Outdoor air pollution causes millions of deaths each year—4.2 million premature deaths globally, according to the World Health Organization. Over time, fine particles of inhaled smog cause cardiovascular and lung issues, like lung cancer and stroke.
These air pollutants may have equally insidious effects on the brain. Over the past decade or so, in both animals and humans, in the lab and in the real world, scientists have documented associations between air pollution and brain-related issues, like anxiety, poor attention, and memory deficits. Children appear to be especially susceptible.
Fine particulate matter, meaning air pollutants measuring 2.5 microns across or less (30 times smaller than the width of a human hair), are the likely culprits. “Once they are inhaled,” said Devon Payne-Sturges, an environmental health researcher at the University of Maryland, “they can reach to the really deep areas of the lung and they can get into the bloodstream and get carried into the brain.” Fine particulate matter is small enough to freely cross the blood-brain barrier that keeps out larger toxins. Alternatively, if inhaled through the nose, the fine particles can get to the brain via the olfactory nerve in the nasal cavity.
Once the particles arrive, in rodent brains at least, neural immune cells called microglia move in to engulf and destroy the particles. A similar process may occur in humans. However, the microglia may not remove all particles, leaving them to accumulate in the brain. These particles can trigger inflammation which may lead to more serious brain disorders and cognitive effects.
Scientists have grown increasingly concerned about these effects in children, who have more permeable blood-brain barriers. “Some researchers are beginning to use MRI scans to look at these neurodevelopmental impacts of pollutants, and showing actual changes in the brain,” said Payne-Sturges.