MONDAY, Sept. 17 (HealthDay News) -- Where you live may affect the type of asthma you develop, new research suggests.

The international study found that youngsters in affluent, developed countries had more allergy-triggered asthma symptoms than did children in poorer, less developed countries.

"The link between atopic sensitization and asthma symptoms in children differs strongly between populations and increases with economic development," the study's authors wrote.

Results of the study are published in the September issue of the American Journal of Respiratory and Critical Care Medicine.

An estimated 20 million Americans have asthma, according to the American Academy of Allergy, Asthma and Immunology. About half of that group -- 10 million people -- has what's known as allergic asthma. That means their asthma symptoms, such as wheezing, shortness of breath and a feeling of tightness in the chest, are triggered by allergens, such as dust mites, mold and pollen.

The exact cause of asthma still isn't known, but some experts have suggested that the rise in asthma incidence may be related to more sterile homes and the use of antibiotics. The "hygiene hypothesis" theorizes that without real challenges -- like those from bacteria and viruses -- the human immune system may begin responding to harmless substances, such as animal dander, causing allergy and asthma symptoms. While this theory is still controversial, some studies have supported the idea, according to the academy.

The new study, while not specifically designed to examine the hygiene hypothesis, sought to assess the differences in allergic sensitization and the types of asthma experienced in different countries.

The study included almost 55,000 children between the ages of 8 and 12 from 22 countries. The countries represented a wide range of living conditions from rural areas in Africa to urban areas of Europe.

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 specialist at the Children's Hospital at Montefiore in New York City. "It's likely that the environment being different -- both indoors and outdoors -- is responsible for the differences in this study."

Khadavi said, "In non-affluent countries, children may have worse nutrition and be exposed to worse pollution, and more of those things could lead to their asthma, instead of allergies."

More information

Learn ways to control your asthma from the American Academy of Family Physicians.

SOURCES: Alan Khadavi, M.D., pediatric asthma specialist, New York University Medical Center, New York City; Sandra Braganza, M.D., pediatric asthma specialist, the Children's Hospital at Montefiore, New York City; September 2007, American Journal of Respiratory and Critical Care Medicine

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