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 City kids have smorgasbord of food choices. But they also face food allergies¹ more than do their country cousins. Researchers mapped food allergies in children across the U.S. And they found more cases per capita in areas with high population densities². The work is in the journal Clinical Pediatrics. The scientists collected allergy³ information on more than 38,000 Americans under age 18, and they sorted the information by zip code. Six percent of children in rural areas had a food allergy. But that number jumped to 10% of kids in urban centers. Peanut and shellfish allergies in particular were more common. Although the frequency allergies varied⁴ by location, their severity did not. What explains this trend? It could be that rural dirt is good. In the country, children immune sytems get challenged early and often, and thus more properly trained. Or, it could be that urban dirt is bad. City children deal with pollutants⁵ that may predispose them toward allergies. Solving this puzzle could help protect kids from common foods that can do them uncommon⁶ harm.