But Hunt counters that there is plenty of corroboration to consider BPA a problem. In response to the Harvard study, she helped to produce a “state of the evidence” paper for Reproductive Toxicology in 2007. Along with 36 other researchers, led by vom Saal, the group analyzed hundreds of government-funded studies and found that 90 percent had concluded BPA was a health risk. It was the dozen or so industry-funded studies, vom Saal says, that failed to replicate other BPA research.
More important than these conspiratorial undertones, Hunt says, is one of communication between toxicology (the way skeptics look at BPA) and endocrinology (the way she looks at it). For instance, according to a statement on www.bisphenol-a.org, a Web site created by the American Chemistry Council (which represents dozens of companies engaged in plastics manufacturing), the toxicology of BPA is “well understood,” and “BPA exhibits toxic effects only at very high levels of exposure.” Current U.S. Food and Drug Administration guidelines, based partly on these findings, set a safe daily exposure to BPA at 50 micrograms per kilogram of body weight.
But according to Hunt, treating BPA like a traditional toxin is dangerous because it “doesn’t play by the rules.” Standard toxicology states that if a chemical is bad, “then higher doses are worse and an even higher dose is even worse,” Hunt explains. But with hormones (and estrogen mimics like BPA), she says, high doses can sometimes “shut down” the body’s response, and low doses are enough to exert effects.
Indeed, her lab rodents show BPA effects at just 20 micrograms per kilogram; other labs have found similar thresholds, making them one-half to one-third the FDA levels. These experiments yield bodily concentrations of BPA in ranges of parts per million, but some recent studies have even found that when BPA interacts with hormone receptors on cell membranes, concentrations of one part per trillion can stimulate physiological responses.
That means basically any exposure to BPA could have consequences, an alarming conclusion, considering that in 2004 the Centers for Disease Control and Prevention found unmetabolized BPA in the urine of 93 percent of more than 2,500 human subjects. According to the National Toxicology Program of the U.S. Department of Health and Human Services, BPA has also been detected in human blood and breast milk.
With such ubiquitous exposure, one might expect to see numerous problems already afflicting humans. And perhaps this lack of any definitive effects most bothers skeptics. “Why do we have to work so hard to try to replicate and show these low doses really have an effect?” Sipes asks. “Why don’t [reactions to BPA] stand out in black and white?”
Hunt is asking the same question. She is now working on a paper about how diet can alter responses to the chemical. It is one of many unstudied facets of the issue that, she says, may be making it difficult for scientists to reproduce their research: “There’s a lot of complexity and a lot of things we just don’t understand.”
While scientists grapple to get a better handle on BPA, the public domain has made up its mind. On April 17 the National Institutes of Health raised concerns about BPA’s established “safe” levels. Four days later Health Canada, the Canadian version of the FDA, announced a ban on polycarbonate baby bottles, citing concerns over BPA. The moves rattled the industry, as consumer outcry led stores such as Wal-Mart and CVS to announce they would phase out some polycarbonate products. And Nalgene, a company synonymous with its popular shatter-resistant bottles, decided to pull them from shelves.