Exposure to bisphenol A, the hormonally active chemical used to make the linings of most tin cans and hard plastic bottles, may be able to alter brain function, impairing the ability to learn and remember, according to a new study by researchers from Canada and the United States.

The study, conducted on monkeys, whose brain development is similar to that of humans, raises the possibility that ailments such as depression, Alzheimer's disease and schizophrenia may be linked to the controversial chemical.

Almost all people living in industrialized societies are exposed to BPA as a result of trace amounts leaking from food and beverage containers.

The researchers, from the University of Guelph in Ontario and Yale University in Connecticut, found that low-level exposure to bisphenol A, or BPA, was able to block the formation of some types of synapses in the brain, the tissue that allows brain cells known as neurons to communicate with each other. The proper development of these synapses is considered crucial for remembering thoughts and experiences, and impairments in them are common in sufferers of depression and other brain-related ailments.

The study, to be published this week in the Proceedings of the National Academy of Sciences, is a significant advance over previous rodent-based findings that BPA is able to impair synapses. That research was open to criticism that what happened in the brains of a mouse or a rat was of limited applicability to the more complex brains of humans.

"If bisphenol A at these kind of low doses is able to interfere with [monkey synapses] then there has to be concern that continuous exposure to bisphenol A is probably not a good thing," said Neil MacLusky, a biomedical science professor at the University of Guelph and one of the study authors.

The researchers were able to cause the harmful effects with a daily dose of 50 micrograms per kilogram of body weight - the human-exposure limit currently considered safe by the U.S. Environmental Protection Agency. (A microgram is one millionth of a gram.) Although Health Canada's limit is half that of the EPA, Dr. MacLusky said standards in both countries are too lax and should be reduced.

"If we're getting a complete blockage of the effect" at the U.S. standard, then Canada's standard "is probably not safe," he said, although he cautioned that the doses used in his experiment were much higher than the exposure people would get from food and beverage containers.

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