Report No DWI0289

Drinking water meeting current standards and gastrointestinal illness: A critique of the work of Pierre Payment, with discussion of its relevance to the UK and suggestions for a future study



Executive summary

  1. In 1991, Pierre Payment and co-workers reported on a randomized controlled trial conducted in Canada (the Montreal study). The paper claimed that the study had shown one third of the episodes of gastrointestinal infection in the population were caused by drinkingtap water, even though the water met current microbiological standards. The Epidemiological Survey Project Group was asked to consider critically the methodology of the study, to re-evaluate its conclusions in the light of any weaknesses found, to assess the relevance of the study to the situation in England and Wales and, if appropriate, to suggest a specification for the design of any study which could be conducted here.
  2. The Group found some problems with the epidemiological design of the Montreal study, principally that participants were not blind to whether they were drinking tap or filtered water and thus the reporting of symptoms might have been biased. Against that possibility, a dose response relationship was reported between rates of gastrointestinal illnesses and variables participants were blind to ie. quality of water in the distribution system, levels of contamination in the filters and quantity of tap water consumed. The study was therefore considered to be sound enough to be taken seriously in spite of methodological limitations.
  3. Other criticisms of the study included the use of reverse osmosis filters, because of the change in the chemical composition of the water and evidence of contamination by bacteria. This did not undermine the validity of the study, but raised questions about the interpretation of results. It was noted that no stool samples were collected; this might have helped to identify the types of organism associated with the gastrointestinal illnesses linked to drinking tap water.
  4. The study was considered relevant to England and Wales because the quality of the tap water appeared to be similar, although the quantity conserved might be lower in England and Wales. Very limited available data suggests the rate of gastrointestinal infections might also be lower than in Canada.
  5. The Group supported the undertakings of a similar study in England and Wales and presented an outline design and costings for a study which would eliminate the methodological limitations of the Montreal study.

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