A recent study has concluded that exposure to air pollution, particularly during the first 10 years of life, could play a significant role in the development of psychiatric disorders. However, not everyone is convinced by the data.

The study, which appears in PLOS Biology, used data from the United States and Denmark to uncover the possible link between environmental pollution and psychiatric disorders. The new research found that rates of both bipolar disorder and depression were higher among those living in areas of poor air quality.

The researchers also concluded that Danish people who lived in polluted areas during their first decade of life were more than twice as likely to have personality disorders and schizophrenia.

With mental health in the spotlight, researchers are keen to understand the factors that influence whether or not someone develops psychiatric illness. There are a multitude of potential causes, including genetics as well as life experiences, so it is not possible to exclude environmental factors. In this new study, the team looked more closely at how a specific environmental factor—air pollution—affects the brain and the likelihood of psychiatric disorders.

Air pollution research

To reach their conclusion, the researchers drew from two large datasets. The pollution information for the United States came from the Environmental Protection Agency's (EPA) air quality measurements, while for Denmark, the researchers looked at the national pollution register.

The EPA tracks 87 different air quality measurements. Although the Danish pollution register monitors fewer measurements, they have a higher spatial resolution.

The team then looked at healthcare data. For the United States, they accessed a health insurance database that included claims that more than 151 million individuals made between 2003 and 2013. For Denmark, they used data for all of the residents who were born in the country between 1979 and 2002 and were living in Denmark on their 10th birthday.

Denmark assigns each person a unique identifying number that links information from national registries. This information enabled the researchers to estimate air pollution exposure during the first decade of life. However, the researchers were not able to be quite so specific with the US dataset, as they were limited to the county level.

According to the authors, the findings showed that air pollution did have links to various psychiatric disorders. Using Denmark's more specific records, the researchers were able to pinpoint that the developing brain during a person's first 10 years of life might be a bit more prone to the effects of air pollution.

"We hypothesized that pollutants might affect our brains through neuroinflammatory pathways that have also been shown to cause depression-like signs in animal studies," says Andrey Rzhetsky, of the University of Chicago, IL, who led the study.

Computational biologist Atif Khan, who is the first author of this study, comments on the findings. He says, "The physical environment—in particular air quality—warrants more research to better understand how our environment is contributing to neurological and psychiatric disorders. 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."

A dose of skepticism

Although the results are interesting, the study does have significant limitations and has caused much debate, as Rzhetsky himself explains. He says, "This study on psychiatric disorders is counterintuitive and generated considerable resistance from reviewers."

In fact, there was so much division that the journal decided to publish a companion article alongside the research paper. Professor John Ioannidis, a scientist who assisted in the journal's editorial process but who is not connected with the original study, is the author.

In the article, he picks apart the data. Among other criticisms, he explains how "results from the US data offer mostly coarse, exploratory hints. Associations may be entirely spurious or, conversely, important associations may be missed because of these deficiencies." Ioannidis eventually concludes that a "causal association of air pollution with mental [conditions] is an intriguing possibility."

"Despite analyses involving large datasets," he adds, "the available evidence has substantial shortcomings and a long series of potential biases may invalidate the observed associations. More analyses by multiple investigators, including contrarians, are necessary."

In conclusion, the theory that pollution impacts mental health will require a great deal more evidence before mainstream scientists begin to take it seriously.