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According to a new recent study, as air quality declines, the prevalence of mental health problems may increase.
By examining data on millions of people in the US and Denmark, researchers found correlations between exposure to air pollution and rates of some psychiatric disorders. In both countries, poorer air quality was associated with a slightly increased risk of bipolar disorder.
No conclusive answers
And in Denmark – where child data were available – people exposed to polluted air during the first 10 years of life were at increased risk of depression, schizophrenia and personality disorders.
The big warning: None of this proves that air pollution is to blame.
"These are just correlations," said Dr. John Ioannidis, a professor of medicine at Stanford University in California. "Inherently, data like this 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 results are an "interesting sighting" that can spur more research.
"If there is really a link, it will be very important," said Ioannidis.
Inflammation of the brain
Andrey Rzhetsky, principal investigator of the study, agreed that the findings suggest an association – and do not constitute proof of cause and effect.
But animal research has provided clues as to how polluted air could affect mental health, according to Rzhetsky, a professor in the Departments of Medicine and Human Genetics at the University of Chicago.
It has been shown, for example, that fine particles of air pollution can travel to rodent brains through the nose. And laboratory animals exposed to air pollutants have shown signs of brain inflammation, impaired learning and memory, as well as behavior similar to depression.
Ioannidis, however, cautioned against doing too much research on animals.
"You can find biology to explain everything you want," he said. But that does not prove that it actually happens in humans.
For the current study, Rzhetsky's team exploited data on two large populations to look for links between pollution and psychiatric disorders.
Stronger associations in Denmark
For the US side, investigators looked at health insurance claims from more than 151 million Americans, noting the mental health diagnoses made between 2003 and 2013. They had no way to measure personal exposure of people to atmospheric pollution. But they had information on the air quality in the country of residence of each person – for the years 2000 to 2005.
The researchers divided these counties into seven (or septic) groups based on air pollution. Overall, the prevalence of bipolar disorder was about 29% higher in the group of more polluted counties than in the group with the purest air.
Danish data, on the other hand, show stronger associations. The Rzhetski team had enough information to estimate children's exposure to air pollution for nearly 1.5 million people born between 1979 and 2002.
Investigators found that people exposed to air pollution during their first 10 years of life were at increased risk of developing bipolar disorder, major depression, schizophrenia, or personality disorder before they became ill. 37 years old.
Again, the most important difference was in septiles living in the most polluted areas compared to those who breathed the purest air: their rates of schizophrenia and personality disorders had more than doubled, while that their depression rate was 50% higher and bipolar disorder higher than 29%. .
Overall, Danish data provided a "stronger signal," said Rzhetsky. But does this mean that children's exposure to air pollution during early brain development is more important than exposure to adulthood?
At this point, it's not clear, said Rzhetsky.
Many other exhibitions
The results leave many questions unanswered, according to Ioannidis.
"Is the exposure of children to air pollution important?" Ioannidis said. "Is it bad, no matter when you're exposed, or does it make no difference?"
This last question concerns a larger issue. Psychiatric disorders are complex, said Ioannidis: they involve a mixture of genetic vulnerability and various environmental influences.
And people living in the most polluted areas can undergo many other exposures that distinguish them from people who breathe the purest air, said Ioannidis.
Of course, it is not possible to conduct a lawsuit where people are assigned to live in polluted areas. But, said Rzhetsky, there are other ways to address the issue of cause and effect: for example, studies might examine whether the prevalence of mental health diagnoses decreases after the improvement of the quality of the air.
It will be important to determine if air pollution is actually a contributing factor, Rzhetsky said, because air quality is one thing we can change.
Ioannidis added another point: there are already many reasons to want cleaner air, because poor air quality is closely linked to heart and lung diseases and higher mortality.
"Air pollution is not good for us," said Ioannidis. "We know that."
Image credit: iStock
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