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Exposure to air pollution is linked to a greater risk of depression and suicide, the first overview of studies on the subject has found.

While the effects on the body from breathing dirty air are well-established, an emerging body of research has suggested pollution may also affect mental health.

Isobel Braithwaite at University College London and her colleagues looked in detail at 25 studies published up to late 2017, for a meta-analysis on the links between the two.

They found that someone living for at least six months in an area with twice the World Health Organization’s recommended limit for fine particulate matter, PM2.5, would have roughly a 10 per cent increased risk of developing depression as a person living in an area that met the limit.

The WHO’s guideline is that PM2.5s shouldn’t exceed This is the average level in the UK. In London the average figure is and in Delhi��.

Mental health concerns

For suicide, an association was found with short-term exposure to a slightly larger type of pollution, PM10. Each 10μg/m3 increase in PM10 a person was exposed to during a three-day period was linked to a 2 per cent greater risk of suicide.

Braithwaite says teasing out the links between pollution and mental health is important because there is still a long way to go on reducing air pollution. She notes the UK hasn’t yet adopted the WHO guidelines on PM2.5, though .

“Knowing it not only affects physical health but it could also be affecting our mental health, which is something that does affect large numbers of people, I think adds to the weight of the argument for cleaner air and policies that achieve it,” says Braithwaite.

Unknown mechanisms

She adds that research on mental health and air pollution is decades behind research into its effects on physical health, but within five to 10 years there could be a clearer picture on whether dirty air is directly causing mental health conditions. “In my view we cannot tell with any certainty yet,” says Ioannis Bakolis at King’s College London, who wasn’t involved in the research.

He adds that one interesting finding is that the link between PM2.5 and depression is similar for people across the world and for different study designs.

The exact mechanisms for how pollution could be affecting our brains aren’t certain, but there is evidence that tiny particulate matter can enter our blood and reach the brain. Air pollution is also known to affect inflammation, which is thought to be implicated in depression, and there is some evidence exposure could affect stress hormones, too.

However, disentangling other factors – particularly the role of noise and living near green spaces – makes drawing links between air pollution and mental health difficult. Braithwaite’s team didn’t find any associations between pollution and psychosis or bipolar disorder, and only limited evidence of links to anxiety.

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