For the first time, a U.S. government agency last week acknowledged "some concern" over the common plastic ingredient bisphenol A (BPA)—used to make the polycarbonate and epoxy resins in everything from DVDs to baby bottles. Three days later, the Canadian government proposed a ban on BPA in baby bottles. Studies have linked exposure to the chemical—which can rapidly leach out of plastic bottles when they are exposed to a hot liquid—to damage in developing brains and tissues as well as a heightened risk of cancer later in life.

"In most instances, negative effects occur at levels much greater than those we are exposed to. That is not the case for newborns and infants. Early development is sensitive to BPA," Canadian health minister Tony Clement said at a press conference held to announce the proposed ban. "We have concluded that it is better to be safe than sorry."

The U.S. National Institute of Environmental Health Sciences's National Toxicology Program (NTP)—established in 1978 to coordinate the federal government's studies of various chemicals' adverse effects—in a new report warned that "the possibility that bisphenol A may alter human development cannot be dismissed."

The reason: a growing number of studies show that exposure to low levels of BPA during pregnancy, infancy and early childhood may affect normal development and "sensitivity to onset of disease later in life, most particularly the potential for mammary and prostate cancer," says NTP associate director John Bucher, a toxicologist. "Exposure to BPA in utero tends to cause the differences between males and females you typically see to diminish."

For example, an area of the rat brain associated with fear—the locus coeruleus—is usually larger in females than males. But males exposed to low levels of BPA—which mimics the hormone estrogen in the body—end up with a female-size locus coeruleus, along with attendant changes in behavior, such as behaving more anxiously.

The U.S. Centers for Disease Control and Prevention (CDC) found levels of the chemical that cause such effects in mice and rats in 93 percent of 2,157 people between the ages of six and 85 tested in 2004. "The doses being used [in the animal studies]," Bucher notes, "are not terribly different from the exposure we're experiencing at the present time."

The U.S.'s Environmental Protection Agency (EPA) and Food and Drug Administration (FDA) both consider BPA to be safe based on a handful of studies largely conducted by the chemicals industry.

"How does [the] FDA hold on to its position [that this is safe even after] it has looked at this literature, which is absolutely, blatantly false?" says reproductive biologist Fred vom Saal of the University of Missouri-Columbia, who was a member of a 2007 panel convened by the U.S. National Institutes of Health that found cause for concern. "What [FDA officials] mean when they say [that they believe it is safe] is they have looked at two corporately funded studies: one discredited and the other not published. What about the other 698 studies?"

In the wake of the NTP report, even industry has called on the agency to re-examine BPA to "put to rest" lingering safety questions, according to a letter sent by the American Chemistry Council to FDA Commissioner Andrew von Eschenbach.

The FDA in written statement said it would take "appropriate regulatory action" if new information "indicates that existing data no longer support the continued safe use of these materials." An FDA spokesperson declined to discuss the matter further.

BPA has been used in the U.S. and worldwide since the 1950s as an essential building block of the polycarbonate plastic in water bottles as well as in epoxy resins used to line cans to prevent corrosion and food contamination; more than 2.3 billion pounds (one billion kilograms) of BPA are manufactured annually in the U.S. and more than three million metric tons of it are used worldwide in a year.

Chemist Steven Hentges, executive director of the polycarbonate/BPA global group of the American Chemistry Council, insists the chemical does not pose a risk for adults and that it remains unclear whether the amounts to which infants are exposed are enough to cause damage.

The NTP found that, contrary to manufacturers' previous claims, humans are exposed to significant levels of BPA. Hentges admits that "there are several studies where there's limited evidence [of harm in animals]," but adds that he doesn't believe "there's a scientific basis to take action."

Part of the problem, Bucher says, is that it is not clear exactly how the human body processes BPA. Scientists believe that it quickly breaks down but there are few studies detailing how the chemical is absorbed, distributed, metabolized and excreted. "We can't account for the kind of exposures that would be required to result in the blood levels being measured in people," Bucher says. In other words, it is not clear whether the levels currently found in Americans tested are due to continuous or short-term exposure to very high levels of BPA—or both.

What is clear is that the basic workings of rat, mouse and human cells (that allow for the uptake and interaction with BPA) are identical—and that fetal and developing rats and mice exposed to it are more likely to display brain abnormalities and to develop cancer later in life. As a result, the Canadian government last week moved to list BPA as a toxic substance and prohibit it from being used in baby bottles. "This is the most important decision about a single chemical in decades," Canada's environment minister John Baird said during the press conference. "It is an internationally significant decision."

The Canadian government did not, however, move to ban the epoxy resins that line canned goods, including some infant formula containers in the absence of viable replacements. "For infant formula cans, there are not alternatives to date to reduce the BPA," Clement said. "We will be working with industry to reduce the level of BPA in the lining of cans and find alternative technologies as soon as possible."

Meantime, Canadian retailers have taken steps to strip all polycarbonate beverage bottles from store shelves in response to consumer demand. U.S. retailing giant Wal-Mart, which has outlets in Canada, says it plans to stop selling all polycarbonate-laced baby bottles in that country as well as in the U.S. by next year.

Under normal conditions, low levels of BPA leaches into contents from polycarbonate plastic bottles, which can be recognized by the number 7 in the recycling code or the letters PC. Recent studies show that the chemical leaks at faster rates after a lot of wear and tear or when exposed to hot liquid. "If this is of concern to parents or pregnant women, then clearly they should use plastics that don't leach BPA," Bucher says. "Don't mistreat polycarbonate-containing plastics by putting really hot things in them or things that would make migration more rapid."

Canadian officials say that more studies will be done, including monitoring some 5,000 pregnant women and their babies for BPA exposure levels. In the U.S. industry, academics and government  officials also plan to conduct more research, including analyses of how BPA interacts with the human body and its effects on development in pregnant macaques.

The House Committee on Energy and Commerce is investigating the potential dangers of BPA and how the FDA and manufacturers concluded that it was safe. Sen. Charles Schumer (D–N.Y.) said he plans introduce legislation today that would bar companies from using the chemical in baby bottles and other children's products as well as in food-packaging containers.

So should people be worried? "I have trouble answering that question right now," Bucher says. "I'm in the camp that is becoming concerned. More and more information is coming out that there's cause for concern."