Food allergies are more common among kids living in cities than among children in less populated areas, a new study finds.

Researchers found that the share of children with any type of food allergy was 9.8 percent in cities, 7.2 percent in suburban areas, and 6.2 percent in rural areas.

Previous studies found that city dwellers have higher rates of other types of allergies, such as asthma, eczema and hay fever.

The new findings, based on a survey of parents, held even after the researchers accounted for factors associated with children's likelihood of having a food allergy, including ethnicity, gender, age, household income and the latitude of where they live.

The study is the first to examine the prevalence of child food allergies by geographical region, the researchers said.

The finding means there may be some factors that come with city living that predispose children to food allergies, said study researcher Dr. Ruchi Gupta, an assistant professor of pediatrics at the Northwestern University Feinberg School of Medicine.

Gupta and her colleagues surveyed more than 38,000 parents who had at least one child no older than 18. The online survey asked whether their child suffered from a food allergy and, if so, when it as diagnosed and how severe it was.

Kids' food allergies were then mapped by ZIP code.

The researchers looked at individual foods and found the allergy rates were consistently higher among city children. For instance, close to 3 percent of children in cities had a peanut allergy, compared with 1.3 percent in rural areas. And 2.4 percent of city children had shellfish allergies, compared with 0.8 percent in rural areas.

Generally, kids living in the southern and middle latitudes were more likely to have a food allergy than those living farther north – though not always, the researchers said.

The states with the highest rates of food allergies in children were Nevada, Florida, Georgia, Alaska, New Jersey, Delaware and Maryland. Children in the District of Columbia had the highest rate after Maryland.

Food allergies were just as severe for kids in rural areas as for kids in cities, Gupta said. Nearly 40 percent of kids in the study had undergone a life-threatening allergic reaction, according to their parents.

Researchers aren't sure why certain types of allergies are more common in cities. One idea, known as the hygiene hypothesis, is that early exposure to the bacteria found in rural areas protects against allergies, the researchers said. Or it could be that pollutants in cities trigger allergies.

Researchers also have wondered whether changes in the food supply, such as an increase in processed foods or a move away from locally grown foods, have played a role in the rise in food allergies in recent decades, Gupta said.

Gupta said she plans to conduct future studies looking into the environmental causes of food allergies.

The study will be published in the July issue of the journal Clinical Pediatrics.

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