Parts of California, Oregon and Montana are facing unhealthy levels of air pollution after several major wildfires, including the Northern California Park Fire, which has become the fifth largest in the states’ history.
Because wildfire smoke can travel hundreds or even thousands of miles, the smoke poses a danger to people in the immediate area and even far away.
The tiny particles in smoke aren’t just bad for your lungs; a number of studies over the past few years have found that it also negatively impacts your brain health, increasing the risk of dementia, cognitive difficulties and mental health problems.
“Most of the research on wildfire smoke has historically focused on our lungs and hearts,” says Stephanie Cleland, an assistant professor at Simon Fraser University who studies the health effects of wildfire smoke. “The shift to focusing on cognitive outcomes and brain health is more recent.”
The latest addition to this body of evidence is research presented Monday at the Alzheimer’s Association International Conference suggesting that wildfire smoke may increase the likelihood of a dementia diagnosis.
The study analyzed the health records of more than 1.2 million seniors aged 60 and older living in Southern California between 2009 and 2019.
The researchers looked at where people lived to determine their exposure to fine particulate matter from wildfire smoke and other types of pollution. Their results showed that for every additional microgram/cubic metre of fine particulate matter from smoke that participants were exposed to over an average of three years, the likelihood of developing dementia increased by 21%.
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 likelihood of developing dementia.
Dr. Holly Elser, an author of the study and a neurology resident at the Hospital of the University of Pennsylvania, said the evidence is preliminary but suggests that chronic exposure to wildfire smoke contributes to cognitive decline.
However, “the threshold at which wildfire smoke begins to pose a risk for dementia is unknown,” he said.
A similar study published in August found that greater 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 intense due to climate change — increasing many people’s exposure to smoke. A recent study found that the frequency of extreme wildfires more than doubled from 2003 to 2023.
Scientists think that the reason wildfire smoke affects the brain is because tiny particles in it can cross the barrier between the bloodstream and the brain, causing inflammation in the central nervous system. The particles can also travel directly to the brain through the nose, which can affect people’s ability to think, learn or remember.
Dementia isn’t the only possible outcome. A 2022 study found that adults who had been recently exposed to wildfire smoke performed worse on a brain-training game that measured skills like memory, attention, flexibility, processing speed, and problem-solving.
“We see significant reductions in people’s ability to pay attention hours and days after exposure to wildfire smoke,” said Cleland, one of the study’s authors.
Another study published the same year showed that students exposed to wildfire smoke during the school year had lower test scores compared to a smoke-free year.
“The more smoke you take in, the worse you do on tests,” said Marshall Burke, an associate professor of global environmental policy at Stanford University and co-author of the study. “The impact on any one student is probably pretty small, but if you add it up across students and schools, it adds up to pretty big aggregate learning losses overall.”
Burke said he has some doubts about the dementia findings announced this week because wildfire smoke and other types of pollution “can’t be compared apples to apples.”
Elser acknowledged that many questions remain about the effects of smoke on the brain. For example, it is unclear whether smoke causes dementia in healthy people or only in those at risk.
“Whether this creates new cases of dementia that would never have occurred or whether it accelerates the onset of clinically apparent dementia is a really interesting question,” he said.
Other ongoing questions concern the link between wildfire smoke and mental health. A February study found that exposure to wildfire smoke in the western U.S. was associated with increased emergency room visits for anxiety. (Air pollution in general has been linked to depression and psychotic disorders such as schizophrenia.)
Wildfire smoke can change the neurochemistry in people’s brains, which can lead to depression and anxiety, Elser said. But it’s also possible that the anxiety and stress of experiencing or being in a wildfire can independently lead to mental health problems.
Other health consequences of wildfire smoke are much better understood. Scientists have known for decades that fine particles from smoke can travel deep into the lungs or enter the bloodstream when inhaled, increasing the risk of asthma, stroke, heart attack, lung cancer and low birth weight in newborns.
Cleland said those living in high-fire risk areas of the U.S. should be concerned about those risks, as evidenced by last summer’s wildfires in Canada that blanketed parts of the Midwest and Northeast in smoke.
“This past summer completely changed the conversation about who is exposed to wildfire smoke,” he said. “Oregon, California, Washington, British Columbia are really experiencing a lot of wildfire smoke, but that doesn’t mean places like the Northeastern U.S. or Ontario are immune.”
To reduce exposure to wildfire smoke, experts recommend that people living in areas with an Air Quality Index of 100 or higher avoid outdoor activities, close all windows, run indoor air filters and wear an N95 mask when they must go outside.
This article was originally published on NBCNews.com