In recent studies, it has been found the certain plastic reusable bottles may be a cancer risk. The risk is due to a chemical called BPA (Bisphenol A) that can be released when the bottle is washed, heated, and/or re-used. This impacts adults as well as children. Here are a very ways to limit the risk of BPA in plastic bottles:
Look for "BPA-free" claims on toys, baby bottles and containers. A lot of companies are starting to roll out BPA free baby bottles, bottle liners, and re-usable containers.
Avoid polycarbonate and PVC (polyvinyl chloride) plastics, both of which contain BPA. At the bottom of the plastic, there should be a recylcing code. Anything with code 7 is at risk. Alternatives include polyethylene plastic (also labeled PETE) and containers marked with recycling code 1, 2 (HDPE) and 4 (LDPE). Polypropylene (recycling code 5, or PP) are also safe.
If you use hard polycarbonate plastics (Nalgene bottles, baby bottles, sippy cups), do not heat or use them for warm or hot liquids. This includes running in the dishwasher. Nalgene just recently did a recall around a lot of their bottles.
Do not wash polycarbonate plastic containers in the dishwasher with harsh detergents.