“Because it involves inefficient combustion of wood, leaves and soil, wildfire smoke contains just an enormous number of chemicals. In many ways, breathing wildfire smoke is similar to smoking unfiltered cigarettes,” says Paul Wennberg, atmospheric chemist at California Institute of Technology.
One thing you are likely to breathe in with both noxious urban air and wildfire smoke is nitrogen dioxide (NO2), a harmful gas that can also react with other compounds in the air to produce secondary pollutants, such as ozone. Then, there are the fine particles found in pollution and smoke: larger ones, called PM10, and finer ones, PM2.5. All of these compounds have been found to negatively affect mental health.
A study published this year in JAMA showed, for example, that across the United States, the more people are exposed to ozone, the higher their risk of developing depression. A British study, meanwhile, showed that people who routinely breathe air with PM2.5 levels of at least 10.6 micrograms per cubic meter have 15 percent higher risk of depression than those who live in areas with less than 9.3 micrograms of that pollutant per cubic meter. To put these numbers into perspective, during the recent Canadian wildfire haze, the air in New York City on June 7 had PM2.5 levels of 196 micrograms per cubic meter.
“It is possible, even likely, that there is a dose-response, with longer exposure to air pollution increasing the chances of depression. However, even acute, short-term exposure to air pollution may be detrimental,” says John Ioannidis, epidemiologist at Stanford University.
As for anxiety, a recent study in China found that young people living in areas with the highest fine particle pollution have a 29 percent higher risk of anxiety than those residing in the cleanest locations. By comparison, according to the World Health Organization, the first year of the pandemic sent anxiety levels up worldwide by 25 percent.
Meanwhile, research conducted recently in California showed that ozone increases the odds of bipolar disorder, self-harm and suicide, while a Danish study linked high levels of NO2 in the air with schizophrenia.
Mental health, behavioral associations
Even less severe mental troubles seem to surge as air quality plummets. Preschool-aged children, for instance, tend to behave worse: They may be quicker to break rules or act aggressively, according to one study. Your adult peers may be harder to deal with, too. A 2020 meta analysis of many studies found that air pollution goes hand in hand with unethical behavior, such as dishonesty and cheating on tests.
Is it safe to exercise outdoors when the air quality is bad?
Granted, such studies often rely on associations rather than show a proven impact, and it’s hard to verify that air pollution is the actual cause of all these mental health troubles. Typically, scientists take data on air pollution from various areas and match it with the number of local residents who develop depression or anxiety. While they try to control for a number of variables that could skew the results — such as the fact that poverty often means both living in polluted areas and a higher risk of depression — they can’t control for every single possible factor that might blur the picture.
“It would be unethical to run a randomized control trial. We cannot randomize people to breathing polluted air and to breathing healthier air. So we are mainly relying on observational evidence on this,” says Ioannis Bakolis, an epidemiologist at King’s College London.
Yet there are good reasons to believe that it is in fact dirty air that causes mental health issues. For one, Bakolis says, “we have plausible biological mechanisms that have been tested in experimental animal studies.” Two such mechanisms include inflammation and oxidative stress, damage to cells and DNA by molecules called free radicals. According to David Eisenman, public health researcher at UCLA, “living in a smoke-filled environment effects brain chemistry.”
Air pollutants reach your brain either through the lungs, where they may get picked up by the blood and carried across the blood-brain barrier, or even more directly through the olfactory epithelium, the tissue inside the nose that helps you smell. Once there, such molecules can inflict serious damage. Studies on rats reveal, for example, that oxidative stress caused by ozone kills off brain neurons that normally produce dopamine, a neurotransmitter involved in motivation and how we respond to rewards, and often dubbed the “pleasure molecule.” (Animal studies do not often reproduce the effect in humans.) Other research shows that certain cells in the brain may attack pollutants, setting off an immune response of the kind that has been linked to depression.
Pollution’s effect on the body’s stress response
Air pollution also seems to affect the stress response. When Canadian researchers made lab rats breathe in ozone, they noticed that it not only flooded the animals’ bodies with stress hormones, but even changed the expression of stress genes in their brains — basically, flipped the stress genes on, like a light switch. Rats are not humans, of course, but a 2022 study suggests that dirty air changes how humans react to stress, too. Several dozen men living in Berlin had to perform complicated math calculations in front of a panel of very critical judges (it was all done to induce stress), while other researchers performed functional magnetic resonance imaging on their brains. The scientists found that brains of those men who live in areas more polluted with PM2.5 particles activate differently in response to stress as compared with brains of people who are lucky to reside in cleaner parts of the city.
Of course, air pollution, and wildfire haze in particular, can affect our mental well-being also on a purely intellectual level: It’s simply hard to feel cheerful when you know that the planet is burning. Psychologists talk of climate anxiety or something they call solastalgia — distress over seeing the natural environment negatively transformed. Already as many as 69 percent of Americans are worried about the climate. The rate of those who describe themselves as “very” worried stands at 29 percent.
The good news, experts say, is that there are some ways to protect yourself from the brain-damaging effects of air pollution, especially if it’s only temporarily elevated by wildfire smoke.
“Stay indoors if possible and use air filters,” Wennberg says, adding that well-fitting N95 or KN95 masks should “filter out most of the PM2.5.” The bad news is that some climate change models predict that PM2.5 pollution levels caused by wildfires may close to double by 2100.
Experts predict that a warming world will bring more wildfires, more orange-hued skies enveloping our cities and, likely, more mental health problems. That is why Wennberg says that if we want to reduce mental health burdens of wildfire air pollution we should focus our efforts on one goal: “Above all, fight climate change.”
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