Parents filled out questionnaires, and allergy tests via skin testing were completed on almost 32,000 youngsters. Another 8,951 children also had blood tests to measure levels of a substance known as IgE that increases in response to allergens.
Overall, children with allergies who lived in non-affluent countries were 2.2 times more likely than their non-allergic counterparts to have asthma. But, youngsters with allergies from wealthier, more developed nations were four times more likely to have asthma than their non-allergic peers.
The areas with the highest levels of allergy-induced asthma were Guangzhou, China (93.8 percent); Hong Kong, China (59.6 percent); and the Netherlands (58.6 percent). The areas with the lowest rates of allergic asthma were Ankara, Turkey (0 percent) and Mumbai, India (2 percent).
"The gist of this study is that, in more affluent countries, asthma seems more related to allergy than to other triggers," said Dr. Alan Khadavi, a pediatric asthma specialist at New York University Medical Center in New York City. "This study seems to support the hygiene hypothesis, because normal gut flora can be altered in affluent countries, with the use of antibiotics and living very cleanly."
However, just because the levels of allergic asthma are higher in more developed nations doesn't mean poorer countries escape the airway disease. For example, in Ankara, Turkey, where there was no incidence of allergic asthma, nearly 11 percent of children reported wheezing during the previous year. Yet, in Guangzhou, China -- the area with a nearly 94 percent rate of allergic asthma -- just 3.2 percent of the children in the study had experienced wheezing during the past year.
"Asthma is a complex disease caused by many different things, and it can present with many different symptoms," said Dr. Sandra Braganza, a pediatric asthma spec
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