Legionella are bacteria belonging to the Legionella sp. genus that occur naturally in the environment, and especially in watery environments (fresh water and artificially created bodies of water). They can easily colonise water systems which provide the ideal conditions for development:

  • stagnant water where there are other microorganisms able to provide the nutrients that their own metabolism cannot produce, as well as scale or corrosion (iron, zinc);
  • water temperature between 25°C (77°F) and 45°C (113°F), optimum 32 - 35°C (90 - 95°F).


Legionella spreads through the air and via the inhalation of microdroplets of contaminated water (aerosols). Contamination in humans can cause:

  • a benign illness with flu-like symptoms (Pontiac fever)
  • a serious illness with pneumonia (legionellosis) which can result in death for vulnerable people. Legionellosis is, therefore, a notifiable disease.

The gravity of the infection depends on the number of microorganisms inhaled, the virulence of the bacterial strain and the state of the infected person’s immune system. Legionella pneumophila serogroup 1 is by far the most virulent strain.    

The most common source of contamination is the shower.

In the event of a positive test result for Legionella:
First of all, the user's risk of exposure must be identified. Installing anti-Legionella filters is the most effective response if one of the following occurs: widespread contamination; contamination levels above 1,000 CFU/L; the serotype has been identified; a case of legionellosis has been diagnosed; the users are vulnerable patients. Chemical or thermal shocks are also effective, but implementing either can be problematic and will be less effective if the cause of the proliferation is not identified.

Find out about our anti-Legionella solutions for Public Buildings.