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Adding boiling water to polycarbonate plastic bottles causes a dramatic spike in the amount of bisphenol A, or BPA, leaching from containers into drinks, according to a U.S. research team.

The finding suggests that parents sterilizing polycarbonate baby bottles by heating them in water or in a microwave may be inadvertently increasing the amount of the estrogen-mimicking chemical leaching from the containers. It also indicates hikers who use the bottles as a thermos to store hot tea or liquids may be doing the same.

The addition of boiling water increased BPA migration rates by up to 55-fold compared with water at room temperature, according to experiments run at the University of Cincinnati. A paper outlining the findings is being released today in Toxicology Letters, a peer-reviewed journal.

The researchers tested both new bottles and old, scratched bottles whose plastic had turned opaque, and found age of the containers didn't influence how much BPA they leached in an hour, but adding hot water to them did.

"In our study, it didn't make a difference. What made the difference was temperature," said Scott Belcher, the lead researcher and associate professor at the university's college of medicine.

Concerns over possible health risks posed by BPA led Mountain Equipment Co-op, one of Canada's largest sporting goods retailers, to remove polycarbonate plastic bottles, such as the popular Nalgene brand, from its shelves late last year, pending the outcome of a continuing Health Canada evaluation into the chemical.

Bisphenol A is being reviewed because of health concerns that exposure to the hormone-like chemical could be a factor in recent health trends, such as increasing rates of prostate cancer, earlier onset of puberty in girls and declining sperm counts.

In a second part of the new research, scientists took water that had been stored in polycarbonate bottles and added it to cell cultures derived from rat brains. They found that the BPA-contaminated water, even at incredibly small concentrations of less than a part per billion of the chemical, had an effect similar to estrogen in killing cells.

Many scientists, however, down play test tube results because they may not necessarily predict what would happen in living things.

Nonetheless, the study takes on added importance in Canada because its gives a glimpse into what Health Canada may be finding through its own research. The federal agency is subjecting polycarbonate plastic bottles to so-called migration tests to see how much BPA they're leaching, but hasn't published results.

In an e-mail response to questions posed by The Globe and Mail, Health Canada said it has conducted initial studies into how much BPA is shed from plastic bottles, but didn't use high-temperature conditions in the experiments, a lapse it is now remedying.

"Health Canada is aware that bisphenol A migration from [polycarbonate] is temperature dependent and in its assessment of BPA is reviewing the results of all available studies from other countries," it said.

The federal agency said it is currently running boiling water tests "and the results, when available, will be considered in the context of different potential consumer-use scenarios."

Bisphenol A is a key building block of polycarbonate plastic, often identified by the chemical industry's recycling symbol of a triangle encasing the number 7. It is also used to make the epoxy resins lining the insides of most tin cans, compact discs, dental fillings and sport helmets, among its dozens of consumer uses.

Although adults can rapidly metabolize and excrete BPA they ingest, most people are exposed to the compound constantly through food and beverage containers, and maintain blood levels of it around the low parts per billion, according to an independent research summary issued last year. A part per billion is equal to one second of elapsed time over 32 years.

Although this is a minute exposure, natural hormones produced by living things are active at even lower amounts, typically around a part per trillion, or a thousand times less.

In the new study, researchers found that boiled water allowed to cool in polycarbonate bottles contained up to 7.7 ppb of BPA after 24 hours. Water added at room temperature had far less, with amounts typically well under one ppb.