Air pollution refers to a wide range of gases, liquids, and solids suspended in the atmosphere. Known to have harmful effects on the respiratory and cardiovascular systems, these contaminants are now implicated in damage to the brain—an organ exposed to the air via multiple pathways.

Laurie o'keefe

1

In through the nose

Contaminants breathed through the nose can come into direct contact with the olfactory bulb, a neural structure in the vertebrate forebrain. Some research in humans and other animals suggests that fine particulate matter smaller than 2.5 micrometers across can reach the olfactory cortex and other brain regions via this route.

2

Inhaled into the lungs

Most gases can easily traverse the epithelium in the lungs to make it into the bloodstream, and some studies in humans and rats suggest that fine particulate matter can do the same. Circulating contaminants may wear down the blood-brain barrier and/or cross it...

Laurie o'keefe

1

In through the nose

Contaminants breathed through the nose can come into direct contact with the olfactory bulb, a neural structure in the vertebrate forebrain. Some research in humans and other animals suggests that fine particulate matter smaller than 2.5 micrometers across can reach the olfactory cortex and other brain regions via this route.

2

Inhaled into the lungs

Most gases can easily traverse the epithelium in the lungs to make it into the bloodstream, and some studies in humans and rats suggest that fine particulate matter can do the same. Circulating contaminants may wear down the blood-brain barrier and/or cross it to directly interact with neural tissue.

3

Via the gut

Pollutants that make it to the gut in swallowed air may be absorbed by the gut wall and into the bloodstream, where they can travel to the brain. But researchers are currently more interested in how pollution may influence the brain via changes to the composition of the gut’s resident microbial community. Recent research shows that high exposure to air pollution is associated with altered gut microbiomes in humans, while research in mice shows that inhaled particulate matter can change the microbiome’s composition within weeks. Microbiome changes have recently been linked to cognitive function, suggesting air pollution could act on the brain via this indirect pathway.

Evidence of harm

Many of the studies on the effects of chronic exposure to air pollution are observational, making researchers wary of saying air pollution causes reduced cognitive function. Nevertheless, evidence from in vivo studies in humans and nonhuman animals, combined with in vitro research and analyses of postmortem brains, is beginning to lend support to the idea.


PARTICULAR PRESENCE

Researchers have discovered magne­tite particles that look similar to those produced by vehicle engines in the frontal cortex of postmortem human brains.


WHITE MATTER

Some studies have found reduced volumes of white matter in people who have been exposed to elevated levels of air pollution.


REDUCED DEVELOPMENT

MRI scans indicate that some regions of the brain, such as the basal ganglia, develop more slowly than normal in children exposed to air pollution.


GETTING INFLAMED

In vitro and in vivo research finds a link between components of air pollution and inflammation-related damage in brain cells.


AMYLOID PLAQUES

A number of research groups have reported a higher accumulation of Alzheimer’s-associated proteins such as amyloid-ß in the brains of humans and animals exposed to air pollution.


Laurie o'keefe

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Catherine Offord is an associate editor at The Scientist. Email her at cofford@the-scientist.com.

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