A man standing on the roof of a house as he watches wildfires burn down nearby treesShare on Pinterest
Wildfire smoke has been linked to an increased risk of experiencing many health problems. David McNew/Stringer/Getty Images
  • Wildfires and agricultural spraying increase the lifelong risk of dementia more than air pollution from other sources, according to a new study.
  • Tiny particulates circulating in air pollution can easily enter the body and affect the brain via inflammation or direct crossing of the brain-blood barrier.
  • Even non-extreme levels of air pollution may be sufficient to increase the risk of dementia.

A new study from the University of Michigan has found that of all types of air pollution, wildfires and air pollution from agricultural spraying are the most damaging when it comes to brain health.

The authors of the study found that the tiny particles carried aloft by these two sources — which we may breathe in — are most strongly associated with the onset of dementia.

The study comes in the wake of July 2023 being designated the hottest month ever recorded and as out-of-control wildfires continue to wreak havoc across North America and the world. Canada, for example, is currently grappling with 236 fires in its Northwest Territories as the government has ordered Yellowknife residents to evacuate the city.

The smoke from such fires can travel great distances, reducing the quality of air in places far removed from the actual conflagrations.

At the same time, in other areas, agriculture continues releasing damaging particulates into the air, where they may also be dispersed over large geographic areas.

Air quality is measured by the amount of PM2.5 — an abbreviation for “particulate matter two and a half microns or smaller” — in the air. PM2.5 is tiny enough to pass through the blood-brain barrier that otherwise effectively protects our brains.

The study estimated that nearly 188,000 new cases of dementia per year can be attributed to PM2.5 exposure in the U.S.

The researchers’ analysis utilized data covering January 1, 1998, to December 31, 2016, from the Health and Retirement Study. Researchers followed for just over ten years the cognitive health of 27,857 individuals over the age of 50 who did not have dementia at the start of the study.

Estimates of the amount of PM2.5 likely to have been present at participants’ home addresses were correlated with new cases of dementia to arrive at the study’s findings.

The study is published in JAMA Internal Medicine.

PM2.5 — which is largely invisible to the eye — may encompass a broad range of chemicals, depending on its source. According to the study’s lead author, Dr. Boya Zhang, environmental epidemiology researcher at the University of Michigan’s School of Public Health, “PM2.5 can get deep into our body due to its tiny size. They could first enter our lungs, cause inflammation there, and then the released inflammation factors may transfer into the blood circulation.”

What makes it so potentially neurotoxic, Dr. Zhang said, is that, eventually, it can contribute to brain inflammation, which is implicated in dementia.

“PM2.5 may also be able to impact our blood vessels that supply oxygen to the brain. In addition, some really tiny PM2.5 can even damage our brains by directly entering the brain through our noses.”
— Dr. Boya Zhang

This is not the first study to suggest a link between PM2.5 and dementia.

“Several prior studies have also found that PM2.5 is statistically associated with elevated risk of [Alzheimer’s disease and related dementias],” said Dr. Kelly Bishop, assistant professor in the Department of Economics and Center for Environmental Economics and Sustainability Policy at Arizona State University, who was not involved in the study.

She noted a recent study, which she authored, and found that “the relationship between PM2.5 and [Alzheimer’s disease and related dementias] is causal, based on quasi-random variation in PM2.5 exposures among more than two million nationally representative seniors over a decade.”

Anything that serves as fuel for a wildfire — trees, homes, cars, factories, stores, and so on — may be converted through heat-related chemical transformations into potentially toxic PM2.5.

“The chemical transformations that occur during wildfires may vary dependent on what has been burned during the events and how long the smoke lingers in the atmosphere as it spreads,” explained Dr. Zhang.

“Wildfires release components that are likely to be highly toxic because they tend to incinerate not only natural but also synthetic materials in an uncontrolled manner,” she added.

The primary source of agricultural PM2.5 is pesticides sprayed onto crops — and into the air. It is not uncommon for such treatments to involve neurotoxins that increase their effectiveness.

“We believed the common application of neurotoxic pesticides or herbicides in agriculture could plausibly explain observed associations between PM2.5 from agriculture and dementia in our study,” said Dr. Zhang.

She also pointed out such chemicals may piggyback on existing PM2.5: “The neurotoxins in pesticides or herbicides used in agriculture may coat particles in the air. These particles may then enter our body through inhalation or other pathways.

The researchers note that the average levels of PM2.5 associated with the onset of dementia were lower than the National Ambient Air Quality Standard. This implies that cognition may be affected even at air pollution levels considered benign by current standards.

Revising those standards, however, may not be that helpful, say both Dr. Bishop and Zhang.

When air quality is tracked, it is not done by measuring PM2.5 at an individual’s exact location but by a somewhat wider area in which they reside.

Therefore, said Dr. Bishop, “we cannot measure the exact air quality that people breathe.”

“Improving the measurement of exposure to PM2.5 and the application of statistical methods used to address bias from measurement error will be important areas for future research in this area,” added Dr. Bishop.

Dr. Zhang noted that while learning about PM2.5’s sources is essential, it is difficult to do so “since there are rarely truly unique markers of sources.”

“Future research,” she suggested, “may need more emphasis on advanced modeling effects for source-specific PM2.5, like the chemical transport model used in our study, rather than re-calibrate the ways we measure.”