New research shows that pollution could be linked to psychiatric disorders and poorer mental health.
The study, published in PLOS Biology, found a link between psychiatric disorders and exposure to environmental pollution.
The researchers looked at data from both the USA and Denmark. Using data from 1.4 million individuals in Denmark and from 151 million health insurance claims in the USA, researchers from the University of Chicago estimated individual exposure to air pollution.
They found that poor air quality was associated with higher rates of bipolar disorder and major depression in both populations. In Denmark, the trend appeared even stronger, with exposure to polluted air in the first ten years of a person's life leading to a two-fold increase in schizophrenia and personality disorders.
The researchers suggest that, although there are various genetic factors contributing to mental illnesses like schizophrenia, environmental factors such as pollution could also be contributing to the onset, severity and progression of the illnesses.
"We hypothesised that pollutants might affect our brains through neuro-inflammatory pathways that have also been shown to cause depression-like signs in animal studies," said leader of the study, Andrey Rzhetsky.
He made clear that although significant associations between air pollution and psychiatric disorders were found in the study, it doesn't necessarily mean that the disorders were caused by air pollution.
Dr Daniel Maughan, associate registrar for sustainability at the Royal College of Psychiatrists, said that the College would: "welcome more research to help increase our understanding of how toxins in the air interact with the brain and how that could affect brain functioning."
The findings were considered so controversial by reviewers that PLOS Biology commissioned a special companion article from Professor John Ioannidis of Stanford University who suggested that the data from the USA and Denmark used for the study was limited.
"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," he said. "More analyses by multiple investigators, including contrarians, are necessary.
Similarly, Dr Ioannis Bakolis at King's College London argued that one study cannot individually prove a link between mental health conditions and pollution.
"The problem is that people who live in more polluted areas tend to be different from those who live in less polluted areas in many ways not caused by the pollution, and it could be that one or more of these other differences is the real cause of differences in rates of psychiatric disorders." These could include higher population density, reduced access to green spaces and poverty. "Because of these issues about what causes what, no one study of this kind can, on its own, establish a causal link between a pollutant and its mental health consequences."
This study was published in PLOS Biology.