Last edited 02 Jun 2021

Legionnaires' disease

Legionnaires’ disease is the most serious of a number of diseases that can be caused by legionella bacteria. It is generally contracted by inhaling water droplets suspended in the air.

Legionnaires’ disease is a form of pneumonia that can be fatal, and is particularly serious for older people, people who smoke, heavy drinkers, people with impaired immunity, and people with illnesses such as chronic respiratory disease, kidney disease, diabetes, lung disease or heart disease.

Legionella is commonly found in rivers, lakes and reservoirs, but its presence is limited and rarely causes legionnaires' disease. However, it can multiply in systems such as cooling towers, evaporative condensers, hot and cold water systems, spa pools, and other plant and systems where there may be a risk during operation or maintenance, for example where:

  • The water temperature is between 20°C and 45°C.
  • Water droplets can be produced and dispersed.
  • Water is stored or re-circulated.
  • Deposits such as rust, sludge, scale, organic matter and biofilms are present.

Measures must be introduced in these systems to prevent or control the risk of legionella by:

The legal framework for the control of legionella is established by:

The Health and Safety Executive has published, an approved code of practice (ACOP) Legionnaires’ disease: The control of Legionella bacteria in water systems, and measures adopted must be at least as effective as those described.

The approved code of practice applies to, ‘...premises controlled in connection with a trade, business or other undertaking where water is used or stored; and where there is a means of creating and transmitting water droplets (aerosols) which may be inhaled, causing a reasonably foreseeable risk of exposure to legionella bacteria.’

It sets out duties for employers and people with responsibilities for the control of premises such as landlords. Dutyholders should take suitable precautions to prevent or control the risk of exposure to legionella, including:

  • Identifying and assessing sources of risk.
  • Where necessary, preparing a written scheme for preventing or controlling the risk.
  • Implementing, managing and monitoring precautions.
  • Keeping records of the precautions taken.
  • Appointing a competent person to help take the measures needed.

Whilst Legionnaires’ disease remains an uncommon and sporadic infection, in January 2019, the European Centre for Disease Prevention and Control ( an agency of the European Union ) published the Annual Epidemiological Report for 2017: Legionnaires’ disease which revealed that in 2017, 30 countries reported 9,238 cases. Of the 6,976 cases with known outcome, 8% were fatal. The report found a 30% increase in cases of Legionnaires’ disease in 2017 compared with 2016.

The increasing in notifications is thought to be driven factors including; improved surveillance, an ageing population, travel patterns and changes in climate and weather factors. It is thought that conditions such as temperature, humidity and rainfall have had an effect on the bacterial ecology and/or an increased use of aerosol-producing devices or installations such as cooling towers.

In 2017, the vast majority of threats monitored were travel-associated cases. In addition, nine countries reported community- or hospital-acquired outbreaks. Thirteen EU/EEA countries reported no outbreaks.

The report recommends regular checks for the presence of Legionella bacteria and appropriate control measures applied to engineered water systems, in particular at tourist accommodation sites, in hospitals, in long-term healthcare facilities and in other settings where sizeable populations at higher risk may be exposed.

You can read the report in full at:

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