When purchasing a “BPA-free” item, you assume that you’re receiving a safer alternative to the Bisphenol A typically used to strengthen plastic products. But according to a new study from researchers at the University of California, Los Angeles, BPA’s common substitute, Bisphenol S (BPS), may not be as safe as originally thought.

Published in the journal Endocrinology, the study found that BPS quickened the development of zebrafish larvae when exposed the chemical, which resulted in premature births.

Nancy Wayne, the study’s lead author and reproductive endocrinologist at UCLA’s David Geffen School of Medicine, considers the results frightening, “…the aquatic version of the canary in the coal mine.”

Researchers exposed the zebrafish to low levels of BPA and BPS, mimicking the concentrations that fish would encounter in polluted rivers. Due to their transparent embryos, zebrafish serve as optimal case studies for researching developmental effects.

BPA acts as a hormone disruptor that imitates effects of estrogen and testosterone. Significant research has linked BPA to reproductive development issues and the Food and Drug Administration (FDA) even banned BPA use in baby bottles and children’s cups.

Within 25 hours of exposure, the UCLA research team saw changes. “The embryos developed much faster than normal in the presence of BPA or BPS,” according to Wayne. Researchers tracked development of the fish’s endocrine neurons within the brain that trigger puberty and fertility. They found up to a 40% increase.

“We’ve raised a red flag here on how safe Bisphenol S is, but the main issue is whether any endocrine-disrupting chemicals can be safe to use at any level—there’s just not enough research yet,” according to Wayne.

The chemical industry’s largest trade organization, the American Chemistry Council, are not convinced of the relevancy the study has for human health. “The study examines effects of BPA and one alternative on zebrafish embryos in water …It is well established through extensive scientific research that humans, including pregnant women, efficiently convert BPA to a substance with no known biological activity and quickly eliminate it from the body.”

Despite the (unsurprising) doubt cast by the chemical industry organization, the study still shows that exposure to these endocrine disruptors easily affects developing organisms. More research should be done to study what impact these chemicals have before we readily accept them as safe for use.