Native-born Americans might be allergic to the United States. New evidence published Monday suggests being born in a foreign country significantly reduces the likelihood of having allergies or asthma after moving to the U.S.
That protection doesn't last forever, however: People who have been in the United States for at least a decade lose that protection and are more likely to develop asthma, hay fever and eczema, according to the study, published in the Journal of the American Medical Society Pediatrics.
"We found that children born outside the United States have significantly lower prevalence of allergic disorders, including asthma, eczema, hay fever and food allergies," writes Jonathan Silverberg, a doctor at St. Luke's—Roosevelt Hospital Center in New York. "The odds of developing allergic disease significantly increased after residing in the United States for one decade or longer."
The study analyzed 69,667 children with data from the 2007-2008 National Survey of Children's Health. It is the first study to link Americans' allergy risk with place of birth, though previous studies have shown that asthma and allergy rates are lower in developed countries. People with parents who were born in other countries also had a lower risk of having allergies or asthma, even if they themselves were born in the United States. According to the study, it did not matter what age a person moved to the United States, but their foreign-born protection dissipated after 10 years. Similar studies have found that immigrants to Italy, Israel and Australia have lower rates of allergies than native-born people, thus "lower prevalence of childhood allergic disease in immigrants seems to occur in other developed nations."
"These data indicate that duration of residence in the United States is a previously unrecognized factor in the epidemiology of [allergies]," Silverberg writes. "Further, this suggests that foreign-born U.S. residents might be at increased risk for later onset of allergic disease."
The study gives credence to theories that suggest early exposure to certain viruses might protect against allergies later in life.
"The findings of the present study are consistent with the broader hygiene hypothesis, which suggests that either infections or certain microbial exposures in early childhood may confer protection against [allergies and asthma]," Silverberg writes.