The danger to humans is less clear. The National Institutes of Health's toxicology program states "some concern" that the chemical could harm fetuses, babies, and young children. And a recent study suggests that adults exposed to relatively high levels of BPA are somewhat more likely than others to develop diabetes, cardiovascular disease, and liver problems. Still, the Food and Drug Administration maintains that products containing BPA are safe.

For many, the verdict is already in. "The science is clear that this chemical is toxic at low doses," says Anila Jacob, M.D., a senior scientist with the Environmental Working Group. "Consumers should avoid it."

If you want to play it extra safe, you have no shortage of options. Nalgene is phasing out BPA in its candy-colored bottles, and all of CamelBak's new bottles are already BPA free. Jacob favors metal bottles without a plastic lining, such as those made by Thinksport and Klean Kanteen.