Legionellosis: what are the risks?


Legionellosis is a potentially fatal disease caused by a bacterium, Legionella, which is quite common in nature. It is found in small quantities in most surface waters. It thrives best in warm stagnant water (± 40°C). It cannot survive temperatures above 60°C and dies relatively quickly.


How do you get legionnaires' disease?

The disease is caused by inhaling airborne microdroplets containing Legionella bacteria. In contrast, there is no real risk when ingesting contaminated water.

The inhalation hazard comes mainly from showers, hot tubs and air conditioning systems in large buildings (hotels, schools, sports centres, hospitals). Cooling towers in buildings and industry are another major source of danger. The risks are increased in hospitals and nursing homes, as these are the places where people are least resistant to the disease.

What precautionary measures should local authorities take?

In order to limit the presence of Legionella in local authority buildings as much as possible, preventive action is of the essence.

  • Design a suitable internal network with sufficient loop flow at all points in the network, without retention zones. Ensure that the pipework is sized correctly so that the calculated flow rates at the correct temperature are actually circulated through each column. This allows a good balance of the network.
  • Keep limescale to a minimum, as this serves as a fixing point for the biofilm, in which the Legionella develops. Pay particular attention to the loop and exchangers.
  • Maintain a sufficient flow temperature (between 55 and 60°C).

If your indoor pipes are insulated, you can maintain a constant temperature above 50°C in the circuit, thereby preventing the growth of the bacteria.

The Scientific and Technical Centre for Construction (BBRI) provides practical recommendations and advice for large buildings subject to specific legal requirements.

Checking water for Legionella

At your request, our laboratory can carry out sampling and analysis of Legionella in domestic hot water systems under ISO 17025 accreditation. Depending on the origin of the tap water, the enumeration of bacteria by culture may be preceded by qualitative detection of negative samples (absence of Legionella pneumophila) by gene amplification (PCR). Results are available within 24 hours. In this case, the culture analysis is then started only in the presence of PCR-detected Legionella DNA.

If your request concerns Legionella analyses on cooling tower water, we subcontract these to an external ISO 17025 accredited laboratory.

What about private individuals?

The risk of catching Legionnaires' disease is not usually a problem in individual dwellings. However, there are some precautionary measures you can take:

  • don’t allow warm water to stagnate;
  • make sure that the hot water circuit is as short as possible;
  • ensure that the hot water is always between 55 and 60°C;
  • place good thermal insulation between the hot and cold water pipes.
Frequently asked questions

What quality parameters must drinking water meet? Who sets these criteria?

Tap water must meet a large number of quality parameters. These are set by the standards of the Walloon Water Code. The water from our network is analysed daily to ensure that it complies with the standards.

What should I do if I have doubts about the quality of my water?

Water quality must meet the legal standards applicable to drinking water. This quality is strictly controlled, but a problem cannot be ruled out completely. 

If in doubt, contact us. We will always look into your query carefully.If necessary, we will send a technician on site for further investigation. They may take water samples. The laboratory analysis will determine any corrective measures to be taken to bring the situation into compliance. 

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