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When it comes to bisphenol A exposure from polycarbonate plastic drinking and baby bottles, it's not whether the container is new or old but the temperature of the liquid that has the most impact on how much BPA is released, new research suggests.The Canadian Press

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Exposure early in life to the chemical bisphenol A (BPA) could increase a child's chances of developing asthma, according to a new study by U.S. researchers.

BPA – a compound used to make hard, clear plastic – is no stranger to controversy. Its chemical structure resembles the hormone estrogen and some scientists have been warning that it could disrupt many biological processes.

But until now, asthma – an immune disorder – has not been the primary focus of these worries.

"This is the first such study linking early childhood exposure to BPA with asthma in school-age children," the lead author, Dr. Kathleen Donohue, said in an e-mail.

Asthma rates have been climbing steadily for the past three decades. This suggests "some as-yet-undiscovered environmental exposures may be implicated. Our study indicates that one such exposure may be BPA," said Donohue, who is an assistant professor of medicine at Columbia University College of Physicians and Surgeons and a researcher at the Columbia Center for Children's Environmental Health.

The findings are based on 568 mothers and their offspring. BPA exposures was determined by analyzing urine samples collected during the third trimester of pregnancy and in the children at ages 3, 5, and 7. When the children were between the ages of 5 and 12, physicians examined them for asthma symptoms, which included pulmonary function tests and a review of their medical histories.

The results, published in the Journal of Allergy and Clinical Immunology, showed that post-natal BPA exposure was associated with an increased risk of wheeze and asthma.

BPA has become extremely commonplace – it is used to line food cans and is found in many cash-register receipts. More than 90 per cent of the children in the study had at least some detectable levels of a BPA metabolite (a breakdown product of the chemical) in their urine. The higher the level, the greater the risk of lung issues.

But the study did contain a least one contradictory trend: A child was less likely to have breathing problems at age 5 if BPA was detected in the mother's urine during her third trimester of pregnancy. That is the opposite of what you might expect if BPA exposure had a consistent effect on asthma risk. Maybe it is the timing of the exposure that is critically important.

Furthermore, the researchers can't explain how BPA might increase the risk of the lung condition. There is no current evidence that indicates it alters the immune system in a way resembling asthma, in which the lungs overreact to certain airborne particles.

That means they have a long way to go to prove there is a real connection between asthma and BPA. Right now it is can be considered only an association.

Also keep in mind that many different theories have been proposed to explain the rise in asthma rates. Just a few weeks ago, one study suggested that the growing number of cesarean-section births may be partly to blame. So there is no shortage of asthma speculation.

Still, BPA remains a cause for concern. Earlier studies have suggested it may play a role in obesity, impaired glucose tolerance and behavioural problems as well as other health issues.

In order to reduce childhood exposure, the U.S. Food and Drug Administration announced in July it would ban the use of BPA in baby bottles and sippy cups. Health Canada took similar action more than a year earlier.

In a press release distributed with the study, the research team repeated the advice of U.S. National Institute of Environmental Health Sciences, which recommends reducing BPA exposure by avoiding plastic containers labelled number 7, eating less canned food and, when possible, choosing glass, porcelain or stainless steel containers, especially for hot food and liquids. BPA is also called polycarbonate.