New research suggests long-term exposure to wildfire smoke may significantly increase dementia risk, posing a greater threat to brain health than other forms of air pollution.

The study, presented this week at the Alzheimer’s Association International Conference, examined a decade of electronic medical records for 1.2 million older adults in Southern California from 2009 to 2019. Researchers found a 21% higher risk of dementia diagnosis for every microgram per cubic meter increase in fine particulate matter (PM2.5) from wildfire smoke in the air, measured over three years.

By contrast, a three-microgram increase in PM2.5 from other sources like vehicle emissions or industrial pollution raised dementia risk by only 3%. The results, currently under peer review, account for variables including age, sex, race, ethnicity, smoking status, and income.

“There is quite a bit of prior literature that demonstrates an association between exposure to ambient air pollution and dementia, but there’s very limited research on how wildfire PM2.5, specifically, might be associated with dementia,” said Dr Holly Elser, lead author of the study and neurology resident at the University of Pennsylvania.

California’s ongoing Park mega-fire, which has burned 392,480 acres and become the state’s fifth-largest wildfire on record, underscores the study’s urgency. Smoke from this blaze now blankets regions where the study’s subjects live.

A separate study published in the Proceedings of the National Academy of Sciences (PNAS) in February highlighted the growing threat, revealing some parts of California endured nearly 18 months of high wildfire smoke levels over 15 years. Rural and tribal areas suffered the worst exposure, with some regions facing 10 times the average smoke particle levels.

These findings indicate that many Californians may face a 5% to 50% higher dementia risk, with economically disadvantaged communities experiencing the greatest danger.

“As wildfire events become more intense, more frequent, and occur in places outside of the Western US, we felt this was an important, specific source of air pollution to consider as a potential risk factor for dementia,” Elser said. “This is a real problem.”

Why wildfire smoke may be more dangerous

PM2.5, the main pollutant in wildfire smoke, consists of airborne liquid and solid droplets smaller than 2.5 micrometres – about 30 times finer than a human hair. These tiny particles can penetrate deep into the lungs, enter the bloodstream, and potentially reach the brain.

Sources of PM2.5 range from coal- and natural gas-fired plants to cars, agriculture, unpaved roads, construction sites, and wildfires. It can take the form of dirt, dust, soot, or smoke.

While researchers are still investigating why wildfire smoke appears more harmful to brain health than other forms of air pollution, several theories have emerged.

Despite sharing the PM2.5 label, particles from wildfires may be more toxic than urban pollution. Their chemical makeup varies widely based on burned materials, unlike the more predictable composition of vehicle or factory emissions.

“There’s a greater concentration of toxic chemicals in wildfire smoke,” said Niki Rubarth, regional director of the Alzheimer’s Association of Northern California. “Wildfires burn everything in their path so that can be anything.”

A 2022 US study of more than 18 million people linked black carbon, a major component of wildfire smoke, to higher dementia risk. Researchers found that each microgram per cubic meter increase in black carbon raised dementia risk by 12%. That same year, a separate study published in Environmental Health Perspectives revealed that even brief exposure to wildfire smoke could impair cognitive functions, including attention.

Another factor is particle size. PM2.5 particles from wildfires are typically smaller than those from urban pollution, potentially allowing for easier penetration into organs and the bloodstream.

The study’s authors emphasize the need for further research to pinpoint which specific components of wildfire smoke might be driving the increased dementia risk.

“I’m really curious to see whether or not we see a similar association with other neurodegenerative processes like Parkinson’s disease or ALS,” Elser added. The link between air pollution and Parkinson’s – a known cause of dementia – is well-established.  

They lost their home in Paradise to the Camp fire in 2018.

Six years later, after relocating to another Chico enclave, Kristy + Michael Daneau have lost a 2nd home to the massive Park fire. (Video of part of their evacuation)

Their tragic story & others:https://t.co/VOmCJ5C7GY pic.twitter.com/e9pgyB9FgW

— Grace Toohey (@grace_2e) August 1, 2024

Experts suggest wildfire smoke’s toxicity isn’t the only factor boosting dementia risk; stress and trauma from these events may also significantly impact cognitive decline.

“Exposure to environmental shocks like wildfires can also uncover cognitive changes already underway,” Elser explained. “The stress and depression associated with these events could potentially unmask neurodegenerative processes that were already present.”

Repeated trauma from losing homes and fleeing wildfires has well-documented links to post-traumatic stress disorder, depression and anxiety across California – all conditions that increase a patient’s risk of dementia.

Similar mental health impacts have been observed globally in relation to extreme weather events. Following devastating floods in the Philippine capital of Manila last year, researchers found evidence of worsened anxiety and distress.

“Repeated chronic stress can cause all sorts of physiological changes,” said Vaibhav Narayan, executive vice president at the Davos Alzheimer’s Collaborative. “Stress is a well-known activator of brain health decline and cognitive issues.” 

“It’s not just the physical impact of these stressors,” Narayan added. “It’s also the psychological impact that is detrimental to brain health.”

Growing evidence links air pollution to dementia risk

The new findings bolster a mounting body of evidence connecting air pollution, particularly high PM2.5 concentrations, to brain health issues.

In its 2024 update released Wednesday, the Lancet Commission on dementia prevention reported that at least nine systematic reviews and meta-analyses since 2019 have established a strong link between air pollution and increased dementia risk.

Studies worldwide demonstrate air quality’s impact on cognitive health. In China, stricter clean air policies slowed cognitive decline in older adults, while another study found 42.4% higher dementia risk in the country’s more polluted northern regions compared to the south.

In 2022, French researchers linked a 12 microgram per cubic meter reduction in PM2.5 levels over 10 years to a 15% lower dementia risk. In the US, meanwhile, older women experiencing larger improvements in air quality over a decade showed lower dementia risk.

High levels of fine particulate matter correlate with increased rates of dementia, mild cognitive impairment, and Alzheimer’s disease in both wealthy and developing nations, the Lancet Commission found. 

Indoor pollution poses additional threats, particularly in developing countries where solid fuels for cooking and heating are associated with higher dementia risk and faster cognitive decline in adults over 45.

While most studies focus on older adults, experts warn that air pollution could affect brain health earlier in life.

“People entering older age now have been exposed to wildfires quite extensively – potentially through their 40s, 50s and 60s,” said Dr Joan Casey, co-author of the wildfire study and a neurologist at the University of Washington. “That might be really important.” 

The Lancet Commission reported growing support for World Health Organization global air quality guidelines, noting, “It is unclear whether any safe concentration of air pollution exists.”

The commission now estimates that addressing “modifiable” risk factors could prevent up to 45% of dementia cases, up from 40% in its 2020 report. These factors include air pollution, less education, hearing loss, hypertension, physical inactivity, diabetes, social isolation, excessive alcohol consumption, smoking, obesity, traumatic brain injury and depression.

“The world can’t afford to ignore the 45%,” said Narayan. “That’s the most scalable way to address this issue.”

Wildfires spread globally 

The study’s findings come amid increasing global wildfire activity. Human-caused climate change has increased atmospheric aridity and fuel dryness, leading to longer, more active fire seasons.

The European Union’s Copernicus Climate Change Service identified significant fire seasons across the world in 2023, from Australia to Bolivia, Algeria, Thailand, North America, Russia, and Chile. 

Canada experienced record-breaking wildfire carbon emissions as climate change more than doubled the likelihood of extreme fire weather in Eastern Canada, while Greece battled the largest wildfire in European Union history. Between 1984 and 2020, the annual area burned by wildfires in the western United States surged by over 300%, with some regions seeing a staggering 1,000% increase. 

Globally, the length of the fire season has lengthened by nearly 20% between 1979 and 2013. 

“The biggest thing will be to slow climate change, stop burning fossil fuels and figure out how we’re going to better manage wetlands so we have reduced wildfire exposures,” said Casey. “Of course, that’s quite complicated.”

Wildfire smoke can impact air quality thousands of miles away. In 2023, Canadian wildfire smoke reached Europe and turned New York City skies orange. Smoke from Algerian wildfires made the reverse journey across the Atlantic, reaching Latin America. 

Experts recommend several strategies to reduce exposure to wildfire smoke, including staying indoors on poor air quality days, using high-efficiency air filters, and wearing N95 or KN95 masks when outdoors during smoke events. However, these measures may not be equally accessible to all populations.

“This is a global problem. Wildfire smoke can travel across oceans and impact everybody,” said Narayan. “But like in everything in life, poor people are always disproportionately impacted.”