Small increases in people's exposure to air pollution are linked to significant rises in depression and anxiety, according to the first such study on the adults of UK.
The researchers found that an incremental increase in nitrogen dioxide, largely produced by diesel vehicles, heightened the risk of common mental disorders by 39%. For tiny particle pollution, which comes from burning fuels, and brake and tyre dust, the risk rose by 18%, reports The Guardian.
The scientists also found that people living in places with higher levels of particle pollution were twice as likely to experience mental health problems as those in the least polluted areas. The researchers acknowledged that other factors were important for mental health, such as genetics and childhood experiences, but added that, unlike these, air pollution could be prevented.
The study followed more than 1,000 adults in south-east London over five years, but the results are relevant for cities and towns across the world. In the UK, almost every urban area has particle pollution levels above the World Health Organization guidelines, and around the globe 90% of people breathe dirty air.
"Air pollution is not the only factor that may have an impact on the presence of mental disorders, but it is a preventable one," said Dr Ioannis Bakolis, a lecturer at King's College London who led the research. "Introducing measures to reduce air pollution may represent a rare and potentially impactful primary health measure for the prevention of psychiatric disorders."
Dr Ian Mudway, a lecturer at Imperial College London and part of the research team, said rates of mental health disorders were known to be higher in urban areas, with suggested causes including deprivation and lack of green space. "But the amazing thing with this study is that when you control for all of those other features of the urban environment and socioeconomic factors, the air pollution signal remains. It's a very robust finding," he added.
The World Bank has estimated that air pollution costs the global economy $5tn (£3.8tn) a year but this includes only the well-known damages caused to heart and lungs by dirty air.
"There is now an increasing body of evidence suggesting air pollution isn't great for the brain either," Mudway said. "We may actually have been underplaying the adverse effects of air pollution on our health from cradle to grave."
Recent research has linked dirty air to increased suicides and indicated that growing up in polluted places increases the risk of mental disorders. Other research found that air pollution causes a "huge" reduction in intelligence and is linked to dementia. A global review in 2019 concluded that air pollution may be damaging every organ in the human body.
The Bakolis-led research is published in the journal Social Psychiatry and Psychiatric Epidemiology, and used standard questionnaires to assess the mental health of people in the London boroughs of Southwark and Lambeth over five years. It used data on individuals to take into account factors including age, socioeconomic status, neighbourhood deprivation, and frequency of drinking, smoking and exercise.
The research improved on previous work by estimating air pollution levels every 20 metres across the area, meaning homes on busy roads could be distinguished from those on back streets. The study also took account of noise levels, which can be hard to separate from air pollution because traffic influences both.
The average NO2 levels in the boroughs varied from 24 to 83 micrograms per cubic metre (µg/m³), but the study found a rise in exposure of just 3 units increased the risk of common mental disorders by 39%. Small particle pollution ranged from 9 to 23 µg/m³ and a 5-unit increase led to the risk of mental health problems rising by 18%.
Epidemiological studies cannot prove a causal link between air pollution and mental health problems and scientists cannot experiment on people. But a broad range of psychiatric conditions have been linked to inflammation in the body , including the brain, and recent work has shown tiny air pollution particles lodged in the brain are associated with molecular damage.
The latest study is important, and accounted for a long list of other factors that may affect mental health, said Prof Antonio Gasparini, at the London School of Hygiene and Tropical Medicine. But he said the broad definition of mental health used mad it difficult to assess potential causal links. "More studies are needed before offering [health] recommendations," he added.
Prof Anna Hansell, at the University of Leicester, said the research was interesting and carefully conducted. She also said more studies using different statistical techniques were needed to confirm the findings.
"However, there are already urgent grounds to reduce air pollution due to its association with chronic disease and premature deaths," Hansell said. She
added that regular exercise, even in polluted places, benefits mental and physical health.
"People should act, not only worry," said Bakolis. The evidence of air pollution's harm was now strong enough that people should ask their elected representatives for more action, he added.