Legionnaire's disease


species are bacteria, which occur naturally in fresh water and the associated environment around the world, including in pristine mountain streams and hot geysers. Legionella pneumophila is the type associated with most cases and outbreaks, (“Legionella” after the first recognized outbreak which occurred following a convention of the American Legion in Philadelphia in 1976 and “pneumophila” which is Greek for lung loving). Legionellae (the plural form of Legionella) will also colonise and grow within constructed water systems and associated equipment which are not well designed and managed.

Legionellosis is the term used to describe infections cause by legionellae which can be in the form known as Legionnaires’ disease (LD), an acute form of pneumonia which can lead to death in approximately 7% of those affected (in reported cases in Europe *). The death rate can be much higher in hospital acquired cases (up to 40%). The time between exposure and developing symptoms of LD (incubation time) is usually within 2 to 10 days, whilst this time can be longer, for most cases it is around 6 days.

There is also a mild form of legionellosis e.g., Pontiac fever; which doesn’t cause pneumonia and is a self-limiting flu like illness which has a short incubation time - the time from exposure to symptoms - of around 48 hours. It does not usually require any treatment or have long-term consequences.

Legionnaires’ disease symptoms

If diagnosed and treated quickly LD responds well to appropriate antibiotics. However, is not possible to diagnose LD by symptoms alone, and clinical tests are essential to ensure an accurate diagnosis and effective antibiotic treatment. Initial symptoms can be similar flu, often with a sudden onset and can sometimes include gastrointestinal symptoms which can confuse the diagnosis. Common symptoms can include:

  • Dry cough
  • High fever
  • Aching muscles
  • Headache
  • Chest pains
  • Shortness of breath.

Legionnaires’ disease risk factors

Legionnaires’ disease has a low infection rate of <5%, so most people who are exposed to Legionella will not become ill. Whilst anyone may be infected with Legionella, those at increased risk, include smokers, those who are immunocompromised because of illness or treatment, those with underlying chronic illnesses e.g., heart and lung diseases, diabetes and increasing age (>45y). For reasons we don’t yet understand, males are around three times more likely to acquire LD than females. To date there is only one probable case of person-to-person transmission of Legionnaires disease, which occurred in Portugal during an outbreak caused by an industrial evaporative cooling tower **.

Modes of Legionnaires’ disease infection

Infection is usually by inhalation of aerosols derived from water droplets. To get deep enough into the lungs to cause infection, the aerosol particles need to be <5µm; at this size most of the water will have evaporated, leaving a remaining infectious aerosol which is not visible to the naked eye, and can remain suspended in air for hours, particularly in warm humid environments. Where systems are not managed effectively infectious aerosols can be formed for example, when water runs over the pack in an evaporative cooling tower, from hot and cold-water systems when turning on a shower, tap or flushing toilets etc. and from exposure to aerosols emitted from spa baths, hot tubs etc. and other aerosol producing equipment.

Aspiration is another mode of infection where, when water is taken into the mouth, it enters the respiratory tract and the lungs rather than the gastrointestinal tract “going down the wrong way”. The risk of aspiration increases with age, those who have had strokes or other conditions which can affect swallowing, so should be considered for cases occurring in healthcare premises and care / nursing homes etc.

Outbreak sources

The three major sources of outbreaks of LD are:

  1. Evaporative cooling towers (i.e. “wet” not dry cooling towers) such as those used for comfort and industrial cooling
  2. Large / complex hot and cold-water systems for example in hospitals, hotels etc. and
  3. Spa pools (also known as whirlpool spas).

Cases and outbreaks have also been associated with a range of water systems and equipment including hot tubs, humidifiers including small ultrasonic devices, jet washers, misting devices used to keep food and vegetables fresh in supermarkets, hotels etc. fountains (particularly those indoors) and many others. LD is also associated with exposure to non-peat based potting compost, usually caused by Legionella longbeachae rather than L. pneumophila; predominantly in Australasia though there have been a few cases in the rest of the world including Scotland.

Legionnaires' disease - the facts

  • Legionella pneumophila is the type associated with most cases and outbreaks of Legionnaires' disease
  • Legionnaires' disease is contracted through inhalation of water droplets infected with the legionella bacterium
  • Legionella bacteria can also cause other flu like diseases such as Pontiac fever and Lochgoilhead fever
  • Legionnaires’ disease has a low infection rate (around 5%) but a mortality rate of around 7% in reported cases.

Legionella in plumbing systems and associated equipment

Drinking water, even when supplied by a public utility, normally contains a range of microorganisms, including potential pathogens such as Legionella. However, where water supply temperatures are below 20°C at the point of supply; the risk of infection is very low as legionellae only actively grow and become infectious in warm water (>25°C and <50°C). However, within buildings systems and equipment, if not designed and / or managed effectively, conditions which allow microorganisms, including Legionella, to grow to levels which can cause illness include:

  • Warm water between >20°C and <50°C,
  • Areas of low flow or stagnant water increasing the potential for biofilm development*** e.g. where water is stored or re-circulated; there are dead legs such as infrequently used outlets (including shower heads, taps and hoses), and / or blind ends etc.
  • Where there are nutrients present which allow the growth of supporting microorganisms such as scale, corrosion, particulates, and sediment. Nutrients can also leach from materials of the system itself and the support microbial growth such as plastics and EPDM lined flexible hose pipe tap connectors, also jointing cutting fluids and compounds etc.
  • Where components fittings etc. are installed which have been leak / pressure tested by the manufacturer, with water and left damp or with water remaining inside or been previously used (e.g. pumps used during commissioning).

Legionella like some other waterborne pathogens, can also be protected inside cysts of protozoa, (single celled microorganisms which occur naturally in water supplied to buildings), and can survive high concentrations of biocide treatment (50 ppm hypochlorite overnight) as well as protection from heat and drying.

Control of Legionella in plumbing systems

The golden rule is “keep systems clean, flowing, the hot, hot and the cold, cold”.

As Legionella are dormant below 20°C and are killed within seconds at 60°C, the traditional approach to control the risk from Legionella is via temperature control. The Health and Safety Executive (HSE) advises the following measures:

  • Hot water storage cylinders should store water at 60°C or higher.
  • Hot water should be distributed at least at 55°C so it reaches the outlet at 50°C (55°C in healthcare premises registered with CQC) within one minute of turning on the outlet.
  • Cold water should be stored and distributed to reach the outlet at less than 20°C, within two minutes of turning on the outlet.

In a well plumbed system these target temperatures should be achievable in seconds. Where a risk assessment identifies there is a scalding risk and a need for thermostatic mixing valves these should need to be fitted as close as possible or within, outlets. Outlets such as those feeding baths and showers (where there is whole body immersion) should always have TMVs fitted.

The risk of managing Legionella must be balanced with the risk of scalding injuries, especially in residential and healthcare properties. 

However, where temperature control cannot be achieved then alternative means of control are necessary, such as the use of biocides including copper silver ionisation or chlorine dioxide, as appropriate and agreed within the Water Safety Plan.

Wherever the Health and Safety Act applies e.g. commercial, and industrial premises, employers, the self-employed and people in control of premises, such as landlords, have a duty to identify and control risks associated with Legionella. Likewise in domestic rented properties - from whole properties to rented rooms - landlords have legal responsibilities to ensure the health and safety of their tenant, including managing the risk from Legionella.

Find out more at:



Empty premises and Legionella

When water is not drawn through a building’s water storage and distribution system, or associated equipment, over an extended period, the water will become stagnant, allowing biofilm microorganisms to grow to levels which can cause infection and be difficult to control. The levels of chemical contaminants from the materials used within the system may also increase (e.g. lead).

It is therefore vital to ensure that water systems are safe to use after a prolonged shutdown. It is recommended that a competent person is appointed to carry out a risk assessment, and oversee any plumbing and heating engineering work to ensure the integrity of the plumbing being re-commissioned.

Key points:

  • Indicators of stagnation - indicate bacteriological growth and/or pipe corrosion - include:
    • A bad or ‘off’ taste
    • Unpleasant odour
    • And/or discoloration.
  • Stagnation can support the accelerated growth of bio-slime, waterborne micro-organisms, and pathogens, such as Legionella, as well as heavy metals which can cause harm to building occupants and users.
  • An empty system that has been drained results in very damp pipe work that contains oxygen. Over a period of time this will increase the corrosion risk inside the pipe work, especially at compression joints and can also support the growth of Legionella and other microorganisms.
  • A Water Safety Plan (WSP) and policy should be developed and implemented following the guidance presented in the BS 8680:2020 Code of Practice to ensure all water systems and equipment containing or using water are safe before use.
  • A risk assessment review to BS 8580-1 should be carried out as part of the WSP development (and BS 8580-2 where there is a risk from other waterborne pathogens such as aeruginosa e.g., in health and leisure premises).
  • A task risk assessment should be developed to ensure the health and safety of those carrying out the flushing and re-commissioning
  • Commissioning following the processes within the WSP should be carried out following a risk assessment (there is a legal requirement this should only be carried out by persons competent to do so see HSE ACOP L8), including pressure testing of all systems.
  • When the system is recommissioned, leaks may occur which can be difficult to trace and especially hard to repair.
  • Flush your water system before your business or building reopens. Flush water through all points of use within the building before re-opening (e.g. showers, sinks, toilets).

A guide to flushing water systems along with other considerations for safe water can be found in Mitigating the Risk of Building Water Systems - a free download from the Chartered Institute of Plumbing and Heating Engineering (CIPHE) and the Construction Industry Council (CIC).

See also ESGLI guidance for managing building water systems during the COVID-19 pandemic 


* Legionnaires’ disease. Annual Epidemiological Report for 2019.(accessed 14/1/2022).

** Correia AM, Ferreira JS, Borges V, Nunes A, Gomes B, Capucho R, Gonçalves J, Antunes DM, Almeida S, Mendes A, Guerreiro M. Probable person-to-person transmission of Legionnaires’ disease. New England Journal of Medicine. 2016 Feb 4;374(5):497-8.

*** Biofilms are a slime layer produced by bacteria and can be attached to all surfaces, (tanks, pipework, within components etc). Biofilms form a supporting matrix which protects them from sheer stresses, heat, and chemical treatment. Scale and corrosion increase the surface area and provides niches for microbial attachment which make them much more resistant to treatment. To control microorganisms growing in biofilms it can up to 1000x longer contact time for a given concentration of biocide than for their free-living counterparts.

Legionella risk assessing in domestic properties

The CIPHE has published a guide to assessing Legionella risk in domestic properties, priced at £18.00 for members or £30.00 for non-members.

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Safe Water Guide: Scald Prevention and Legionella

The CIPHE's Safe Water Guide: Scald Prevention and Legionella is priced at £20.00 for members and £35.00 for non-members.

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