For many years, people have been circulating hoax emails claiming that drinking water from plastic bottles can cause cancer, especially if they have been left in cars, or frozen and reused. There is no good scientific evidence to support these claims. Many of these emails credit the warnings to Johns Hopkins University or the Walter Reed Army Medical Center, but both institutions have denied any involvement. The emails also claim that heating, freezing or reusing plastic water bottles releases chemicals from the plastics that could cause cancer, including dioxins. However, the plastic in water bottles contains no dioxins at all. All in all, water in plastic bottles should be safe to drink, and no less so than water in metal bottles or any other type of container. If they have contaminants in them, they are likely to be found at tiny concentrations. However, bacteria and fungi can grow in bottled water, so if they have been left for a long time, it is best to clean them out with hot, soapy water, and leave them to dry before refilling them.