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?
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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.