As air quality declines, the prevalence of mental health conditions may rise, a large, new study suggests.

Looking at data on millions of people in the United States and Denmark, researchers found correlations between air pollution exposure and rates of certain psychiatric disorders. In both countries, poorer air quality was linked to a slightly heightened risk of bipolar disorder.

No conclusive answers

And in Denmark – where childhood data was available – people who were exposed to polluted air in the first 10 years of life had increased risks of depression, schizophrenia and personality disorders.

The big caveat: None of that proves air pollution is to blame.

“These are only correlations,” said Dr John Ioannidis, a professor of medicine at Stanford University in California. “Inherently, data like these will never give you conclusive answers.”

Ioannidis wrote an editorial published with the study in the journal PLOS Biology.

At this point, he said, the findings are an “interesting observation” that can spur more research.

“If there really is a link, that would be very important,” Ioannidis said.

Brain inflammation

Andrey Rzhetsky, senior researcher on the study, agreed that the findings point to an association – and are not proof of cause and effect.

But animal research has offered clues about how polluted air might affect mental health, according to Rzhetsky, a professor in the departments of medicine and human genetics at the University of Chicago.

It’s been shown, for example, that fine particles in air pollution can travel to the rodent brain via the nose. And lab animals exposed to air pollutants have shown signs of brain inflammation, along with impaired learning and memory, as well as depression-like behaviour.

Ioannidis, however, cautioned against making too much of the animal research.

“You can find biology to explain whatever you want,” he said. But that doesn’t prove it’s actually happening in humans.

For the current study, Rzhetsky’s team mined data on two large populations to look for links between pollution and psychiatric conditions.

Stronger associations in Denmark

For the US portion, the investigators looked at health insurance claims for more than 151 million Americans – noting mental health diagnoses made between 2003 and 2013. They had no way to gauge people’s personal exposure to air pollution. But they did have information on air quality in each person’s county of residence – for the years 2000 to 2005.

The researchers broke those counties into seven groups (or septiles) based on air pollution. Overall, the prevalence of bipolar disorder was about 29% higher in the most-polluted group of counties, compared to the group with the cleanest air.

The Denmark data, meanwhile, showed stronger associations. There, Rzhetsky’s team had enough information to estimate childhood air-pollution exposure for nearly 1.5 million people born between 1979 and 2002.

The investigators found that people exposed to air pollution during their first 10 years of life had increased risks of developing bipolar disorder, major depression, schizophrenia or a personality disorder by age 37.

Again, the biggest difference was between the septile living in the most polluted areas, versus those breathing the cleanest air: Their rates of schizophrenia and personality disorders were more than doubled, while their rate of depression was 50% higher and bipolar disorder 29% higher.

Overall, the Danish data provided a “stronger signal”, Rzhetsky said. But does that mean that childhood exposure to air pollution during early brain development is more important than adulthood exposure?

At this point, it’s unclear, Rzhetsky said.

Many other exposures

The findings leave many unanswered questions, according to Ioannidis.

“Is it childhood exposure to air pollution that matters?” Ioannidis said. “Is it bad, no matter when you’re exposed? Or does it make no difference at all?”

That final question touches on the broader issue. Psychiatric disorders are complex, Ioannidis pointed out: They involve a mix of genetic vulnerability and a range of environmental influences.

And people living in the most polluted areas may have many other exposures that set them apart from people who breathe the cleanest air, Ioannidis said.

It’s not possible, of course, to conduct a trial where people are assigned to live in polluted areas. But, Rzhetsky said, there are other ways to approach the cause-and-effect question: For example, studies might look at whether the prevalence of mental health diagnoses goes down after air quality improves.

It will be important to figure out whether air pollution is, in fact, a contributor, Rzhetsky said, because air quality is something we can change.

Ioannidis made another point: There are already plenty of reasons to want cleaner air, since poor air quality is strongly linked to heart and lung disease, and to higher mortality.

“Air pollution is not good for us,” Ioannidis said. “We know that.”

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