That sippy cup with the “BPA-free” sticker might not be as safe as you think. There’s scientific evidence that other plastics' ingredients could potentially have hormone-disrupting effects as well.

BPA, or bisphenol A, is a compound used in plastics since the 1960s, and often turns up on the insides of water bottles and cans of food. It’s a useful chemical for making clear, strong plastic, but it has been linked to a wide range of health conditions, thanks to its ability to mimic the behavior of estrogens. Due to its structural similarity to the hormone estradiol, BPA can activate certain estrogen receptors. That creates the potential for a world of harm; our body’s hormonal system is a finely tuned machine, and disrupting its normal function, especially early in development, is thought to have wide-ranging effects.

When a plastic is subjected to certain conditions, particularly heat, compounds like BPA can leach out and potentially be ingested by people. BPA shows up in the urine of just about everyone in the U.S. (or at least the ones sampled by the U.S. Centers for Disease Control). In recent years, studies in humans and animals have found connections between BPA exposure and a host of diseases and conditions, from obesity to cancer to neurological issues. The case isn't yet totally closed -- most health officials haven't found the evidence compelling enough to institute general bans, but several countries have moved to ban BPA from baby bottles and sippy cups.

Meanwhile, many manufacturers have seized the chance to promote “BPA-free” plastic products. But some studies show that non-BPA plastic ingredients could have the same endocrine-disrupting effects. In 2011, the journal Environmental Health Perspectives published a study from a team led by George Bittner, a researcher at CertiChem Inc. The researchers tested hundreds of plastic products with common-use stresses like microwaving, UV radiation, and employed specially designed human breast cancer cells that will quickly multiply in reaction to estrogen.

“Almost all commercially available plastic products we sampled -- independent of the type of resin, product, or retail source --leached chemicals having reliably detectable EA [estrogenic activity], including those advertised as BPA free,” Bittner and colleagues wrote. “In some cases, BPA-free products released chemicals having more EA than did BPA-containing products.”

In a feature published recently in Mother Jones, writer Mariah Blake delves into efforts by Tennessee-based Eastman Chemical Co. to push back against the CertiChem team’s research. One of Eastman’s products is Tritan, a plastic marketed as “EA-free.” CertiChem started testing Tritan in 2009 and found that it had even more estrogenic activity than some BPA-containing plastics.

In 2008, Eastman signed a two-year contract with Sciences International, a consulting firm that had a long relationship with the tobacco industry. Sciences International had a history of marshaling studies to bolster industry-friendly claims, such as the “16 Cities Study,” which argued that workplace secondhand smoke exposure was a negligible factor. The Alexandria, Va., firm advised Eastman to try out a computer model that uses chemical structures to predict whether a plastic has estrogenic ingredients. The ensuing study suggested that Tritan ingredient triphenyl phosphate (TPP) was even more estrogenic than BPA, according to Mother Jones.

Subsequent Eastman tests with breast cancer cells also showed signs of estrogenic activity, but this didn’t prevent the company from marketing the plastic as free of synthetic estrogens starting in 2010. When customers such as baby bottle maker Philips Avent asked to have an outside lab run tests, the company strongly tried to dissuade them, internal emails obtained by Mother Jones show.

In 2012, Eastman commissioned another study of Tritan, but one that examined only select ingredients, and not TPP. Later that year, Eastman sued CertiChem and sister company PlastiPure to try and block them from publicizing their findings that Tritan has estrogenic activity.

Eastman maintains that its own tests show that Tritan doesn't have estrogenic activity, and says CertiChem and PlastiPure were engaging in unfair commercial competition (PlastiPure develops its own line of plastics marketed as EA-free). In late July, a jury found for Eastman, and the judge in the case barred the two defendants from talking about Tritan findings in a commercial setting. CertiChem and PlastiPure have appealed, according to the nonprofit Food Packaging Forum.

Meanwhile, the eventual health legacy of plastics remains unknown.

"We know that there's a cost when we mess with the levels of these hormones in our bodies, regardless of how we do it," Laura Vandenberg, a University of Massachusetts-Amherst health scientist who wrote a review of the literature on endocrine-disrupting chemicals, told Mother Jones. "Even small changes early in life can alter brain and organ development and set us up for disease later on.”