Worsening air pollution linked to poor mental health

By Dr. Ananya Mandal, MDAug 22 2019

According to a new large-scale study, rising air pollution can be associated with not just physical problems such as heart and lung diseases but also mental health problems such as depression, schizophrenia and bipolar disorder. The study titled “Environmental pollution is associated with increased risk of psychiatric disorders in the US and Denmark,” was published in the latest issue of the journal PLOS Biology this week.

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The study reveals that in both the United States and Denmark, rising levels of air pollution and declining air quality is associated with an average 29 percent rise in the incidence of bipolar disorders. The authors explained that there has been advanced research trying to assess the genetic risks of mental health disorders in addition to the environmental factors. They write, “Despite some advances in identifying genetic variants associated with psychiatric disorders, most variants have small individual contributions to risk. By contrast, disease risk increase appears to be less subtle for disease-predisposing environmental insults.”

The team of authors use the term “environmental insults” to describe the declining air quality breathed by these populations. For this study the team included two independent data sets from the US and Denmark. The former included 151 million individuals (151,104,811 to be exact) and the latter included 1.4 million (“1,436,702 individuals born between January 1, 1979 and December 31, 2002”). These individuals from the US had sought insurance claims and individuals from Denmark were on the databases or registers. In addition the researchers collected data from the “Environmental Protection Agency (EPA) county-level environmental quality indices (EQIs) in the US and individual-level exposure to air pollution in Denmark”.  The hypothesis for this study according to the researchers was, “pollutants affect the human brain via neuroinflammatory pathways that have also been shown to cause depression-like phenotypes in animal studies.”

The 151 million individuals from the US were studied between 2003 and 2013. The data sets, were single entry data of the individuals and were correlated with the environmental data. The Danish participants were recruited between 1979 and 2003. Their polluted air exposure was recorded till they turned 10 years of age. The impact of this exposure on mental health was studied in adults however.

Results revealed that Denmark had a strong association with air pollution and mental health problems. The results from Denmark revealed that those breathing in polluted air were 162 percent more likely to have personality disorder compared to those who were breathing the better quality air. There was a 50 percent increase in prevalence of depression among persons breathing polluted air compared to those breathing better air. It was also noted that the risk of schizophrenia rose by 148 percent among those breathing foul air compared to those breathing the best air in the Denmark data set.

The authors write, “The Denmark analysis suggests that poor air quality during the initial years of an individual’s life increases the risk of all 4 psychiatric disorders studied here (bipolar disorder, schizophrenia, personality disorder, and major depression). In the US data, we see a similar trend for bipolar disorder (and to some extent for major depression) as that in Denmark, but the signal for schizophrenia and personality disorder is absent.”

Lead author Andrey Rzhetsky, a professor of medicine and genetics at the University of Chicago, said in a statement, “The major takeaway is that the environment matters. And it’s involved in psychiatric disease.” The team explains that the brain is exposed to the pollutants in air in various ways. This could be as simple as directly inhaled via the nose or could be indirectly reaching the lungs via the blood stream. Either way, the authors write, the pollutant chemicals reach the brain and also cause neuroinflammation. Rzhetsky said, “There are several ways in which pollutants can enter the brain. Once they are in the brain, they cause abnormal processes, such as inflammation.”

Results also showed that as the Earth warmed, air pollution tends to get worse with increased formation of smog. The researchers found that persons of colour as well as those belonging to lower income groups in the US lived near to regions where pollution was greater. These individuals also had poorer access to healthcare and thus were also at a greater risk of poor mental health.

According to Rzhetsky and colleagues, this study could be a warning bell so that governments and researchers focus on the environmental impacts on psychiatric health and take necessary measures to improve air quality.

Authors concluded, “...we observed a strong positive association between exposure to environmental pollution and an increase of prevalence in psychiatric disorders in affected patients...However, these strong associations do not necessarily mean causation; further research will be needed to assess whether air pollution’s neuroinflammatory impacts share common pathways with other stress-induced conditions.”

This study was funded by the NordForsk project 75007 titled, “Understanding the Link Between Air Pollution and Distribution of Related Health Impacts and Welfare in the Nordic countries (NordicWelfAir)” and the DARPA Big Mechanism program and National Institutes of Health grants.

John Ioannidis, professor of medicine and health research and policy at Stanford University, in an accompanying article with the study in PLOS Biology said that the US data set was not representative of the whole population. He said n a statement, “I see this study as opening a new avenue of research. The current data need to be seen with great caution, nothing is proven yet regarding a cause-and-effect relationship between air pollution and mental disease. But the observation cannot be ignored, it needs to be pursued further and examined with additional data.” “This research could have policy implications (e.g. regulation of air pollution) and it could change the direction of where we search for mental health causes. Most mental health research is focusing and spending research money on biology and neuroscience rather than the environment and social/societal factors,” he added.