Canadians and Americans are alike in so many ways, but when it comes to levels of the plastic-making compound bisphenol A in their bodies, there is a huge difference.
For reasons unknown, typical Americans have about twice as much BPA, as the estrogen-mimicking compound is also known, than Canadians.
The big difference has prompted notice among scientists, who are searching for clues to why two populations often thought to be nearly identical when it comes to lifestyles and eating habits, and have similar demographics, show wildly different exposures to the toxic chemical.
BPA is used in everything from the inside plastic linings of most food cans to the polycarbonate in lenses on eyeglasses. Traces of the chemical leach out of consumer goods and end up in people and the broader environment.
Details of the difference between the two countries were published on Tuesday in the Canadian Medical Association Journal, and were based on a review of surveys done by U.S. and Canadian governments into the levels of common contaminants in their respective populations.
The U.S. researcher who pinpointed the unusual gap between Americans and Canadians says she is at a loss to explain the finding. "It's a huge mystery," said Laura Vandenberg, a postdoctoral fellow at Tufts University in Massachusetts.
Dr. Vandenberg also compared Canadian children and teenagers with those in Germany, a country that has done extensive testing of its young people, and found that Canadians have substantially less of the compound. However, American children were much more contaminated than those in Germany.
Although there is no clear explanation for the differences, Dr. Vandenberg speculated that it could be due to a dietary factor, such as Canadians not eating as much canned food as Americans. However, there are no data to support this conclusion and supermarkets in both countries stock many of the same brand-name items, often made in the same processing plants.
Health Canada said in a written response to questions that it could not comment on the study until it had a chance to review the research, but it said population-monitoring surveys are taken at different times, using different analytical procedures and age groups, which could explain the findings.
Government regulators, in their risk assessments on BPA, have assumed that most exposure is due to diet, with the vast majority coming from canned food and beverages. The data on the differences between Canadians and Americans hint that this assumption might be wrong because it implies that Canadians eat only about half as much canned food as Americans.
Canada doesn't have any petrochemical plants that manufacture BPA, another difference, but Dr. Vandenberg said U.S. production is centred in only five states, and there is no evidence that people living near these facilities have different levels of the compound.
BPA is also found on many cash-register receipts, from which the chemical could be absorbed into the skin or ingested through hand-to-mouth contact while eating. But, once again, Canadians and Americans use the same cash-register receipts, which are supplied to the global market by three main manufacturers, one of which no longer uses the compound.
"All of these things are pointing to the fact that as much as we know about how we're being exposed to this chemical, there is a ton that we don't know," Dr. Vandenberg said.
Canada added BPA to the country's list of toxic substances last year, the first country in the world to take this action, and it has banned baby bottles made from it, partly as a precaution to reduce exposure among toddlers. It's also asked makers of infant food to get the chemical out of formula cans.
Scientists are worried about BPA because exposure amounts to an extra dollop of estrogen, a powerful hormone that influences brain and breast development and the timing of puberty, among other health impacts.
The survey of BPA in Canadians, conducted from 2007 to 2009, found that 91 per cent of people had detectible amounts of the compound, on average just over one part per billion in urine. Although this is only a trace, it is still about a thousand times higher than natural levels of the female hormone. On some cell receptors, BPA and estrogen have roughly the same potency, but on others it is far weaker than the natural hormone.
Still, Dr. Vandenberg said the BPA concentrations found among Canadians are in the same range as those that have been shown in laboratory experiments to cause alterations in mammary glands, brain structure and reproductive tracts, particularly among animals exposed during fetal development.
She said Health Canada should consider broader curbs on BPA in consumer products to protect women during their pregnancies. "The fetal period is really a very sensitive period to this chemical," she said.
While Health Canada said it believes that "the general public need not be concerned" about BPA, it is studying human fetal exposure to the chemical and the results will be used to determine "whether or not this chemical poses a health risk to unborn children."