Exposure to air pollution is linked to an increased severity of mental illness. At least, according to the results of a study involving 13,000 people in London, England.
The study found that a relatively small increase in nitrogen dioxide exposure led to a 32% increase in the risk of needing community-based care and an 18% increase in the risk of hospitalization.
The researchers say the findings may apply to most cities in developed countries, and thus reducing air pollution could benefit millions of people.
"Air pollution can be modified, and on a large scale as well, reduce population-level exposure," said Joanne Newbury, of the University of Bristol.
"We know there are interventions that can be used, such as expanding low-emission zones. Mental health interventions at the individual level are actually quite difficult," he added.
This study used the frequency of visits to the hospital or to community doctors and nurses as a measure of severity. The researchers calculated that a small reduction in just one pollutant could reduce disease and save the UK's National Health Service (NHS) budget by tens of millions per year.
Air pollution levels in London have fallen in recent years but there is no safe level, said Ioannis Bakolis, of King's College London, who led the research.
'Even at low levels of air pollution, you can observe this kind of very important effect,' he said.
Recent research has shown that small increases in air pollution are associated with significant increases in depression and anxiety.
The data also links dirty air with increased suicide attempts and suggests that growing up in polluted air increases the risk of mental disorders. Other studies have found that air pollution causes a significant decline in intelligence and is linked to dementia. A global review in 2019 even concluded that air pollution can damage every organ in the human body.
The study, published in the British Journal of Psychiatry, tracked patients in south London from their first contact with mental health services and used high-resolution estimates of air pollution in their homes.
Quarterly average NO2 levels in the study areas varied between 18-96 micrograms per cubic meter (µg/m³). The researchers found that people exposed to pollution levels of 15µg/m³ or higher had an 18% higher risk of hospitalization and a 32% higher risk of requiring outpatient care after one year.
NO2 is mostly emitted by diesel vehicles, but some comes from small particulate pollution produced by burning fossil fuels. The levels of small particles varied from 9 to 25 g/m³ and this increased exposure increased the risk of hospital admission by 11% and the risk of outpatient care by 7%.
The scientists reassessed the patient data seven years after the first treatment, and found the association with air pollution was still clear. This finding was not explained by various other possible factors including age, sex, ethnicity, deprivation or population density, although unidentified factors may still play an important role.
"Identifying modifiable risk factors for disease severity and relapse could inform early intervention efforts and reduce the human suffering and high economic costs caused by long-term chronic mental illness," the researchers said.
This study was not designed to prove a causal relationship between air pollution and the severity of mental illness because it required difficult experimental work. But according to the researchers, the association makes biological sense as air pollutants are known to have strong inflammatory properties and inflammation is believed to be a factor in psychotic and mood disorders.
