To find out more about the underlying cause of those behavioral changes, Nelson compared the brains of mice that had been exposed to dirty air with brains of mice that hadn't. He found a number of striking differences. For starters, mice exposed to particulate matter had increased levels of cytokines in the brain. (Cytokines are cell-signaling molecules that regulate the body's inflammatory response.) That wasn't entirely surprising, since previous studies investigating the cardiovascular effects of air pollution on mice had found widespread bodily inflammation in mice exposed to the pollution.

More surprisingly, Nelson also discovered physical changes to the nerve cells in the mouse hippocampus, a region known to play a role in spatial memory. Exposed mice had fewer spines on the tips of the neurons in this brain region. "Those [spines] form the connections to other cells," Nelson says. "So you have less dendritic complexity, and that's usually correlated with a poorer memory."

The changes are alarming and surprising, he says. "I never thought we'd actually see changes in brain structure."

Changes in the brain chemistry or composition are likely more plausible channels between air pollution and cognition. It is beyond the scope of this paper to test the exact mechanism, so we leave it as agenda for future research.

> It's not clear precisely how pollution impacts cognitive decline, though Chen suggests that pollution may have a negative effect on the brain's white matter, which could vary between men and women. As NPR has reported, "the brain's white matter coordinates communication among brain regions."

> James Hendrix, the director of Global Science Initiatives at the Alzheimer's Association, was skeptical of the claim that pollution impacts white matter. "I think that's speculation. I think we don't have a direct cause and effect that can be proven," he said.