BPA-free labels, blazoned on baby toys and beverage holders, are supposed to allay fears about the notorious chemical, previously used in sturdy plastics and epoxy resins. After all, bisphenol A (BPA) has been shown to impersonate hormones such as estrogen, and it is associated—though not definitely linked—to a broad range of health problems, including cancers and cardiovascular disease.

But the "BPA-free" label may simply be a meaningless marketing ploy.

A growing number of studies suggest that manufactures are swapping BPA for chemical cousins that have the same troubling activities in humans and animals. In a new study in Endocrinology, for instance, researchers found that a common BPA stand-in, bisphenol S (BPS), has nearly identical hormone-mimicking effects as BPA in zebrafish, a model organism used to study genetics and development. In the study, researchers found that BPS, like BPA, altered nerve cell development, changed the activity level of genes involved in developing the reproductive system, and caused early hatching (the fish equivalent of premature birth).

The study follows others that found that BPS and another BPA-alternative, 4-hydroxyphenyl 4-isoprooxyphenylsulfone (BPSIP), are widely used as substitutes for BPA and can soak into human bodies as BPA does. Other studies have found that BPS, like BPA, can alter heart rates in rats, as well as brain development and behavior in zebrafish.

“BPA-free plastic products are not necessarily safer than products containing BPA,” the authors of the new study in Endocrinology concluded.

Concern about BPA mounted after years of research suggested that BPA acts like a hormone in humans; it typically acts like estrogen, creating elevated levels of signaling that may harm health. In animal studies, researchers have repeatedly found that low-level exposure to BPA can bungle the precisely orchestrated, delicately timed development of the brain and reproductive system, among other things. In humans and animal studies, BPA exposure has been linked to cardiovascular problems, cancers, obesity, early puberty, and behavioral disorders, such as attention deficit and hyperactivity disorder (ADHD).

In 2012, the Food and Drug Administration banned BPA from baby bottles and sippy cups, as did the European Commission and Health Canada. Though few other restrictions exist in the US, public concern solidified and manufacturers voluntarily started using alternatives. There are now BPA-free versions of nearly all the products in which BPA is typically included, such as hard plastic water bottles, coatings used on the inside of food cans, and on thermal paper used for sales receipts.

But BPA's links to human health effects remain mostly correlations, and researchers in the US continue to debate what level of exposure is necessary to cause harm in humans—and how that level compares to lab animals’ exposure in experiments.

Meanwhile, BPA and its chemical cousins remain ubiquitous in products, people’s homes, and the overall environment. In 2008, 2.7 billion kilograms of BPA were produced worldwide, and the need for BPA and its relatives was expected to increase—some researchers estimated that 5.5 billion kg would be made by 2015. In 2007, 2.5 million kg of the chemical ended up in the environment. Studies from the US, China, Japan, Germany, the Netherlands, and Spain generally found around 8 micrograms per liter or less of BPA in river water.

Whether it's in the environment or consumer products, BPA and its ilk readily make their way into humans, leaching into food from containers or getting absorbed through the skin.

In an August study in Environmental Health Perspectives, researchers found that BPA, BPS, and BPSIP can be found in the blood and urine of cashiers who handled receipts coated with at least one of the compounds. For those who worked with BPS-laden receipts, the levels of BPS in cashiers' urine doubled after a shift. The data follows a 2012 study suggesting that BPS was widely used in receipt papers and could be found on many other papers, such as currency. Another 2012 study that examined 315 urine samples collected from the United States, China, India, Japan, Korea, Kuwait, Malaysia, and Vietnam found that 81 percent tested positive for BPS.

In an Environmental Health Perspectives study last year that looked at effects in rats, researchers found that low doses of (10–9 Molar) BPA and BPS both caused faster heart rates and irregular beats in female rats. Also last year, a study in the Proceedings of the National Academy of Sciences found that at low levels (levels similar to BPA levels in waterways) both BPS and BPA caused early development of nerve cells in the brains of developing zebrafish. When the fish grew up, they were hyperactive, swimming in frantic circles in their tanks.

The new study in Endocrinology found similar nerve-cell-altering effects from low doses of BPA and BPS in zebrafish. However, the study added new evidence—findings suggested that, in addition to the chemicals acting like estrogen doppelgängers, they may also interfere with thyroid hormone signaling. That hormone is involved in development and metabolic regulation and can influence heart rates.

As researchers call for more studies to understand how the animal data relates to human health, the evidence casts significant doubt that BPS and other chemical relatives are truly alternatives to BPA.

Endocrinology, 2015. DOI: 10.1210/en.2015-1785 (About DOIs).