Bisphenol A (BPA) is a component of some plastics and is found in food containers, plastic water bottles, dental sealants, and thermal receipt paper. In the body, BPA is a mild synthetic estrogen, one of the gigantic class of chemicals called "endocrine disruptors", even if they are 1/20,000th able to bind as well as the actual estrogen.
Environmentalists claim that despite the FDA doing an exhaustive four-year study and finding no effects, even in pregnant mice, despite feeding them 1,000 times as much BPA as a human can get, and affirming that humans process and excrete BPA far more efficiently than rodents, that a magical hormesis effect is still causing problems.