Study: Wildfire smoke increases dementia risk more than other pollutants

A landmark study of more than 1.2 million California residents suggests that exposure to wildfire smoke increases the likelihood of being diagnosed with dementia compared with exposure to other air pollution. The study, presented Monday at the Alzheimer's Association International Conference in Philadelphia, is the largest and most comprehensive look to date at the effects of wildfire smoke on brain health, the authors say.

“We expected to see an association between wildfire smoke exposure and dementia,” said study author Dr. Holly Elser, an epidemiologist and neurology resident at the University of Pennsylvania, “but the fact that we saw a much stronger association with wildfire smoke exposure than with non-wildfire exposure was somewhat surprising.”

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The discovery has major health implications, especially in Western states where air pollution from wildfires is severe. It currently accounts for half of all particulate pollution. This figure is on the rise as wildfires grow and intensify. Climate change and Legacy of fire suppression and industrial logging have Changed the composition of many western forests.

The researchers looked at a type of particulate matter pollutant called PM2.5. These particles are 30 times smaller than the width of a human hair and are small enough to penetrate deep into the lungs and enter the bloodstream, causing inflammation. Exposure has been shown to increase the risk of dementia and many other diseases, including heart disease, asthma and low birth weight.

“It's becoming increasingly clear that PM2.5 is associated with almost every health outcome that we look at,” said study author Joan Casey, an associate professor of public health at the University of Washington.

Elser, Casey and their team analyzed health records for more than 1.2 million Kaiser Permanente Southern California members age 60 and older from 2009 to 2019. None had been diagnosed with dementia at the start of the study.

The researchers estimated each person's exposure to PM2.5 based on where they lived, and then categorised it into wildfire and non-wildfire pollution using air quality monitoring data, satellite imagery and machine learning techniques.

Next, the researchers looked at how many participants were ultimately diagnosed with dementia. Unlike past studies, the researchers were able to determine this using patients' complete electronic medical records, rather than relying on hospitalization history as a proxy for diagnosis.

When the researchers looked at participants' average wildfire PM2.5 exposure over a three-year period, they found that for every increase of 1 microgram of particulate matter per cubic meter of air, the odds of being diagnosed with dementia increased by 23%. For non-wildfire PM2.5 exposure, a 3% increase in risk of being diagnosed with dementia was recorded for every increase of 3 micrograms of particulate matter per cubic meter of air.

“The bottom line is, what is it about wildfire smoke that's so different?” Casey said.

More research is needed to know what that is exactly, but possibilities include the fact that wildfire particles are produced at higher temperatures, have higher concentrations of toxic chemicals, and are smaller on average than PM2.5 from other sources.

Casey said these ultrafine particles can travel from a person's nose to the brain via the olfactory bulb.

“Normally the brain is protected by the blood-brain barrier, but here ultrafine particles can get directly into the brain and cause some of the problems we see in people with dementia,” she said.

The way people are exposed to wildfire smoke also differs from other types of fine particle pollution, the researchers said. Background, or ambient, fine particle pollution levels are typically relatively constant over time in a given location. But wildfire particulate matter tends to fluctuate widely, resulting in greater exposure in a short period of time that can overwhelm the body's defenses.

“Of the nearly 5,500 abstracts submitted to the Alzheimer's Association International Conference, this abstract stood out for its novelty, importance and impact,” said Claire Sexton, PhD, senior director of scientific programs and outreach at the Alzheimer's Association.

“Previous studies have looked at different types of contamination, but this study was unique in terms of the scope and methods of analysis,” she said.

The researchers found that Asian, black and Latino people and those living in high-poverty areas were particularly affected. Casey said the hardest hit were groups the researchers classified as “other” because they didn't include enough people to distinguish further. That group included Native Americans, Pacific Islanders and people of unknown race.

“Unfortunately, as we often see with environmental exposures, these disparities are emerging again,” she said, “but the levels observed here were pretty shocking.”

Casey believes these disparities are due to different exposures depending on where populations live, and previous studies have shown that Native Americans in California By far the highest levels of exposure to particulate matter from wildfiresOther contributing factors include poorer quality housing, lack of access to air filtration equipment, jobs that don't allow people to stay indoors during wildfires and people with pre-existing high blood pressure or diabetes responding differently to the same amount of pollution, she said.

“All of this is driven by social determinants of health,” she said. “The fact that we need to allocate additional resources to these people and places to protect their health and reduce health disparities going forward is really significant.”

The researchers didn't distinguish between dementia subtypes, such as Alzheimer's disease, the most common form of dementia, because they relied on diagnostic codes rather than brain imaging or postmortem studies. This is important to know and a key area for future research, because to best protect people, clinicians need to understand what underlies the relationship between wildfire smoke and different dementia subtypes, Elser said.

Still, the study is notable for its large sample size and careful approach that took into account sociodemographics such as comorbidities and census tract poverty, said Rachel Whitmer, director of the Alzheimer's Disease Research Center at the University of California, Davis, who was not involved in the research.

Dementia rates are on the rise as baby boomers age, but environmental factors may also be contributing to the increase, she said.

Studies like this lay the foundation for future research, she said.

“With the increase in wildfires, this is a really important issue and I think they've done a very rigorous job looking at it,” she said.

PM2.5 levels had been declining since the Clean Air Act was enacted in 1970, but wildfires in California have reversed that trend. Hindering efforts to reduce emissionsIn recent years, wildfire smoke has also affected the Midwest and East Coast. In 2023, smoke from Canadian wildfires is expected to cause major damage to the U.S. Covered the Atlantic coastAir quality warnings were issued and outdoor events were forced to be cancelled.

“It's a big problem in areas where wildfires are endemic,” Elser said, “and as wildfires become more and more frequent, our concern is that they're going to affect more people, more frequently, across a wider geographic distribution.”