Children born outside the United States have a lower risk of asthma, skin and food allergies, and living in the United States for a decade may raise a person's allergy risk, said a study on Monday.
The research in the Journal of the American Medical Association suggests that certain environmental exposures could trigger allergies later in life, overcoming the protective effects of microbial exposure in childhood.
The study examined records from 2007-2008 phone surveys of nearly 92,000 people in the United States, where food and skin allergies have been on the rise in recent years.
Conditions reported in the survey included asthma, eczema, hay fever, and food allergies.
"Children born outside the United States had significantly lower prevalence of any allergic diseases (20.3 percent) than those born in the United States (34.5 percent)," said the study led by Jonathan Silverberg of St. Luke's-Roosevelt Hospital Center in New York.
"However, foreign-born Americans develop increased risk for allergic disease with prolonged residence in the United States," it said.
Children who were born outside the United States but came to live in the United States for longer than 10 years showed "significantly" higher odds of developing eczema or hay fever but not asthma or food allergies, said the research. It did not matter what age they were when they arrived.
Foreign-born children who resided in the United States for 10 years or more had nearly five times higher likelihood of eczema and more than six times higher odds of hay fever compared with foreign-born children who lived in the US for up to two years.
Silverberg said that the asthma levels seen in the study just missed the cutoff for being statistically significant.
"That is, the results may have become more significant if the study included a few thousand more children," he said in an email to AFP.
The study concluded that "duration of residence in the United States is a previously unrecognized factor in the epidemiology of atopic disease" and "foreign-born US residents might be at increased risk for later onset of allergic disease."
Previous research has shown that children who grow up in developing nations tend to have lower rates of allergies, and experts believe this is because they are exposed to more infections and microbes that build up their immune systems.
Allergies are essentially a symptom of a hypersensitive immune system, reacting to substances that should normally be considered harmless.
The JAMA study noted that the United States may not be alone in this phenomenon, since previous studies have found that immigrants in Italy, Israel, and Australia had lower allergy rates than natives.
More research is needed to find out if allergies increase in those countries among immigrants who live there for long periods, said the study.
"It's not that living in the US is bad," said Punita Ponda, a doctor in the division of pediatric allergy and immunology at Cohen Children's Medical Center of New York.
"Living in developed countries is probably thought of as a risk factor compared to living in developing countries," explained Ponda, who was not involved in the study.
"When you live in the US, you tend to lose some of that bacteria that is in the gut that protects you from allergies, and you take up the bacteria of the people around you."
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JAMA Pediatr. Published online April 29, 2013. doi:10.1001/jamapediatrics.2013.1319