Since 1989, prospective epidemiologic studies have been undertaken to evaluate if tap water can be a significant source of illness. These studies have compared groups of families consuming tap water to families drinking tap water further processed by reverse osmosis to eliminate microorganisms that could have evaded treatment. Studies have focused on a single water treatment plant and distribution system in order to reduce confounding factors. The water produced by the treatment plant meets all North-American regulations (Québec, Canada and United States). All studies are based on long observation periods (16 to 18 month) of randomly chosen families distributed randomly in the observation groups. The rate of gastrointestinal illness average was 0.7 episodes/person-year and varied between 0.2 and 3.0 episodes/person-year according to the observation periods. We have observed that 10 to 30% of gastrointestinal and respiratory symptoms were apparently associated to the consumption tap water. The annual rate of hospitalisation for gastrointestinal or respiratory diseases was 0.7% and the medical visit rate was 19%. Furthermore, 25% of the participants lost at least one day of work due to these symptoms. The costs associated to these health effects could reach several millions of dollars each year for Canada with a population of 30 million people.

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