Kids and teens who are born abroad and immigrate to the United States are about half as likely to have asthma and allergies as those who are born in the U.S., according to a new study.
Researchers surveyed the parents of 80,000 children in one of six languages and found that association held even after they took into account where families lived and how often they moved, as well as their race and income.
"This is definitely something we see clinically and we're trying to better understand, what is it in our environment that's increasing the risk of allergic disease?" said Dr. Ruchi Gupta, who studies allergies at the Northwestern University Feinberg School of Medicine in Chicago but wasn't involved in the new research.
"Food allergies have increased tremendously," she told Reuters Health. "We do see people who come from other countries don't tend to have it, but immigrants who are maybe second generation, they're identical (to U.S.-born people)."
It's not obvious what explains that pattern, researchers said.
According to Gupta, two possible culprits are the so-called hygiene hypothesis - which suggests kids in the U.S. are too clean, and their immune systems never get exposed to common allergens - or the poor quality of American diets.
Lead researcher Dr. Jonathan Silverberg from Beth Israel Medical Center and St. Luke's-Roosevelt Hospital Center in New York said climate, obesity and various infections might also be playing a role.
"The results of the study suggest that there are environmental factors in the U.S. that trigger allergic disease," he told Reuters Health in an email.
"Children born outside the U.S. are likely not exposed to these factors early in life and are therefore less likely to develop allergic diseases."
Surveys were completed by parents of kids and teens in 2007 and 2008. Just over 20 percent of children born outside the U.S. had any type of allergic disease - including asthma, eczema, hay fever or food allergies - compared to between 34 and 35 percent of those born in the U.S.
What's more, the risk of allergies increased with the more time foreign-born children spent in the U.S., Silverberg and his colleagues wrote in JAMA Pediatrics.
For example, 27 percent of foreign-born kids who had immigrated more than a decade earlier had any type of allergy, according to their parents' reports, versus 17 to 18 percent of those who had moved to the U.S. within the past two years.
"You acclimate to wherever you are and you pick up whatever is going on there," Gupta explained. "The findings here are very interesting - and not surprising."
Silverberg said he hoped the results would lead to further discoveries of what puts U.S. kids at risk for allergies and how to prevent them.
For now, Gupta recommended parents make sure their children eat a diet with plenty of fruits and vegetables. And allowing them to play in the mud a little bit probably wouldn't hurt, she said - with proper hand-washing afterward, of course.