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.
Like a 'Sex Hormone Patch'
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:
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.
Findings Are Disputed
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.
Researchers, meanwhile, defended their methods as in line with or more conservative than human exposure to BPA.
"There are too many positive findings by reputable laboratories to ignore," said Prins.
Prins and Ho's study showed precancerous prostate lesions in 100 percent of male rats exposed to BPA but in only 40 percent of the control group.
BPA exposure affected a number of genes, said Ho, including one that normally fuels cell growth in development. This gene stayed "turned on" later in life, leading to precancerous growth.
"Our environment is very artificial right now," Ho said, and it could have complex effects on disease. She works in an emerging scientific discipline called epigenetics, which examines how environmental and lifestyle factors reprogram our genes.
What Should Be Done?
While San Francisco has banned BPA in certain products, the FDA has taken no action for now, said Mike Herndon, a spokesman for the FDA in Washington, D.C.
"At the present time, FDA has no reason to change its opinion that the dietary exposure to BPA ... is safe," he said.
Dr. Durado Brooks, director of the prostate cancer division within the American Cancer Society, called the BPA correlation with prostate cancer "debatable" and cautions against extrapolating from rodent studies to humans.
He said the scientific community is a long way from making any recommendations to the public.
But vom Saal suggests taking the steps he uses for his own family: Buy water filters, avoid heating plastics and throw away old or cracked plastics. Pregnant women and parents of young children should be especially cautious, he said.