Parts of California, Oregon and Montana are struggling with unhealthy levels of air pollution as a result of several large fires, including the Park Fire in Northern California, which has quickly become the fifth largest fire in state history.
The smoke can be dangerous for people in the immediate vicinity, but also for people who live far away. The smoke from a forest fire can travel hundreds or even thousands of kilometers.
The tiny particles in that smoke aren’t just bad for your lungs. A series of studies over the past few years have shown that they also negatively impact your brain health, increasing your risk of dementia, cognitive problems and mental health issues.
“A lot of research on wildfire smoke has historically focused on our lungs and our hearts,” says Stephanie Cleland, an assistant professor at Simon Fraser University who studies the health effects of wildfire smoke. “A shift to focus on cognitive outcomes and brain health is more recent.”
The latest addition to this 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 medical records of more than 1.2 million seniors aged 60 and older in Southern California from 2009 to 2019.
The researchers looked at where people lived to determine their exposure to particulate matter from bushfire smoke and other forms of pollution. Their results showed that the risk of dementia increased by 21% for every additional microgram per cubic meter of particulate matter from smoke that participants were exposed to over an average of three years.
In contrast, the same increase in exposure to particulate matter from other sources, such as cars or factories, was associated with a 3% increase in the risk of developing dementia.
The evidence is preliminary, but it does show that chronic exposure to wildfire smoke contributes to cognitive decline, said Dr. Holly Elser, a study author and neurologist in training 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 unknown.”
A study published in August also found that higher exposure to particulate matter from various sources, including wildfires, was linked to a higher risk of dementia.
Wildfires are becoming more frequent and intense due to climate change, which also increases many people’s exposure to smoke. The frequency of extreme wildfires has more than doubled from 2003 to 2023, a recent study found.
Scientists believe that the reason that wildfire smoke affects the brain is that the small 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. That in turn can affect people’s ability to think, learn or remember.
Dementia isn’t the only possible consequence. A 2022 study found that adults who had recently been exposed to wildfire smoke performed worse on a brain-training game that measures skills like memory, attention, flexibility, processing speed and problem solving.
“Within hours and days of exposure to wildfire smoke, we see significant declines in people’s ability to concentrate,” said Cleland, one of the study’s authors.
Other research published the same year found that students performed worse on tests when exposed to wildfire smoke during the school year, compared to a year without smoke.
“The more smoke you get, the worse you do on tests,” said Marshall Burke, an associate professor of global environmental policy at Stanford University who co-authored that study. “The effect on an individual student is probably quite small, but when you add it up across students and schools, it adds up to quite large total learning losses.”
Burke said he has some doubts about the dementia findings released this week because wildfire smoke and other forms of pollution are “not an apples-to-apples comparison.”
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 people who were already at risk.
“It’s a really intriguing question whether this is creating new cases of dementia that otherwise would never have occurred, or whether it’s simply accelerating the onset of clinically apparent dementia,” she said.
Other questions that remain unanswered concern the connection between wildfire smoke and mental health. A February study found that exposure to wildfire smoke in the western U.S. was linked to more emergency room visits for anxiety. (Air pollution in general has been linked to depression and psychotic disorders such as schizophrenia.)
Elser said it’s possible that wildfire smoke changes the neurochemistry in people’s brains, which can cause depression and anxiety. But it’s also possible that the anxiety and stress of experiencing or living through a wildfire could independently lead to mental health problems.
Other health effects of wildfire smoke are much better understood. Scientists have known for decades that when fine particles from smoke are inhaled, they can travel deep into the lungs or 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 U.S. who need to worry about these risks. This was evident, for example, in the wildfires in Canada last summer that ravaged parts of the Midwest and Northeast.
“This past summer, our conversation about who gets exposed to wildfire smoke completely changed,” she said. “Oregon, California, Washington, and British Columbia are really affected by wildfire smoke, but that doesn’t mean places like the Northeast or Ontario are immune.”
To limit exposure to wildfire smoke, experts advise people in areas with an air quality index of 100 or higher to avoid outdoor activities, close all windows, use home air filters, and wear an N95 mask if they must go outside.
This article was originally published on NBCNews.com