslaughter charges often brought, such as in the case of Barrow in Furness in the UK in 2002. (


Legionellosis, including Pontiac fever tends to affect men more than women, smokers, people with compromised immune systems and the elderly. This makes detection and prevention particularly important when considering buildings where a susceptible population will be found. Examples of this include care homes where there are a high concentration of elderly residents and healthcare buildings such as hospitals and clinics.


The guidance in the UK is particularly stringent for hospitals and heath care facilities with the government publishing a separate technical document alongside the Associated Code of Practice (ACoP L8), HTM-04-01 aimed at duty holders in the healthcare setting.


In the US the OSHA has a technical memorandum in regards to legionellosis ( Here protocol is laid out into investigative procedures, sampling, preventative measures and conducting employee awareness programs.


Due Diligence & the Legionella Risk Assessment


In many countries such as the United Kingdom it is a legal requirement under the Health & Safety at Work Act to carry out a legionella risk assessment in order to identify the risk present from a buildings hot and cold water systems. The OSHA in the US lays out guidelines for preventing legionella and treatment within a water system. A study conducted by Paola Borella et al in Italy illustrates the prevalence of legionella bacteria in water systems with 22.6% of hot water systems surveyed having legionella bacteria present. (


It is the responsibility of the duty holder, i.e. the person ultimately in charge of the building such as the managing agent or landlord to appoint a suitable person to undertake a legionella risk assessment.


The risk assessment is the starting point of a water treatment control program and should be considered when:


  • There is an open water system present with the potential to create aerosol
  • Legionella bacteria has been detected in the system. There has been a significant change or upgrade to the water system
  • The management pathway has changed such as during a company take over or there has been a significant change in responsible personnel


The risk assessment should include:

  • A detailed overview of the buildings water system with schematic diagram showing water outlets
  • The management pathway showing responsible personnel with their names and contact details
  • A written scheme detailing the sites control measures
  • A temperature profile of the sites hot and cold water services
  • The significant findings of the survey clearly detailed and an action plan to address any issues found


Legionella in the Environment and Proliferation


Legionella bacteria can be found throughout the natural environment in the soil, rivers and lakes. Rapid growth within man-made water systems can commonly occur if the right environmental conditions are present with many domestic and industrial systems representing a foreseeable risk in regards to legionella growth.


Systems of particular concern include open cooling system such as cooling towers and evaporative condensers, air conditioning systems, whirl pool spas and multiple shower systems often found in leisure centers and hospitals.

Systems should be flushed regularly with cold water storage tanks and hot water storage vessels inspected annually and treated with an appropriate biocide such as chlorine if deemed necessary. Shower heads and spray taps need to be kept clean and a quarterly de-scaling program is recommended.

Industrial Boilers
Figure 1. Water tanks and hot water vessels should be inspected annually and cleaned if necessary
Open cooling systems need meticulous control to prevent bacterial growth. Biocides such as chlorine and bromine should be utilized alongside anti-scaling and anti corrosion compounds in order to maintain a clean environment. Thermostatic mixers should be regularly serviced and should comply with ISO-9001 guidelines. Spray nozzles must be kept clean and free of scale.


UV (Ultra-violet) treatment of water should be considered where there is a perceived risk of bacterial growth. UV technology is often utilized in hospital settings such as kidney dialysis wards or in large hotels where large volumes of water need to be treated. All installations should comply with current ISO-9001 guidelines.

The diagram below illustrates the main factors needed that may result in an outbreak of legionnaires disease within a building or local environment, explored in more detail below.
Legionella Outbreak Diagram
Figure 2. Legionella Outbreak Diagram.

Nutrient Source


For the legionella bacterium to thrive a suitable nutrient source must be available within the water system. Typically nutrient sources include sludge in the base of water heaters and calorifiers, where a low flow environment and ideal temperature range can give rise to legionella proliferation if the right conditions are present.

Other nutrient sources include biofilm in water systems and iron and scale deposits such as often seen on showerheads and within industrial cooling tower systems.

Stagnant or Poor Water Flow

Bacteria can build up and thrive in areas of the water system that are not subject to regular water flow.

Areas of the buildings water system should be checked for dead legs (redundant pipe work) or little used outlets and these should be noted in the risk assessment. Ideally these should be removed or the little used outlet flushed weekly to prevent bacterial growth.


Aerosol Creation and Dispersal


Any open water system has the potential to create an aerosol and therefore distribute legionella bacteria if it is present. The risk assessment should identify potential high-risk outlets and how to manage these. Systems such as multiple showers and open cooling systems with high volumes of water and the potential for multiple little used outlets are of particular concern.
Poor Building Management & Communication

A lack of a suitable and sufficient management control program is often one of the biggest causal factors when considering an outbreak of legionella. Appropriate responsible personnel should be appointed and a deputy person should be made available to cover tasks such as temperature monitoring and testing when the responsible person is not available.

Poor communication between maintenance staff and managers will lead to gaps in record keeping and lapses in system control; all tasks should be clearly defined with records kept up to date.


Biofilm and Legionella



Legionella bacteria thrive in a temperature range of 20-45°C (68-113°F) and this should be avoided by ensuring water heaters are heated to at least 60°C (140°F) at all times. Temperature monitoring of the building should be carried out on a monthly basis ensuring an appropriate temperature profile of the building is maintained.

Where temperature control is not possible due to limits in system design a residual form of disinfection should be considered such as chlorine dioxide or UV sterilization.




It is important to remember that where there is an open water system with potential for aerosol creation there is the potential for legionella proliferation and distribution to occur.

When considering larger water systems especially when there is a high-risk population present such as hospitals, care homes, health centers and dental clinics a more robust defense against legionella should be considered. Here UV (Ultra Violet) disinfection systems coupled with micro filtration are a viable and recommended option for removing bacteria such as legionella and pseudomonas from the water supply before entry into the buildings water systems. The presence of a UV filter will kill off any bacteria held within the water supply entering the building and is a viable option to consider when extra protection against pathogens is required. Micro filtration is advised by the OSHA in regards to dental water lines that can harbor multiple pathogens. Water should be filtered at the point of use with FDA approved 0.22-microns pore sizes filters in order to help protect patients and staff at the dental clinic.


By following the appropriate guidelines the risk can be minimized, however meticulous management control is required as lapses in monitoring and testing regimes can allow an outbreak to occur.