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We know dirty air is bad for our bodies, causing the equivalent of millions of deaths worldwide each year, making it a bigger killer than smoking. But could air pollution be bad for our minds too?

A study has found psychotic experiences, which can involve hearing or seeing something that others don’t, are more common among teenagers in the UK’s most polluted areas.

However, the association doesn’t mean that breathing in air pollution leads to psychosis in teenagers, because there could be other explanations. The study doesn’t show causation, says Helen Fisher of King’s College London, one of the study’s authors.

Fisher and her colleagues found that 30 per cent of a group of 2000 18-year-olds reported having at least one psychotic experience in their teens – other research on young adults has reported similar figures. However, when the teenagers’ addresses were mapped against air pollution, those in areas with higher pollution were more likely to have reported a psychotic experience.

In areas with the highest levels of nitrogen oxides (NOx) – pollutants produced by diesel cars – 12 teens reported psychotic experiences for every 20 teens who didn’t, with the number falling to seven for every 20 in cleaner areas.

It isn’t known how air pollution could be linked to psychotic experiences. One speculative mechanism put forward by the team is of a cumulative build-up of pollutants directly influencing the brain.

Studies have linked air pollution with inflammation and degeneration in the frontal cortex and the part of the brain that gives us our sense of smell, the olfactory bulb. Inflammation of the brain .

A simpler explanation could be that it is not the dirty air itself, but the noise from the cars emitting pollution. Noise pollution can increase stress and disrupt sleep, two factors associated with psychotic experiences.

Cutting through the smog

“There is no evidence that pollution necessarily causes psychosis, or whether this is one of many factors or acting in isolation,” says Sophie Dix at mental health charity MQ. But the study provides a starting point for a possible link between pollution and psychosis, she says.

There is an emerging body of research looking at possible links between air pollution and mental health. For example,  has been found between polluted areas and Swedish children being given medication for psychiatric disorders.

There is a compelling case for further investigation into toxic air’s potential connection to mental health, says Stefan Reis at the Centre for Ecology and Hydrology in the UK.

But there is limited evidence so far, says Fisher. “It’s really preliminary. We don’t know very much.”

Fortunately, the world doesn’t need to wait until any link is proven between air pollution and mental health to act on what we pump into the air, because we know so much about air pollution’s harmful physical impacts. Tiny particulate matter can penetrate deep into the lungs and bloodstream, , including infections and aggravating asthma. It has also been associated with low weight and premature babies.

“There’s a huge weight of evidence on the physical effects,” says Fisher.