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Assessment of the health risks related to the presence of microorganisms in water intended for human consumption

The microbiological pathogens that may be present in water intended for human consumption (WIHC) are mainly bacteria, viruses and protozoa, most of which come from human or animal excrement. Their presence in tap water is due to insufficient protection of the supply, a defect in water treatment or a backflow of wastewater into the drinking water system. They can cause infectious disease (primarily acute gastroenteritis in mainland France), and they are the primary short-term health risk related to contamination of drinking water.


In accordance with Directive no. 98/83/EC on the quality of water intended for human consumption (WIHC), and Article R. 1321-2 of the French Public Health Code, WIHC must:

  • not contain an amount or a concentration of microorganisms, parasites or any other substances representing a potential danger to human health;
  • comply with the quality limits, based on microbiological and chemical parameters(1).

In practice, health controls of the microbiological quality of WIHC are primarily based on an investigation and count of faecal contamination indicators, as well as efficacy indicators for treatments used to eliminate or inactivate microorganisms that are pathogenic to humans. Quality references or limits are set at the water collection facilities and during distribution. The microbiological quality of the water should be maintained until it reaches the consumer’s tap.

The investigation of waterborne acute gastroenteritis (AGE) outbreaks in France between 1998 and 2006 found that the water collection facilities and the distribution network were equally responsible as the point of entry of the contamination. In the ten AGE outbreaks investigated by the French Institute for Public Health Surveillance (InVS), the pathogens identified were bacteria from the genus Campylobacter, parasites from the genus Cryptosporidium, and viruses, predominantly noroviruses(2). In an epidemic context, lists of pathogens to be tested for in water are used, in addition to the standard microbiological indicators(3).

Furthermore, some genera of bacteria that are naturally found in aquatic environments (such as Legionella, which causes Legionnaires’ disease and can lead to severe pulmonary disease) can survive and multiply in domestic hot water systems. Contamination in humans occurs through inhalation of an aerosol composed of fine water droplets contaminated by Legionella, and not through ingestion of contaminated water.

The seasonal development of cyanobacteria in some surface water that is used as a resource for WIHC production is also a health concern due to the production of toxins from these bacterial species.


For each microbiological agent evaluated, the Agency considers the following points:

  • identification and characterisation of the hazard (taxonomy, characteristics, effects, etc.);
  • assessment of the risk associated with the hazard according to available data (dose-response, exposure, etc.);
  • if necessary, development of a metrological aspect: sampling/analyses.

The role of ANSES

The Agency assesses the health risks related to the presence of potentially pathogenic microorganisms in WIHC, regardless of whether they are regulated or not. For example, it assessed the health risks for humans related to the presence de viruses, Cryptosporidium sp., cyanobacteria and their toxins, Pseudomonades and Bacillus in water intended for human consumption.

These expert assessment studies are mainly intended to provide scientific and technical support to risk managers under local-level contamination circumstances. They may also be used for both national and EU regulatory reviews.

(1) Ministerial Order dated 11 January 2007 relative to the quality limits and references for raw water and water intended for human consumption listed in Articles R. 1321-2, R. 1321-3, R. 1321-7 and R. 1321-38 of the French Public Health Code.
(2) InVS (2008) – Report on waterborne disease outbreaks investigated in France since 1998 and main recommendations
(3) InVS - Guide to investigation of infectious outbreaks related to the ingestion of water from the public supply