How Wildfire Smoke May Harm Brain Health

A growing body of research suggests that exposure to wildfire smoke can have detrimental effects not only on the heart and lungs but also on the brain, including raising the risk for dementia and other neurologic disorders.

The number of wildfires in the United States hasn’t increased, but fires here and elsewhere in the world have become more intense, larger, and more destructive in recent decades. Warming temperatures, earlier snowmelt, and prolonged dry periods have extended the fire season.

“Because of climate warming, the fire season is starting earlier and ending later each year,” Brian Harvey, PhD, MS, with the School of Environmental and Forest Sciences, University of Washington, Seattle, told Medscape Medical News.

Drying of vegetation and fuels has made landscapes more flammable, and historical fire suppression policies have led to the accumulation of fuels in forests, he explained.

“Every time that fire burns, it removes fuel for the next fire,” Harvey said. But fire suppression policies have led to significantly more fuel, making fires more intense and harder to control.

And all of this may spell trouble for brain health.

Is Wildfire Smoke More Toxic?

Wildfire smoke can travel thousands of miles and affect air quality in communities far from the actual fire source. Last summer, when vast numbers of wildfires were burning in eastern Canada, thick plumes of smoke drifted south into many areas of New Jersey and New York, triggering widespread air quality alerts and some flight restrictions.

According to the nonprofit Climate Central, the average person living in the United States breathed in more harmful wildfire smoke in 2023 than in any year since 2006. Studies suggest that exposure in the United States has increased 27-fold over the past decade.

Wildfire smoke is a complex mix of pollutants, but fine particulate matter (known as PM2.5) accounts for about 90% and is the main threat to human health.

As previously reported by Medscape Medical News, wildfire smoke is generally considered more toxic than other types of smoke due to the smaller, more deeply penetrating particulate matter it contains, which can cause significant respiratory and cardiovascular issues, especially when compared with typical air pollution from sources like traffic or industry.

In this study, which was presented at the European Respiratory Society 2024 Congress, researchers in Spain used a large dataset of daily mortality data from 32 European countries collected through the EARLY-ADAPT project. The data analysis indicated that the relative risk per unit of PM2.5 is substantially larger for wildfire-related PM2.5 than for non-fire PM2.5.

Specifically, when employing exposure-response functions specific to wildfire smoke, investigators found that the attributable deaths from all causes of wildfire PM2.5 were approximately 10 times larger than those calculated using total PM2.5 exposure estimates.

What’s the Link to Dementia?

Research on the impact of wildfire smoke–specific PM2.5 on neurologic outcomes is lacking. However, one of the few studies that did look at this was published last month in JAMA Neurology.

The study, led by Holly Elser, MD, PhD, and Joan Casey, PhD, with the University of Washington School of Public Health, Seattle, found that exposure to wildfire smoke was associated with an increased risk for dementia.

The study team analyzed electronic health records of more than 1.2 million Kaiser Permanente Southern California members aged 60 years or older who did not have dementia at baseline.

They estimated total PM2.5 concentrations by census tract from 2006 to 2019 using air quality and weather data. They also used data on wildfires to separate wildfire PM2.5 from other sources of PM2.5.

During the study, more than 80,000 participants were diagnosed with dementia. They found that for every 1 μg/m³ increase in average wildfire PM2.5, the odds of receiving a dementia diagnosis increased by 18%.

In contrast, a 1 μg/m³ increase in average PM2.5 from other sources was associated with only a 1% increase in the odds of a dementia diagnosis.

“This study is the first to carefully track wildfire PM2.5 exposure patterns over time in a large population,” Casey told Medscape Medical News.

Just what level of exposure poses the greatest risk for dementia is an area of ongoing study, Elser told Medscape Medical News.

Exposure to wildfire PM2.5 is unique in that there are brief periods of very intense exposure. Yet for the majority of days, individuals aren’t exposed to any wildfire PM2.5, Elser explained.

“Our study explores this to some extent and suggests the odds of dementia diagnosis are increased in association with exposure to smoke waves (2 or more consecutive days with a mean daily wildfire PM2.5 concentration > 15 μg/m³), but more work is needed that explores how the intensity and chronicity of wildfire smoke exposure matter for the public health,” Elser said.

Impact on Cognitive Function

Dementia risk aside, a recent study showed that wildfire smoke exposure was associated with reduced cognitive performance on a brain-training game within hours and days of exposure. Investigators evaluated associations between daily and hourly PM2.5 and wildfire smoke exposure and cognitive performance in adults playing “Lost in Migration” on the Lumosity platform.

This game challenges a player’s focus by quickly flashing different patterns of flying bird icons on the screen. The player’s job is to quickly identify the direction the center bird is pointed in, assessing its focus, and forcing it to disregard the distracting extra information on the screen.

The results of the study revealed “striking” effects of wildfire smoke and pollution particulates on brain performance, according to a statement from the Environmental Protection Agency (EPA). A 10 μg/m³ increase in PM2.5 in the 3 hours prior to gameplay was associated with a 21-point decrease in score. Medium and heavy smoke densities were also negatively associated with score. Heavy smoke density the day prior to gameplay was associated with a 117-point decrease in score relative to no smoke.

“This is one of the first epidemiologic studies to identify the link between daily and subdaily PM2.5 exposure and cognitive performance in the working-age population and to show that PM2.5 is associated with reduced attention within hours of exposure,” researchers wrote. “It is also the first to identify an association between wildfire smoke density and decreased cognitive performance.”

Exposure to wildfire smoke has also been tied to lower test scores among US students aged 8-14 years. Authors of a 2022 study published in Nature Sustainability analyzed standardized test scores from 2009 to 2016 for nearly 11,700 school districts and satellite-derived estimates of daily smoke exposure.

Compared with a school year without smoke, exposure to average smoke-related PM2.5 during the school year (around 35 μg/m³) was associated with a small decrease in test scores (about 0.15% of SD).

“Our work contributes to a growing body of evidence demonstrating the cognitive, health, and social harms of air pollution in general, and wildfires specifically, and shows how disparities in these impacts across socioeconomic and racial/ethnic groups can emerge even when there are negligible differences across groups in ambient exposures,” researchers wrote.

What About Attention-Deficit/Hyperactivity Disorder (ADHD) and Mental Illness?

Exposure to air pollution, including wildfire smoke, has also been associated with an increased risk for ADHD in children.

In a meta-analysis of 14 studies, researchers found a significant role for some pollutants, in particular heavy metals and phthalates, in the increased risk for ADHD symptoms.

A separate study showed a greater number of days with PM2.5 air pollution levels above the EPA standards was associated with increased symptoms of mental illness in children, aged 9-11 years, both during the year of exposure and up to 1 year later.

In this sample of more than 10,000 youth from the Adolescent Brain Cognitive Development Study, each additional day of exposure at unsafe levels boosted the likelihood of a youth having symptoms of depression, anxiety, and other internalizing and externalizing symptoms. This was the case for both boys and girls.

Links to Parkinson’s Disease (PD) and Multiple Sclerosis

Increased exposure to PM2.5 has also been linked to an increased risk for PD and dyskinesia, findings from a September JAMA Network Open study showed.

Investigators used data from the Rochester Epidemiology Project on 346 patients with PD and 4813 matched control individuals. Mean annual exposures to PM2.5 and nitrogen dioxide (NO2) within a square kilometer area of participants’ residences were estimated from 1998 to 2019 and from 2000 to 2014, respectively.

Increased exposure to PM2.5 and NO2 was associated with a 14% and 13% higher risk, respectively, for PD among those with the highest exposure vs those with the lowest exposure. Increased exposure to PM2.5 was also linked to a 36% higher risk for the akinetic rigid subtype of PD.

Air pollution is also a potential environmental risk factor for multiple sclerosis. As previously reported by Medscape Medical News, investigators leading a large cohort study of almost 550,000 individuals living in Italy found that those living in areas with high levels of pollutants had a significantly greater risk for multiple sclerosis than those who lived in areas with low levels of pollutants.

What’s the Potential Mechanism?

Just how wildfire smoke exposure harms the brain is unclear.

“This is a fairly new area of research so there are still more questions than answers,” Claire Schollaert, PhD, environmental health researcher and exposure scientist with the University of California Los Angeles Fielding School of Public Health, told Medscape Medical News.

“We don’t know specifically how smoke exposure harms the brain, but based on what we know about total air pollution impacts the brain, we might expect fine particles from smoke to be able to cross from the bloodstream through blood brain barrier and directly interact with brain tissue,” Schollaert said.

A 2023 study in mice showed that wildfire smoke can trigger inflammation in the hippocampus, the brain region involved in learning and memory, which can persist for a month or more after exposure.

A recent review reported that both extreme heat and wildfire smoke air pollution (especially PM2.5) induce neuroinflammatory and cerebrovascular effects and oxidative stress.

“The interaction of wildfire smoke–associated air pollution and extreme heat forms a potentially complex and potent threat to neurological health,” review author Anthony White, with the Mental Health and Neuroscience Program, QIMR Berghofer Medical Research Institute, Herston, Australia, wrote.

From exacerbating neuroinflammation to compromising cognitive function and blood-brain barrier integrity, “the intertwined impacts of these environmental stressors that are increasing with climate change necessitate comprehensive research and practical interventions,” White concluded.

What to Tell Patients

The EPA sets guidelines for safe air quality levels, which are measured using the Air Quality Index (AQI):

• AQI 0-50 (Good): Air quality is considered safe for everyone.
• AQI 51-100 (Moderate): Air quality is acceptable, but some pollutants may pose a minor health concern for sensitive groups.
• AQI > 100 (Unhealthy): Health impacts increase for sensitive groups and, as levels rise, for the general population.

While any level of wildfire smoke exposure poses a risk, minimizing exposure and following public health guidance can reduce the potential for harm.

“Clinicians can counsel older adults to check the AQI (airnow.gov) and try to stay indoors, reduce exercise, run air filtration, and wear a mask (N95 or KN95) outdoors when the AQI exceeds 100,” Casey told Medscape Medical News.

In general, people at higher risk from wildfire smoke exposure include individuals with respiratory or cardiovascular conditions, children and older adults, pregnant individuals, and those with preexisting health conditions.

Harvey, Casey, Elser, and Schollaert reported no relevant disclosures.