PhotoAsthma and other allergy-related diseases are serious business in the U.S., with the numbers of people affected increasing every year. And now a new study finds that children born overseas are less likely to suffer from allergies than U.S.-born children, but their risk of allergy diseases rises when they move to the United States.

The study by researchers at St. Luke's-Roosevelt Hospital Center in New York, looked at more than 91,000 children and measured the  prevalence of allergic disease, including asthma, eczema, hay fever, and food allergies. The study was published Online First in JAMA Pediatrics.

According to the study results, children born outside the United States had significantly lower odds of any allergy disorder than those born in the United States, including asthma, eczema, hay fever, and food allergies.

Children born outside of the United States whose parents were also born outside the United States had significantly lower odds of any allergy  disorders than those whose parents were born in the United States.

Children born outside the United States who lived in the United States for longer than 10 years when compared with those who resided for up to 2 years had significantly higher odds of developing allergic disorders, including eczema and hay fever, but not asthma or food allergies.

“In conclusion, foreign-born Americans have significantly lower risk of allergic disease than US-born Americans. However, foreign-born Americans develop increased risk for allergic disease with prolonged residence in the United States,” the study said.

What's the cause?

Why? The study didn't address that question.

A leading theory for rising rates of allergies is known as the "hygiene hypothesis." The idea is that when things are too clean, the immune system doesn't get a chance to build up defenses against viruses, bacteria and so forth.

While this may turn out to be true for allergies, most leading authorities now think it is not true for asthma, which is not always related to allergies.

The reason the hypothesis has fallen out of favor, according to a recent Scientific American article, is that asthma rates are skyrocketing in urban areas that are not particularly clean. It may even be that developing childhood illnesses set the stage for asthma, rather than helping to prevent it, many researchers now think.

Serious problem

Whatever the cause, it's no small matter, as the number of people with asthma in the United States continues to grow. One in 12 people (about 25 million, or 8% of the U.S. population) had asthma in 2009, compared with 1 in 14 (about 20 million, or 7%) in 2001, according to the American Academy of Asthma Allergy and Immunology (AAAAI).

About 1 in 10 children and 1 in 12 adults had asthma in 2009, the AAAAI said. It cited figures showing that 185 children and 3,262 adults died of asthma in 2007. 

Among adults, women are more likely than men to have asthma while among children boys are more likely than girls to suffer from the disease.

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