CHICAGO — Children in urban areas have a higher incidence of food allergies than those in rural America, according to a new study believed to be the first to assess allergic reactions in kids based on geography.

One of the urban centers tracked in the research is Cook County, Ill., where 9.8 percent of children have food allergies compared with 6.2 percent 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 percent versus 1.3 percent. Shellfish sensitivity, too, is more prevalent in urban areas at 2.4 percent 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 United States, 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.

In Chicago, life-threatening allergies made headlines in December 2010 when Katelyn Carlson died after eating Chinese food that had been cooked in peanut oil during a party at her school, Edison Regional Gifted Center.

The seventh-grader’s death led to a change in Illinois law authorizing school officials to give a shot of epinephrine — commonly delivered via EpiPens — to any student suffering an anaphylactic reaction, even if the child has not been diagnosed with an allergy, without fear of legal recourse.

This fall, Chicago Public Schools will stock an epinephrine auto-injector.

Still, few adults can recall “peanut-free” zones or bans on home-baked treats during their own youth, which is why many people still refuse to take allergies seriously, experts said.

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 7-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.

Finding the source of food allergies is not just the pediatrician’s professional passion, it’s a personal mission as well.

Her 5-year-old daughter is allergic to peanuts and tree nuts and she knows that trouble can lurk at every birthday party or sports event. But she also has a 10-year-old son who is not affected and chafes at being denied his peanut butter and jelly sandwiches.

“I’ve been on both sides … I’ve been that parent subjected to the rules, but I also understand why parents are extra-cautious and worried,” she said.

She ticks off numerous reasons why people still do not treat allergies as the life-threatening conditions they are, starting with the fact that many associate the word with sneezing and itchy eyes, not death. Also, parents react differently, depending on the severity and the ubiquitousness of the food.

“No one brings a fin fish to school, so that parent can be very lax … but if your child is allergic to wheat? Well, that’s a lot harder to avoid. When you see parents that aren’t as strict, it’s easy to think someone else is overreacting. You can even get push-back from family members.”

Even so, she thinks that with education and awareness, attitudes are changing

“Everyone understands this one simple fact: that all these parents want is for their kids to be safe.”