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In a groundbreaking study, researchers have shown why a chemical once thought to be a safe alternative to bisphenol-A, which was abandoned by manufacturers of baby bottles and sippy cups after a public outcry, might itself be more harmful than BPA.

University of Calgary scientists say they think their research is the first to show that bisphenol-S, an ingredient in many products bearing “BPA-free” labels, causes abnormal growth surges of neurons in an animal embryo.

The same surges were also found with BPA, though not at the same levels as with BPS, prompting the scientists to suggest that all structurally similar compounds now in use or considered for use by plastic manufacturers are unsafe.

“A lot of the alternative chemicals have not been adequately tested because they don’t have to be,” said lead author Deborah Kurrasch. “A compound is considered safe (by the Food and Drug Administration) until proven otherwise.”

The disruption of prenatal cellular activity in zebra fish, which share 80 percent of their genes with humans and are considered a good model for studying human brain development, seemed to result in hyperactivity, according to the Canadian study, which was published Monday in the Proceedings of the National Academy of Sciences.

“Finding the mechanism linking low doses of BPA (or BPS) to adverse brain development and hyperactivity is almost like finding a smoking gun,” Hamid Habibi, one of the authors of the study, said in a news release.

BPA is an industrial chemical found in many polycarbonate plastics as well as epoxy resins, which are used to coat the inside of food cans. Over the past few years, dozens of studies have linked BPA, which mimics estrogen, with prostate cancer, infertility, asthma, heart disease and a number of neurodevelopmental disorders.

In the new study, the scientists expressed surprise that the early abnormal growth of brain cells they observed in the fish embryo specifically affected male hormones, potentially indicating why more boys than girls are diagnosed with certain neurodevelopmental disorders such as autism.

“We know there’s a fourfold increase in autism between boys and girls,” Kurrasch said. “There are lots of possible explanations for that, but it is interesting to speculate since we do know small changes in hormone levels prenatally can have consequences on brain development.”

The American Chemical Council questioned the relevance of the zebra fish study.

“The authors claim the results are directly relevant to humans, in particular to women during the second trimester of pregnancy,” said Steven G. Hentges of the council’s Polycarbonate/BPA Global Group. “In contrast, humans are exposed to only trace levels of BPA through the diet, and it is well known that humans efficiently convert BPA to a substance with no known biological activity and quickly eliminate it from the body. Although the authors attribute great significance to their results, it would not be scientifically appropriate to draw any conclusions about human health based on this limited experiment.”

BPA, however, is ubiquitous in the environment, according to the Centers for Disease Control and Prevention, which found that 90 percent of Americans have traces of the synthetic compound in their system at any given time, even though it does not linger in the body. BPS, too, is found in the coating of everything from store receipts to recycled paper.

Kurrasch said her lab’s research suggests that pregnant women in particular should limit their exposure to plastics and sales receipts.

One of the most significant findings is that low doses can be more harmful than larger ones.

“I think this is a very important paper that gives evidence that compounds like BPA and BPS have very detectable effects at low dosages on developing vertebrates,” said George Bittner, a professor of neurobiology and pharmacology at the University of Texas at Austin.

In 2012, the FDA said BPA could no longer be used in the manufacture of sippy cups and baby bottles; the ruling came after manufacturers had stopped using it and didn’t apply to its use in other products. Last month, the FDA also reiterated its belief that BPA is safe to use in the lining of canned foods and beverages.