CHICAGO — Could the very air we breathe have an impact on our mental health? That’s the suggestion coming out of a new international study conducted in the United States and Denmark. After analyzing long-term data sets from both countries, researchers from the University of Chicago say they have identified a possible link between exposure to environmental pollution, specifically polluted air, and an increase in the onset of psychiatric and mental health problems in a population.

According to the findings, air pollution is associated with increased rates of depression and bipolar disorder among both U.S. and Danish populations. That association was actually found to be even greater in Denmark, where poor air quality exposure during the first 10 years of a person’s life was found to predict a two-fold increase in the likelihood of developing schizophrenia or a personality disorder.

“Our study shows that living in polluted areas, especially early on in life, is predictive of mental disorders in both the United States and Denmark,” explains computational biologist Atif Khan, the study’s first author, in a media release. “The physical environment – in particular air quality – warrants more research to better understand how our environment is contributing to neurological and psychiatric disorders.”

Of course, it would be wholly inaccurate to suggest that air pollution alone can cause someone to develop schizophrenia. Most mental illnesses, schizophrenia included, develop due to a complex mix of genetic predispositions and unique life events or experiences. However, scientists have long theorized that purely environmental factors and conditions also play a role in the severity, time of emergence, and progression of mental disorders.

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Regarding air pollution specifically, it is believed that small particulate matter (the fine dust found in polluted air), makes its way to the brain after being breathed in through the nose and lungs. In numerous studies, animals exposed to polluted air have exhibited cognitive impairment and depressive symptoms.

For the study, researchers analyzed two population data sets. The first was a U.S. health insurance claims database housing 11 years worth of claims across 151 million people. The second data set included all 1.4 million people born in Denmark between 1979-2002 who were still alive, and still living in Denmark by their 10th birthday.

Air pollution levels in specific areas were measured using the air quality standards set by both countries, respectively. For example, for the U.S. the EPA’s air quality measurements were used. As far as estimating each person’s exposure to polluted air, it was a bit easier for researchers to track individual Danes because they had access to each Danish participants’ citizen ID number. For Americans, air pollution exposure estimates were limited to county areas.

“We strived to provide validation of association results in independent large datasets,” comments study leader Andrey Rzhetsky.

The study has ruffled some feathers in the scientific community and has been met with a bit more criticism than usual. In fact, PLOS Biology, the study’s publishing journal, received so much negative feedback on the study that they published a companion piece from professor John Ioannidis of Stanford University.

“A causal association of air pollution with mental diseases is an intriguing possibility. Despite analyses involving large datasets, the available evidence has substantial shortcomings and a long series of potential biases may invalidate the observed associations,” asserts Ioannidis in his companion article. “More analyses by multiple investigators, including contrarians, are necessary.”

Additionally, the study’s own authors admit that just because they observed an association between air pollution and mental disorders, that does not necessarily indicate causation. Moving forward, they would like to conduct further research on the matter, in order to determine if air pollution’s impact on the brain is similar to other stress-causing conditions or outside factors.

The study is published in PLOS Biology.

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