Long-term exposure to particulates from wildfire smoke linked to dementia risk, new study finds

Smoke permeates everything and impacts everyone. The visible stew of carbon and particulates typically from emission sources travels in the air, shrouds buildings, suffocates birds, and penetrates deep into the lungs. Now researchers believe wildfire smoke may impact the brain too.

Scientists found that people living in areas with high levels of fine particulate matter, or PM2.5, could have a greater risk of developing dementia in their late stage of life. “We saw specifically that emissions from agriculture and wildfires may be more harmful to the brain,” said Boya Zhang, the lead author of a new study published Monday in JAMA Internal Medicine. “It’s really intriguing to us,” the doctoral student at the University of Michigan’s School of Public Health in Ann Arbor told STAT.

It took a while for scientists to find evidence for a link between air pollution and neurodegenerative diseases like dementia. But which specific air pollution sources contribute to this association remains an enigma. The new study is among the first to examine whether PM2.5 from different emission sources carry different risks and show a strong link between exposure to wildfire-specific PM2.5 and neurodegenerative disease.

“It’s a great study, a great population, and it’s got terrific data,” said Marc Weisskopf, a professor of environmental epidemiology and physiology at Harvard T.H Chan School of Public Health. “They’re going the next step to parse out what are the different components of air pollution that matter more than others.”

Weisskopf, who studies how environmental factors affect the nervous system, told STAT that the study advances the field and could especially help public health interventions for dementia prevention. “It’s a great step forward,” said Weisskopf, who was not part of the study.

The latest findings come out of the Environmental Predictors of Cognitive Health and Aging study that Zhang joined in 2019.

As a child growing up in China’s sprawling city of Beijing, infamous for its poor air quality, Zhang experienced terrible pollution levels. Soot was everywhere and she watched Beijing’s skies turn yellow, covered in gritty dust. The hazy scenes never left her memory and sparked her interest in air pollution research.

When wildfires rage, they generate smoke, conjuring PM2.5 and harmful particulate matter far and wide. PM2.5 are tiny bits of particles that can hang in the air for long periods of time. Once a person breathes in these airborne particles, they can bypass the body’s nasal defenses and pump themselves deep into the lungs. With a size about 1/20th of the human hair, they float in the bloodstream and ferry into other vital organs including the brain, damaging cells and causing inflammation.

With Health and Retirement Study data from a nationally representative group of Americans older than 50, Zhang and her team conducted cognitive assessments on nearly 30,000 people with no dementia but who were exposed to different air pollution sources in areas across the U.S. They analyzed the study participants’ exposure levels to PM2.5 between 1998 and 2016, due to emissions from sources including agriculture, road traffic, industry energy, coal combustions, and wildfires. Those who had higher residential PM2.5 levels were linked with increased risk of developing dementia.

“The strength of the observed associations differed across emission sources, with the strongest and most robust associations for PM2.5 from agriculture and wildfires,” Zhang and her co-authors wrote.

The study showed that chemicals in fine particulate air pollution differ with various sources. In agricultural pollution, a key precursor of PM2.5, ammonium, may be more damaging to the brain, the researchers found. They estimated that nearly 188,000 new cases of dementia each year were attributable to total PM2.5 exposure in the U.S.

This summer, stretches of thick smoke fueled by Canadian wildfires swallowed many cities across the U.S. for days and weeks, leaving over 100 million Americans exposed to some of the unhealthiest air on the planet. Scientists are racing to understand the prolonged health dangers of wildfire smoke, but the effects of exposure day after day remains unclear.

With the global burden of dementia projected to increase, Zhang said the study suggests interventions that target specific air pollution sources could be an effective way to cut down the dangerous PM2.5 particle levels among populations in the U.S. Weisskopf agreed. “It sort of helps us to identify the best levers to pull from a regulatory perspective to try and reduce levels of dementia,” he said.

The unprecedented increase in wildfires in the U.S. make interventions, including regulations and technologies, aimed at wildfire-specific PM2.5 all the more necessary to help promote healthy cognitive aging.

Weisskopf told STAT that developing interventions that could help prevent people from getting exposed to the smoke when wildfires occur may have a significant impact on reducing dementia in the U.S. “If there are ways to keep people away from the smoke when it happens, then that would lessen the impact on dementia.”