Avenida Paseo de la Reforma in smoggy Mexico City. [Credit: Carlos Aranda | Unsplash]
My first job after college was smack in the middle of traffic-choked Mexico City. The work was stressful, and when I needed to calm down, I would step outside onto the fire escape and take some deep breaths. It probably wasn’t a great idea. Little did I know I was making things worse.
It turns out, inhaling polluted air not only stresses your lungs and heart. It can damage your mental health, too.
The science on the topic is relatively recent, says Isobel Braithwaite, a public health researcher at University College London. “People tend to think much more about the heart and the lungs. Sometimes the brain and the way we feel doesn’t get as much attention or research interest,” she says, adding that this is finally changing.
While scientists haven’t worked out all of the ways that inhaling pollutants such as carbon monoxide and dust particles can trigger mental health problems, the cascade of recent research has strengthened the hypothesized links. Scientists are detecting pollution particles in brain tissue and tracing how they got there.
There are several routes inhaled pollutants can take to reach our brains, explains Jonas Miller, a psychologist at the University of Connecticut who studies environmental influences on children’s mental health. Some particles cross into the bloodstream through the lungs and circulate throughout our bodies, ultimately penetrating our brains if they are small enough to pass through the blood-brain barrier. Pollutants can also reach the brain through the nose’s olfactory tract, which includes the nerve responsible for transmitting smell information to the brain.
Researchers know pollutants reach our brains because they have found nanoparticles there that could have only been formed by high-temperature combustion – the kind that occurs in engines and boilers, not the human body, Miller explains. “They most likely originated from cars and industry.”
Once they reach the brain, air pollutants can trigger inflammation, which may explain why studies have found that the autopsied brains of people who lived in polluted cities are more inflamed.
That’s important because neuroinflammation is associated with mental health problems. Some research shows that the brains of people struggling with depression are often in a more inflamed state, which makes sense because inflammation is one of the body’s most important protective responses to injury or sickness. Inflammation could turn out to be the biological mechanism behind some depressive behaviors, like sadness and isolation, Miller says.
There is another way breathing polluted air can influence how you feel: by increasing your levels of stress hormones, which regulate the body’s “flight or fight” response in the face of danger.
When your body perceives a threat, your hypothalamus, a structure deep in your brain, releases a hormone that signals the adrenal glands at the top of your kidneys to release two other hormones – first adrenaline, and then cortisol – into the bloodstream. The rush of cortisol then increases your glucose levels, giving you the extra energy you need to fight or flee. The shot of adrenaline, meanwhile, accelerates your heartbeat and breathing so you can run.
When inhaled pollutants cross into our bloodstream, they can activate this cascade of stress responses. “The lungs send a signal to the brain to say, ‘Hey, we need to respond to the stressor,’ which leads to that release of stress hormones, explains Errol Thomson, a toxicologist at Canada’s national health department.
Stress hormones are essential for keeping us alive, but releasing them too often can cause health problems. “There is mounting evidence that chronic or sustained activation of this thing, particularly in contexts absent of threat, can take a toll on your body,” Miller says.
Thomson agrees. “If you are chronically stressed, you’re more likely to get sick, you’re more likely to put on weight, you might be more likely to have a heart attack, right?” he says. “You’re also more likely to be depressed.”
Stress hormones are also responsible for letting your body know when a threat has passed, and it does not need an inflammation response anymore. But they may be less able to send that inflammation-dampening signal if they are activated too often, making it much more difficult for the brain to get back to normal after being inflamed – and an inflamed brain might be more likely to be a depressed brain. Some studies also have found an association between long-term air pollution exposure and anxiety symptoms, even when measuring anxiety with different instruments.
There is still a lot we don’t know about the links between mental health and pollution. Like many other health issues associated with unhealthy air, brain inflammation and chronic stress tend to emerge gradually in people who are exposed to pollutants over extended periods. This time lag makes the health consequences harder to detect and also more difficult to study.
“You can do the short-term exposure [studies], exposing people to a bit of air pollution and monitoring short-term changes and maybe cortisol levels,” Braithwaite explains. But depression and other effects of long-term, chronic exposures can’t be replicated in a laboratory experiment. Instead, she says, researchers look for ways to improve the air people are already exposed to and then see if their mental health improves. In China, for example, researchers installed air purifiers in college dormitories to evaluate how cleaner air affected the students’ brains. It turned out that higher exposure to one specific pollutant (particulate matter) activated the brain area that regulates the body’s stress response. The purifiers led to a reduction in stress hormones.
Researchers hope these studies will spur new efforts to curb air pollution. “[Air quality] seems to me something that we have to decide as a group that this is important to us, and we’re going to do something about it,” Thomson says.
Paul Villeneuve, an epidemiologist at Carleton University, is optimistic. “If you see the air pollutants levels in the ’90s or ’80s, you will see we have come a long way,” he says. “I think we can keep moving forward.”