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PROBLEM: Why are urban kids are more likely to have food allergies than rural kids, and American children more likely to have a host of allergic diseases, including asthma, than those growing up in developing countries? Most scientists attribute it to the "hygiene hypothesis": when kids are exposed to potential allergens early and often, they're better able to develop immunity against them. But does that protective effect hold when people move to places where the natives are more prone to allergies?
METHODOLOGY: Jonathan Silverberg and colleagues at St. Luke's-Roosevelt Hospital Center in New York used survey data to study the prevalence of physician-diagnosed asthma, eczema, hay fever, food allergies among over 90,000 U.S. children under the age of 18. They looked only at whether the child was born outside of the country -- not where they were from -- and at how long it had been since they had immigrated.
RESULTS: Foreign-born American children were 59 percent less likely to have developed one or more of the allergic diseases studied. They were 53 percent less likely to have ever had asthma, 34 percent less likely to have it currently, 43 percent less likely to have eczema, and 39 percent less likely to have hay fever. Race and ethnicity, as well as the age at which they entered the U.S., did not appear to affect these results.
Children who were born in the U.S., but whose parents were not, were also significantly less likely to develop allergies, and being born outside the U.S. to foreign-born parents increased the apparent protective effect.
Compared to children who had been here for 2 years or fewer, those who had been in the country for longer than a decade were over three times more likely to develop allergies, including eczema and hay fever. They were not, however, significantly more likely to develop asthma or food allergies.
IMPLICATIONS: The study, writes Silverberg, is the first to show that increased time spent living in the U.S. increases the risk of developing allergies. If the relatively sterile environment of the U.S. makes American kids more prone to allergies, then among foreign-born children, there appeared to be a protective effect to being exposed to toxins early in life. But that protection didn't appear to be life-long.
Silverberg also hypothesizes that aside from environmental exposure, the way they were raised by their parents may have contributed to children's likelihood of developing allergies. For example, foreign-born parents may have fed their children a healthier diet, perhaps one that included cultural foods like green tea or the spice curcumin, which have properties that are thought to protect against allergies and inflammation.
"Prevalence of Allergic Disease in Foreign-Born American Children" is published in JAMA Pediatrics 
