Air pollution poses risk to mental health

Air pollution is linked to an increased risk of developing psychiatric conditions such as bipolar disorder, major depression, schizophrenia and personality disorders, a new study finds.

The study showed that regions in the United States with the greatest levels of air pollution were associated with a 27% increase in bipolar disorder, as well as a 6% increase in major depression, compared to counties with the worst levels of air quality. 

This study falls in line with similar work in China, South Korea and London. All demonstrated a similar link between air quality and mental health.

“There are a few known risk factors for psychiatric disorders, but pollution is a new direction,” wrote Atif Khan, computational biologist at the University of Chicago and lead author of the study, in an email.

The study was carried out after the researchers realized that “genes could only tell us part of the story” of how psychiatric disorders develop, says Andrey Rzhetsky, co-author of the study and geneticist at the University of Chicago.

The researchers used large datasets from the U.S. and Denmark to generate their findings, published in PLOS Biology. They compared insurance claims from 151 million Americans between the years 2003 and 2013 against the average levels of air pollution for each county. Insurance claims included individuals with bipolar disorder, major depression, schizophrenia and personality disorders. They also included neurological disorders, such as Parkinson’s and epilepsy. 

The researchers found correlations between air pollution and psychiatric disorders, even after accounting for variables such as sex, age, socio-economic background and whether the participants lived in an urban environment. 

“[This work] augments the idea that there is a ‘signal,’” an actual link, between pollution and mental health, says Ioannis Bakolis, biostatistician at King’s College London who was not involved in the study. 

Yet, he also stresses caution: “It’s hard to say if there’s an established causal link.” 

Since counties in the U.S. can be very large, and air quality within counties can vary significantly, the dataset makes for a crude measure of individual exposure to pollution, Bakolis says. Rzhetsky adds that relying on insurance claims automatically excludes poorer individuals who cannot afford health insurance.

To validate their results, the U.S. team contacted researchers at Aarhus University to see if they could obtain similar findings in Denmark. 

“Denmark is much smaller, and has a rich dataset where we could include everyone,” says Oleguer Plana-Ripoll, postdoctoral researcher at Aarhus University, who was in charge of the Danish dataset. Largely for this reason Plana-Ripoll believes “Denmark is able to answer questions that no-one else can.” 

For decades, Denmark has recorded detailed information about its citizens in data sets available to researchers. The Danish team used these registers to look at health data from 1.4 million Danes born between 1979 and 2002 for the first 10 years of their lives. They then computed the accumulated exposure of Danish citizens by tracking their address and comparing it to data on air quality with a precision of 1 square km.

The study found that Danish citizens exposed to the lowest air quality had a 51% higher rate of major depression compared to those living in the highest air quality. Likewise, rates of bipolar disorder, schizophrenia and personality disorder were 29%, 148% and 162% higher, respectively, in areas with the lowest air quality. Exact numbers comparing how many more people suffered from psychiatric conditions with worsened pollution levels were not available. 

To explain their findings, the researchers point to several animal experiments showing how exposure to pollutants may result in inflammation and damage to brain cells. 

But Bakolis says there is still “no strong biological basis” for the link between pollution and psychiatric disorders. 

Future experiments will work on identifying the suspected routes by which pollution may reach and damage the brain, Khan says. He adds that more experiments need to be conducted to see if the link between air pollution and psychiatric conditions truly is causal.

Plana-Ripoll is optimistic about how his research may influence policy change in Denmark. “It’s moving in the right direction,” he says.