What is Legionella?

When hearing the word Legionella, many Dutch people still think back to the Legionella disaster (25 February 1999) at the Flora in Bovenkarspel, West Friesland, in the north of the Netherlands. 32 people died back then from the consequences of a Legionella infection. On the website of the Veterans’ Disease Foundation, you can find all about it. Because of this incident, the Dutch legislation for legionella prevention is one of the most progressive in the world.

Building owners in the Netherlands are responsible for the control and monitoring of legionella in the building. Legionella can be controlled by allowing the water to flow or by regularly flushing it with hot water (thermal disinfection). Capturing Legionella by means of ultrafiltration is one of the so-called alternative prevention methods.

Legionella is a disease-causing bacterium that is found in water. At present, 52 different species (serotypes) of legionella have been identified. Most infections are caused by the legionella pneumophila. The non-pathogenic variant is legionella non-pneumophila. Legionella colonies grow very quickly in stagnant water between 25°C and 40°C.

Legionella infection can only occur through the inhalation of aerosols containing a high concentration of legionella. Aerosols are small water droplets that are released when water is atomised, such as in showers, fountains, humidifiers, air conditioning systems, whirlpools and cooling towers. It is therefore not possible to become infected with legionella by drinking water contaminated with legionella.

The concentration of legionella in water is expressed in cfu/L. These are the number of Colony Forming Units. The norm for Dutch drinking water is a maximum of 100cfu/L.