A new experiment by scientists at the U.S. Food and Drug Administration has found that bisphenol A does not affect the health of rats fed low doses.

The study adds to the ongoing scientific dispute over whether traces of BPA – an industrial chemical found in polycarbonate plastic, some canned foods and beverages, paper receipts and dental sealants  – can harm people.

Rats exposed in the womb and as newborns to the two highest doses of BPA had lower body weights, abnormal female reproductive development and altered hormone levels. But there were no such effects when the rats were exposed to low doses that people are routinely exposed to.

Some scientists not affiliated with the study said that the findings are flawed. FDA scientists didn’t look for all relevant health impacts, such as effects on the developing brain. Also, the experiment lasted for 90 days, so it is unknown if the rats’ health was affected later in life. Some effects, such as altered glucose levels, might occur after continuing exposure.

In addition, critics said that the findings are skewed because all the rats in the study – even those that were supposed to be unexposed controls – had BPA in their bodies.

"It's a flawed study," said Laura Vandenberg, an assistant professor of environmental health at the University of Massachusetts Amherst.

The peer-reviewed study, which was published in the journal Toxicological Sciences, was written by 12 FDA scientists, led by Barry Delclos, a pharmacologist. A companion piece had eight FDA authors, led by Daniel Doerge, a chemist.

Delclos and Doerge did not respond to multiple phone calls and emails seeking responses to the critics.

An FDA spokesperson said in an email that the agency would not “accommodate an interview” on the research. "The study reported no effects of BPA at any dose, except at the very highest levels, and is consistent with the FDA’s current position that BPA is safe at the very low amounts that occur in some foods," the spokesperson said.

The research is part of a two-year FDA project – funded by the FDA and the National Institutes of Health – to determine the toxicity of BPA, which is critical to setting any future regulations on its use in food products. A group of academics from several universities working with the FDA will be conducting more studies on the rats over the next year.

FDA examined the rats' body weights, sexual development, mammary glands, glucose and insulin levels, reproductive hormones, organ weights and cell counts.

"Our interpretation of the results of the present study is that BPA in the 'low dose' region from 2.5 to 2,700 μg/kg bw/day did not produce effects in the evaluated endpoints that differ from normal background biological variation," the scientists, led by Delclos, wrote in their study.

Other recent animal studies conducted at universities have linked low-dose BPA exposure to an array of health impacts, including some of the effects that the new study did not find. These other experiments found mammary gland abnormalities, altered male and female sexual developmentchanges in metabolism, insulin and glucose, impaired learning and memory, stress and obesity

Vandenberg said the biggest problem with the new findings is that all of the animals were exposed to BPA.

The rats that were supposed to be controls  or unexposed  had BPA in their blood at about the same level as the rats in the lowest-dose group, according to the FDA study.

In a separate published report, FDA scientists reported that they could not figure out why the control rats had traces of BPA.

"Potential exposure to BPA from the vehicle, cage materials, bedding, and drinking water was also carefully evaluated and found to be minimal, and the additional source(s) of exposure has not been identified," wrote Doerge and his co-authors.

But the authors of the study reporting the contamination said that this did not affect their results because neither group of rats had any effects at the low doses. The FDA spokesperson said in an email that even though the “source of this exposure could not be identified … interpretation of the toxicological effects, observed only at the highest BPA doses, was not compromised.”

Vandenberg, however, said it sounds like human error. “That’s a problem. When you have contamination like that, you cannot just look at the higher-dose groups and make conclusions,” she said.

The discovery of BPA exposure in the control rats could call into question the relevancy of other studies that have linked low doses to health impacts, the authors wrote.

But Vandenberg disagreed. “Just because they couldn’t keep their controls from being contaminated doesn’t mean that other people can’t. That’s an illogical conclusion. It’s not how science works,” she said.

More than 90 percent of Americans have traces of BPA in their bodies. The FDA, based on studies that had not found low-dose effects, previously determined in 2008 that a safe dose for people is 5 milligrams of BPA per kilograms of body weight. 

The FDA has long maintained that the BPA levels in people are safe. After a review of the science, the National Toxicology Program announced in 2008 that there is “some concern” about developmental and reproductive problems in infants and children exposed to BPA. After that, the FDA altered its stance on the chemical to match the other government agencies by saying there is "some concern" about human health effects. However, the agency updated its website last June to say “BPA is safe at the very low levels that occur in some foods” and “the use of BPA in food packaging and containers is safe.”

The American Chemistry Council, which represents chemical companies that manufacture BPA, called the new study important and well-designed.

It “provides further support for FDA’s current position on the safety of BPA in food contact use. In its most recent statement from June 2013, FDA answers the question, ‘Is BPA safe?’ with one word: Yes,” Steve Hentges, a representative at the American Chemistry Council, said in a prepared statement.

But Joe Braun, an epidemiologist and visiting scientist at Harvard University, said the “the jury’s still out” on some potential health impacts on the rats. This study did not look for neurological effects such as changes in learning, memory and behavior.

“What needs to follow is whether these exposures are causing neurobehavioral changes,” Braun said. “Other research hints that animals exposed to BPA show increased anxiety, and we see similar results in children whose mothers were exposed.

“Hopefully they [the FDA] will address that down the road," he said.

One criticism of the FDA study is that previous research that used brains from the same types of rats found estrogen receptors in parts of the brain were triggered by low doses. The FDA study did not look for those effects.

Two years ago, 12 scientists, including Vandenberg, published a report that concluded “whether low doses of EDCs [endocrine disrupting chemicals] influence certain human disorders is no longer conjecture, because epidemiological studies show that environmental exposures to EDCs are associated with human diseases and disabilities.” Pete Myers, founder of Environmental Health News and chief scientist at Environmental Health Sciences, was the senior author of the report. 

The scientists criticized the EPA’s decades-old strategy for testing chemicals in the environment and consumer products.

In response, the EPA concluded that current testing of hormone altering chemicals is adequate to protect human health.

“There currently is no reproducible evidence” that the low-dose effects seen in lab tests “are predictive of adverse outcomes that may be seen in humans or wildlife populations for estrogen, androgen or thyroid endpoints,” the EPA report said.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.