Plastic bottles are made from many chemicals and some of these, called dioxins, have been found to cause cancer in animal experiments. It is thought that exposing water to the bottles, especially if heated by the sun, can cause the chemicals such as BPA to flow out into the water and increase the risk of cancer. As yet there is no correlation between Breast Cancer and these chemicals in humans. Most one-time bottles used for water do not contain BPA but to be on the safer side, you can opt for bottles labelled BPA-free, or labelled five, four, two or one in the recycling emblem on the bottom. Or better yet, use glass bottles, which are also better for the environment.