More and more Americans are flocking to cities. From 2010 to 2012, many major cities grew more rapidly than they had in the previous decade. But some of the fastest growing urban areas—Houston, Dallas, and San Francisco—are also some of the country’s most polluted. We already know how bad that can be for people's physical health. But a new study suggests that pollution may be hurt kids' intelligence, too. 

The study, published Tuesday in PLOS Medicine, finds that air pollution from traffic hinders the cognitive development of school children. The team of researchers followed 7- to 10-year-olds at 39 different schools around Barcelona, Spain. Using computerized tests, the researchers measured change in working memory (which is akin to short term memory) and inattentiveness of over 2,700 children over the course of a year.

The team found that kids attending a school in a high pollution area showed less cognitive development over the course of the year than their counterparts in less-polluted areas. For example, while children in low-pollution schools showed an 11.5 percent improvement in working memory, students at highly polluted schools only showed a 7.4 percent improvement. The study also looked at schools with comparable socioeconomic statuses to ensure there were not other factors causing the difference in development.

The paper also noted that the primary school age, or when children are about 6 to 10 years old, is “a particularly vulnerable time window for executive function development,” which is essential for learning. Previous studies have shown temporary inflammatory responses in the brain after exposure to traffic-related pollutants; however, this study shows that long-term exposure to polluted air can also have longer-term effects.