Parts of California, Oregon and Montana are facing unhealthy levels of air pollution as several large fires burn, including the Park Fire in Northern California, which quickly became the fifth largest in state history.
The smoke poses a hazard to people in the immediate vicinity and even some far away, as wildfire smoke can travel hundreds or thousands of miles.
The tiny particles in that smoke aren’t just bad for your lungs – a range of studies over the last few years have found that it also has a negative impact on brain health, raising the risk of dementia, cognitive challenges and mental health issues.
“A lot of the research on wildfire smoke has historically focused on our lungs and our hearts,” said Stephanie Cleland, an assistant professor at Simon Fraser University who studies the health impacts of wildfire smoke. “There is a more recent shift in focus on cognitive outcomes and brain health.”
The latest addition to this body of evidence: Research presented Monday at the Alzheimer’s Association International Conference suggests that wildfire smoke may increase the chance of a dementia diagnosis.
The study analyzed the health records of more than 1.2 million older adults — age 60 and older — in Southern California from 2009 to 2019.
The researchers looked at where people lived to determine their exposure to fine particulate matter from wildfires and other forms of pollution. Their results showed that the chance of developing dementia increased by 21% for every additional microgram per cubic meter of fine particulate matter from smoke that participants were exposed to, on average, over three years.
In contrast, the same increase in exposure to fine particulate matter from other sources — such as cars or factories — was linked to a 3% increase in the risk of dementia.
The evidence is preliminary, but it makes the case that chronic exposure to wildfire smoke contributes to cognitive decline, said Dr. Holly Elser, study author and neurology resident at the Hospital of the University of Pennsylvania.
However, she said, “The threshold at which wildfire smoke starts to pose a risk for dementia is not known.”
A study published in August similarly found that higher exposure to fine particulate matter from a variety of sources, including wildfires, was associated with higher rates of dementia.
Wildfires are becoming more frequent and more intense due to climate change – which also increases many people’s exposure to smoke. The frequency of extreme wildfires doubled from 2003 to 2023, a recent study found.
Scientists think that the reason wildfire smoke affects the brain is that the tiny particles within the barrier between the bloodstream and the brain can cross, causing inflammation in the central nervous system. The particles can also travel directly to the brain through the nose. This, in turn, could affect people’s ability to think, learn or remember.
Dementia is not the only potential consequence. A 2022 study found that adults who had recently been exposed to secondhand smoke performed worse in a brain training game that measured skills such as memory, attention, flexibility, processing speed and problem solving.
“Within hours and days of exposure to wildfire smoke, we’re seeing significant decreases in people’s ability to pay attention,” said Cleland, one of the study’s authors.
Other research published the same year showed that exposure to wildfire smoke during the school year lowered students’ test scores compared to a smoke-free year.
“The more smoke you get, the worse you do on the tests,” said Marshall Burke, an associate professor of global environmental policy at Stanford University, who co-authored that study. “The impact on any individual student is probably small, but if you add up across students and add up across schools, those add up to pretty big total aggregate learning losses.”
Burke said he has some skepticism about the dementia findings released this week, however, because wildfire smoke and other forms of pollution are not “comparing apples to apples.”
Elser acknowledged that many questions remain regarding the effects of smoke on the brain. It is not clear, for example, whether smoke triggers dementia in healthy people or just in those who were already at risk.
“It’s an interesting question, is this creating new cases of dementia that never happened before, or is it just accelerating the onset of clinically apparent dementia,” she said.
Other ongoing questions relate to the link between wildfire smoke and mental health. A February study found that exposure to wildfire smoke in the western US was associated with increased emergency department visits due to anxiety. (Air pollution in general has been shown to be associated with depression and psychological disorders such as schizophrenia.)
Elser said it’s possible that wildfire smoke changes the neurochemistry in people’s brains, which can drive depression and anxiety. But it is also possible that the anxiety and stress associated with experiencing or living through a wildfire can lead to mental health challenges independently.
The other health effects of wildfire smoke are much better understood. Scientists have known for years that when fine particles from smoke are inhaled, they can travel deep into the lungs or enter the bloodstream, increasing the risk of asthma, stroke, heart attack, lung cancer and low birth weight in newborns.
Cleland said it’s not just people in fire-prone areas of the United States who need to worry about these risks, as evidenced by last summer’s Canadian wildfires that sent smoke into parts of the Midwest and Northeast.
“Last summer completely changed our conversation about who is exposed to wildfire smoke,” she said. “Oregon, California, Washington, British Columbia really get a lot of wildfire smoke, but that doesn’t mean places like the Northeast US or Ontario are immune.”
To reduce exposure to wildfire smoke, experts recommend that people in areas with an Air Quality Index reading of 100 or higher avoid outdoor activities, close all windows, run indoor air filters and wear an N95 mask if they must go outside.
This article was originally published on NBCNews.com