Unit of measurement
World Health Organisation Standards (1993)
Not mentioned
Standards for private water supplies in England (2016)
Not mentioned
Standards for private water supplies in Scotland (2006)
Not mentioned
Standards for private water supplies in Wales (2017)
Not mentioned
European Union Drinking Water Directive (1998)
Not mentioned


Legionella is a gram-negative pathogen which can cause Legionnaires' disease and Pontiac fever. This bacteria is commonly found in natural water sources such as rivers, lakes and streams but usually in small quantities. They can also be found in purpose-made water systems such as those found in pools.

As it lives in both natural and man-made water sources, legionella is helped to survive and multiply by the various other organisms that also make the water their habitat. Due to this, legionella is able to live in drinking water.

Humans usually have the bacteria enter their system by breathing in mist. This mist, which is small water droplets suspended in the air, have the bacteria trapped inside. The mist may be generated from such places as showers, hot tubs, saunas and air conditioning units. Most people who are exposed to legionella suffer no ill effects but there can be more of a risk for those:

  • Who are 50 or over
  • Are smokers
  • Drink heavily
  • With a chronic lung condition
  • That have a weakened immune system
  • With kidney disease
  • Who have diabetes, lung and heart disease.

For individuals that do become unwell from breathing in the legionella bacteria, the results can be severe and life-threatening. Legionnaires' disease is one consequence of legionella bacteria.

The symptoms of Legionnaires' include:

  • High temperature, fever and chills
  • Muscle pain
  • A cough
  • Pneumonia
  • Confusion
  • Diarrhoea

As previously mentioned, legionella can also cause Pontiac Fever, a mild illness, which isn't dissimilar to flu. Pontiac Fever can be contracted by previously healthy individuals but unlike Legionnaires' it does not have the ability to develop into pneumonia.

The only way to tell if your drinking water is contaminated with legionella is to test it. If it is found that your water does contain legionella, it can be treated either focally or systemically. Focal treatment is directed at a specific part of your drinking water system whereas systemic treatment methods seek to disinfect the entire system. The main treatment methods for drinking water contaminated with legionella are UV light filters, hyperchlorination and copper-silver ionisation.

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