And water bottles sure can be a haven for those bacteria. In a 2002 study published in the Canadian Journal of Public Health, researchers from the University of Calgary took 76 samples of water from water bottles of elementary school students; some of the bottles were reused for months on end without being washed. They found that nearly two-thirds of the samples had bacterial levels that exceeded that of drinking water guidelines, which may have been the result of "the effect of bacterial regrowth in bottles that have remained at room temperature for an extended period," researchers wrote in the study.