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The study, conducted on 13,000 patients in south London, England, since they began using mental health services, found the link between air pollution and the severity of mental illness was most pronounced with NO2 – a pollutant largely emitted from diesel vehicles.
In the study area, the average quarterly NO2 concentrations ranged from 18-96 μg/m³. The research team found that those exposed to pollution concentrations greater than 15 μg/m³ had an 18% higher risk of hospitalization and a 32% higher risk of requiring outpatient treatment after one year.
The research team re-evaluated the patient data over seven years and found that the association with air pollution remained very clear. These findings cannot be explained by factors such as age, gender, ethnicity, poor living conditions, or population density, although unknown factors may still play a significant role.
Although this study did not focus on proving a causal link between air pollution and the severity of mental illness, the research team suggested that this link is "biologically plausible," as air pollutants have strong inflammatory properties, and inflammation is thought to be a factor in mental disorders.
Several recent studies have also indicated that increased air pollution has a significant impact on the rise of depression and anxiety. Furthermore, dirty air is linked to an increase in suicide rates; growing up in polluted areas increases the risk of mental disorders. Other studies have found that air pollution causes a sharp decline in cognitive function and is associated with dementia. A 2019 global review concluded that air pollution can harm all organs in the human body.
According to the research team, these findings can be applied to most cities in developed countries, and reducing air pollution could benefit millions of people.
