One of the urban centers tracked in the research is Cook County in Illinois, where 9.8% of children have food allergies, compared with 6.2% in more bucolic ZIP codes.

"That's a big discrepancy," said Dr. Ruchi Gupta, an assistant professor of pediatrics at Northwestern University Feinberg School of Medicine and a lead author of the study. "What we've found for the first time is that population density and environment have an impact."

The study, which followed almost 38,500 children under age 18, will be published in the July issue of Clinical Pediatrics. The researchers surveyed a representative sample of U.S. households with children about food allergies and mapped them based on their ZIP codes in every state.

Other key findings include: Peanut allergies are twice as common in urban centers as rural communities, 2.8% versus 1.3%. Shellfish sensitivity, too, is more prevalent in urban areas at 2.4% versus .08% in the country.

"The big question now is what in the environment is the trigger?" Gupta said.

Childhood food allergies have become a growing health concern for well over a decade, affecting 1 in 13 children under age 18 in the U.S., according to a 2011 study published in Pediatrics. An estimated 150 kids die each year due to an anaphylactic reaction, resulting in a drop in blood pressure, swelling of the throat and trouble breathing.

The growing body of research like Gupta's will help raise awareness and understanding, said Jennifer Jobrack, the Midwest director of the Food Allergy Initiative, a nonprofit founded in 1998, which provided financial support for the study.

"This is indeed a real public health issue that affects families and decision-making every day," said Jobrack, whose seven-year-old son is allergic to nuts. "It's not just about where you eat out ... but your interactions with camps, schools, even bus drivers. You always have to be vigilant on behalf of your child."

One possible hypothesis for the food allergies problem: Americans -- with their hand sanitizers, anti-bacterial gels and body washes -- have actually become too hygienic, causing the body to "fight things it shouldn't be fighting," Gupta explained.

"But right now, this is just a theory. Nothing is proven," said Gupta, who is also on the faculty at the Ann & Robert H. Lurie Children's Hospital of Chicago.

 From the June 8, 2012, Prepared Foods’ Daily News