Air Pollution Linked To Alzheimer's Related Brain Changes, In New Study
Most air quality advice is seasonal, suggesting you watch the index when wildfire smoke drifts in, keep the windows shut, then go back to normal once the pollution has past.
But there is also the unremarkable pollution exposure, the kind from traffic and combustion outside your door year after year. Most people think of these things as only affecting respiratory health, but pollution's effect on brain health is getting a closer look.
Looking at brain scans & pollution exposure
A new study from researchers at the USC Neuroimaging and Informatics Institute at the Keck School of Medicine drew on brain scans and residential air pollution estimates1 from 1,484 adults who did not have dementia or a history of stroke. They involved two separate groups: 387 men from the Vietnam Era Twin Study of Aging (62 years old on average), and 1,097 women from the Women's Health Initiative Memory Study (78 tears old on average).
Using residential history, the team estimated each participant's exposure to air pollution in the three years before their MRI scan to two widespread outdoor pollutants: fine particulate matter, known as PM2.5, and nitrogen dioxide, or NO2.
- PM2.5 particles are less than 2.5 micrometers wide, small enough to travel deep into the lungs and potentially enter the bloodstream. They come from vehicle exhaust, power plants, wildfires, and other kinds of combustion.
- NO2 is a gas produced largely by burning fossil fuels. It is commonly associated with traffic.
Indoor air matters too. Research on common indoor air pollutants has looked at much shorter-term effects on how the brain functions.
What the researchers measured
The team measured the thickness of the cortex, the folded outer layer of the brain involved in memory, language, and decision-making. The cortex generally thins with age, and faster-than-expected thinning in certain areas can be a sign that brain cells are breaking down.
Researchers first looked at four areas of the cortex clustered in the temporal region, where Alzheimer's tends to show up earliest, combined into a single measure, then across 34 regions covering the rest of the brain.
Older women: more exposure, thinner cortex
Among the women tested, higher exposure to both pollutants was associated with a thinner cortex in those four Alzheimer's-vulnerable areas.
For each additional microgram per cubic meter of PM2.5 exposure, the estimated difference in thickness was comparable to about 13 months of aging. For each additional part per billion of NO2, the difference was comparable to roughly three months of aging.
The pattern extended past the four main areas measured. Higher PM2.5 exposure was linked to a thinner cortex in 23 of the 34 brain regions examined, spanning all four lobes.
Younger men: more exposure, thicker cortex
In the group of men, the association was the opposite. Greater pollution exposure was associated with a thicker cortex in the same Alzheimer's-vulnerable areas.
This relationship shifted with age, gradually fading between 55 and 64, then flipping to a thinner cortex after roughly 65. However, this reversal was weak enough that researchers can't rule out chance as an explanation.
Why a thicker cortex isn't automatically a healthier one
A thicker cortex is often read as a sign of healthier tissue. But emerging research suggests the cortex may temporarily thicken during the earliest stages of some disease processes, before thinning as brain cell loss progresses. Possible explanations include inflammation, swelling or enlargement of brain cells, or early changes tied to the buildup of amyloid, a protein involved in Alzheimer's disease. The study did not measure amyloid, tau, or inflammation, so it can't confirm whether any of those processes explain what showed up on the scans.
"Our findings raise the possibility that pollution-related brain changes are not linear," said Lauren Salminen, PhD, lead author of the study and assistant professor of research neurology at USC, in a press release. "A thicker cortex at one point in life could represent an early biological response, while thinning later in life could reflect accumulated damage."
The caveats worth knowing
Because the two groups differed in both age and sex, the researchers can't say which factor, or which combination, explains the opposite results. The analysis also captures a single point in time rather than following people throughout their lifetime, so it can't establish that air pollution caused the brain differences.
Salminen noted that following people over time will be essential for determining what these patterns mean for Alzheimer's risk, ideally alongside measures of inflammation, amyloid, tau, and changes in memory and thinking.
What this means for your brain health
Arthur W. Toga, PhD, director of the Stevens INI, put the bigger picture in context: "Air pollution is a widespread and potentially modifiable risk factor that affects people across entire communities." Unlike your age or your genetic risk for Alzheimer's, the air you breathe is something you, your community, and policymakers can act on.
- Check the air quality index: A quick look at the air quality before a long walk or run pays off most during wildfire season and on high-traffic days. Most weather app's have an air quality index feature, and it can help you decide when to avoid outdoor exposure.
- Move your workout away from traffic: Vehicle exhaust is a major source of both pollutants, so a park loop or side street beats a busy road.
- Filter the air where you sleep: A HEPA air purifier in the bedroom, the room most people spend a lot of their time in, is a good investment. Researchers have connected cleaner indoor air to better brain performance.
- Cut down on indoor sources: There are plenty of low-effort ways to clear your home's air, from swapping certain cleaning products to improving ventilation.
- Consider what's on your plate: Some research has explored whether eating fish regularly offers the brain some protection against air pollution.
- Keep your foundational habits going: Daily brain health habits around sleep, movement, and nutrition still do the heavy lifting.
The takeaway
You may have thought air quality is only something to worry about when it comes to wildfire season and lung health, but air quality is a year-round factor into how your brain ages.
This research shows that air quality is a modifiable lever for brain health, alongside many of the habits you already track. If you haven't dug in to limiting your air pollution exposure, this is your sign to get started.
