Scientists recreated the airborne pollutants that come from the burning of fossil fuels and the weathering of car parts and pavement, and exposed mice to the harsh air for 15 hours per week over 10 weeks.
The tiny air particles were "roughly one-thousandth the width of a human hair, and too small for car filtration systems to trap", but exerted massive damage on the brains of the exposed mice, said the study on Thursday.
"You can't see them, but they are inhaled and have an effect on brain neurons that raises the possibility of long-term brain health consequences of freeway air," said senior author Caleb Finch of the University of Southern California.
Scientists found that the exposure resulted in "significant damage" to neurons involved in learning and memory, and they detected "signs of inflammation associated with premature aging and Alzheimer's disease."
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The study is published in the peer-reviewed journal Environmental Health Perspectives.
More research is necessary to determine if the same effects could be seen in humans.
"Of course this leads to the question, 'How can we protect urban dwellers from this type of toxicity?' And that's a huge unknown," Finch said.
