Legionella pneumophila bacteria (first identified in July 1976) are found in all types of nutrient-rich fresh water environments, including rivers and streams. From these sources they enter and colonise man-made water systems such as humidifiers, cooling towers and hot and cold water systems. The bacteria are dormant below 20°C and do not survive above 60°C. Legionella only become a risk to health if the bacteria multiply rapidly, which occurs when temperatures are between 20-45°C and nutrients are available.

Legionnaires’ disease is a potentially fatal type of pneumonia, contracted by inhaling airborne water droplets containing viable Legionella bacteria; therefore the plumbing system needs to create fine mists of water to be dangerous. Anyone can develop Legionnaires’ disease, but the elderly, smokers, alcoholics and those with cancer, diabetes or chronic respiratory or kidney disease are at more risk. Because the symptoms are similar to pneumonia it is often argued that the number of cases are far higher than the official figure of 400 a year in the UK. The original outbreak was from a cooling tower in the US (plenty of warm moist air and nutrients, spread in a fine mist) but since then whirlpool baths and showers have also led to outbreaks and in the UK concern about Legionella is high.

How to prevent legionella proliferation

The risk of legionella increases if the water temperature in all or some parts of the system is between 20–45°C; water droplets can be produced and dispersed; water is stored and/or re-circulated; there are deposits that can support bacterial growth, such as rust, sludge, scale, organic matter and biofilms; and the building houses ‘at risk’ people who may be exposed to the contaminated aerosols. Control and prevention of the disease is through treatment of contaminated water systems and eliminating bacterial growth in the first place through good design. As treatment either requires the administration of biocides or a temperature regime that results in a negative impact on energy and water efficiency, good design is of paramount importance.

Keep cold water temperatures below 20°C

Inhaling cold water droplets is rarely dangerous as the bacteria remain dormant below 20°C. Ensure thermal gain is kept to a minimum by adequate lagging and the separation of cold water services pipework and components from hot water services and heating systems. Use direct cold water supplies or, if stored cold water is required, minimise storage to 24 hour’s use and ensure no stagnation can occur by the position of the inlet and outlet pipework.

Ensure hot (not warm) water at all outlets

Hot water systems are considered a greater risk, predominately because of the presence of warm water within the system, both as the cold water is heated up in the cylinder and as it cools down in the pipework. Hot water should be stored at 60 °C as a minimum and distributed so that it reaches a temperature of 50°C (55°C in healthcare premises) within one minute at the outlets. Therefore it is the position of hot water outlets compared to the position of the main heating store that requires the most thought. If hot water outlets cannot be centrally grouped, then point of use water heaters should be specified. Do not to oversize the hot water storage and ensure that cylinders have fast recovery heat coils. In larger buildings a secondary return is required and dead legs should be as short as possible.

Don’t specify Thermostatic Mixing Valves (TMVs)

TMVs supply warm water to mixer taps, at a temperature of 38°C to 46°C; the blended water provides a potential environment in which legionella can multiply, and the valve has to be removed to flush the system through with hot water as part of a control regime. The most recent HSE directive recommends that if the risk of scalding is low TMVs are not required. This has now lead to the removal of many TMVs in public toilets and subsequent “Beware. Hot Water” signage.

Specify water softeners in hard water areas

Scale increases the surface area and therefore the potential for microbial colonisation and can provide protection from the effects of biocides. A water softener will reduce the risk of scale being deposited at the base of the cylinder and on heating coils, and will save energy as well as reducing the risks of legionella.

Specify showers but not spas

Showers in any situation are considered as high risk, including electric showers even though they heat the water instantaneously! Spray nozzle outlets have to be inspected, cleaned and descaled as part of a legionella control regime. Hotels routinely throw out shower heads and replace with new ones as that is cheaper than disinfecting them. Unfortunately, there is not a lot that you can do to design out showers where they are required. Spas are a prime source for legionella and waste lots of water and energy, so just don’t specify them.

Are those hot taps really necessary?

All of the above can be utilised in new build situations or retrofits. In a retrofit situation, if the water efficiency is improved (e.g. WCs upgraded) the amount of storage may also need to be reduced. If hot taps in toilets are causing a legionella risk then they are better removed. School pupils and office workers are at low risk of legionella and most of the HSE Directive on legionnaire’s disease is about the risks of legionella in hot water systems, with cold water systems seen as causing little concern if designed well. ‘Maintaining regular movement of cold water in sections prone to stagnation and guarding against excessive heat gain is the most effective legionella control measure in CWDS. For most buildings, carrying out these measures is all that is required. ‘ The less hot water outlets the less you need to worry, and staff kitchens can be provided with a point of use water heater.

This column first appeared in Green Building Magazine Autumn 2014 issue


  1. The current (2104) HSE directive ‘Legionnaires’ Disease, Part 2: The control of legionella bacteria in hot and cold water systems’ runs to 65 pages.
  2. Where TMVs are fitted with low flow rate spray taps on hand washbasins, the risk is increased.
  3. HSE directive on Legionnaires’ Disease, Section 2.60

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Legionella – the risks and the solutions