From food-storage containers to disposable silverware, plastic products are such a part of our lives that it's easy to forget they contain chemicals that could harm us.
But last month, San Francisco banned a type of sturdy, hard plastic made with a molecule known as bisphenol A , or BPA. Any toys, bottles and pacifiers made with BPA must be replaced, according to the law the mayor signed in June.
Why did the city take such drastic action? BPA, like many other man-made chemicals, is now detectable in most people's bloodstreams and could cause dangerous hormonal changes in children.
BPA -- sometimes indicated by a number 7 on products -- is found mostly in strong plastics, such as nondisposable water bottles, baby bottles and in the lining of canned foods.
But whether BPA poses a real danger depends on whom you ask.
While the Food and Drug Administration and the American Plastics Council insist BPA is safe, an outspoken biology professor and other scientists believe it may bring all kinds of harm -- such as cancer, early puberty, obesity and even attention-deficit disorder.
Frederick vom Saal, a biology professor at the University of Missouri at Columbia, stumbled onto BPA in 1997 while studying fetal development. He found that BPA passed through the protective placenta from mother to baby, mimicking the behavior of the natural hormone estrogen.
Vom Saal, who has studied BPA ever since, said there is so much BPA in the environment it is as if we are all wearing "a sex hormone patch."
Six billion pounds of polycarbonates are produced each year, and much of it ends up in landfills, water systems and the air, vom Saal said. "You're breathing it, you're absorbing it."
He also insists that even though the products may be labeled as dishwasher- and microwave-safe, heating this type of plastic may cause the chemical to leach out into the body at a much higher rate.
A number of recent studies support vom Saal's view that BPA could be unsafe:
A study in the June issue of Cancer Research showed a correlation between rats that had early BPA exposure and those that developed prostate cancer in later life. The study was done by Shuk-mei Ho, head of environmental health at the University of Cincinnati, and Gail Prins, physiology professor at the University of Illinois in Chicago.
A study in this month's issue of the journal Endocrinology found that young female mice exposed to BPA showed brain and behavior traits more typical of male mice.
Other animal studies, such as one at Tufts University, have found possible links between bisphenol A and breast cancer. Japanese researchers have connected it to miscarriages, and Spanish and Mexican researchers have linked it to diabetes.
And, according to vom Saal, there is a big discrepancy between 130 independent studies that identified harmful BPA effects and industry-backed studies, none of which found any danger.
However, Steve Hentges, a chemist and director of the American Plastics Council, disagrees. In the research as a whole, Hentges said, "the most consistent finding is no effect."
He criticized the recent findings, saying even "low dose" studies use BPA levels that are too high to represent real-world situations.
A person would have to consume more than 500 pounds of food or drink in contact with BPA in order to exceed the Environmental Protection Agency's acceptable dose, according to Hentges.