Mexico City wears a thick coat of air pollution that clogs lungs and takes a toll on hearts and blood vessels. But that’s just the beginning — the metropolis’s dirty air may have contributed to brain inflammation and intellectual deficits in at least some school-age children, a new study suggests.

Among healthy children aged 7 to 18, lifelong Mexico City residents scored lower than their peers from Polotitlán — a Mexican city with low levels of air pollution — on tests of memory, flexible thinking, novel problem-solving skill and the ability to monitor and change one’s behavior during challenging tasks, scientists report in an upcoming Brain and Cognition. These tests make up part of standard IQ measures for school children.