One of the entry routes for this pollution is through the nose.

A number of studies have shown that chemicals and particles — especially fine and ultrafine particles — can enter nerves in the nose and travel to the olfactory bulb, which is a structure in the front of the brain that is involved in olfaction, or the sense of smell.

From there, the particles enter the rest of the brain through the entorhinal cortex and hippocampus, both of which play major roles in learning, memory, and behavior.

Another direct route into the brain is by way of the cranial nerve that supplies sensation to the inside of the mouth and tongue; this is called the trigeminal nerve.

Particles and dissolved gases can follow the course of these nerves and enter the brain stem.

While those are major pathways directly to the brain, of even more concern is that air pollution can affect the brain by causing widespread inflammation throughout the body.

Studies have shown that exposure to inhaled air pollution, especially pollution containing fine and ultrafine particles, triggers inflammation deep within the lungs and can penetrate the bloodstream.

From there inflammation spreads throughout the body, generating immune chemical messengers called cytokines and chemokines, which can penetrate the brain.

This activates the brain’s immune system, which is controlled by cells called microglia.