One reason bisphenol A (BPA) hasn't been banned in the U.S. is that most tests showing that it is harmful have been done on animals. But that may change now that a team of French researchers has found exposure to low concentrations of BPA is harmful to human male reproductive tissue.

Not only are concentrations of BPA similar to those commonly found in the human bloodstream harmful to fetal human testicles, say researchers, such tissue is 1000 more times sensitive to BPA than similar tissue from rats and mice.

The U.S. banned BPA from baby bottles and sippy cups in July 2012. But adults and especially children are still exposed to BPA from many sources, including canned foods.

Compared to human fetal tissue, tissue from mice and rats was far less sensitive to BPA exposure. It took roughly 1000 times more BPA before these effects were seen on mouse and rat Leydig cells.

The researchers kept fetal testicle tissue growing for days on a culture medium inside a Petri dish. Using tissue from humans, rats and mice, they tested the effect of added BPA on cells called Leydig cells present in the tissue. Leydig cells secrete testosterone and another hormone called insulin-like 3 (INSL3), both of which are required for the testicles to descend during normal male development.

Adding BPA to the culture medium at a concentration of about 2 micrograms per liter lowered both the amount of testosterone and the amount of INSL3 produced by the human Leydig cells. This is about the same BPA concentration as is generally reported in people's blood, urine or amniotic fluid. Compared to human fetal tissue, tissue from mice and rats was far less sensitive to BPA exposure. It took roughly 1000 times more BPA before these effects were seen on mouse and rat Leydig cells.

There have been several human studies linking high levels of BPA in blood or urine to physical and behavioral abnormalities. One recent study even links BPA to obesity in children. But that type of study can only point out a correlation with obesity and BPA exposure, it can't even come close to showing that BPA is responsible for harm. The French study offers much more direct evidence of BPA's toxicity.

And in another study, even newer materials designed to replace BPA also act as endocrine disruptors, just as BPA does. Researchers at the University of Texas Medical Branch at Galveston found that a widespread BPA replacement, bisphenol S, alters the way cells respond to estrogen, causing changes in hormone release, cell growth and cell death. And while the study, published in Environmental Health Perspectives, was done on rat pituitary cells cultured in the lab, similar studies are what first started scientists wondering whether BPA was a human health hazard.