Psychotic experiences - such as hearing voices or extreme paranoia - are more common in teenagers living in cities with high pollution than those in rural areas, a study of young people in England and Wales suggests.
Scientists say that their research could provide possible clues about why children in urban areas are more likely to get psychotic disorders later on.
But they caution that much more work is needed to be certain of the link.
The study appears in JAMA Psychiatry.
Scientists from King's College London tracked some 2,000 teenagers living in urban, semi-urban and rural areas.
Almost a third (623) reported they had been through at least one psychotic experience between the ages of 12 and 18 - for example feeling like people were spying on them or hearing voices no-one else could.
'Noisy city life'
Researchers matched their responses to detailed estimates of the air pollution each adolescent faced over a year.
- In areas with the highest levels of pollutant gases (for example inner-city areas near busy roads), there were 12 teenagers who reported psychotic experiences for every 20 who did not
- In areas with lower levels of nitrogen oxide gases, there were only seven who reported psychotic experiences for every 20 who said they had not had them
Scientists say the findings remained firm even when they factored in other issues that can contribute to psychotic episodes - including a family history of mental health issues, social deprivation and alcohol and drugs.
Lead researcher, Dr Joanne Newbury, from the Institute of Psychiatry, Psychology and Neuroscience, said: "We found that adolescent psychotic experiences were more common in urban areas."
She added that while the study could not show pollutants caused the episodes to take place, "our findings suggest air pollution could be a contributing factor in the link between city living and psychotic experiences".
How could air pollution be connected to mental health?
Researchers say that while the study does not prove that air pollution causes psychotic experiences, it adds to growing evidence that air pollution may have wider effects on the body beyond the heart and lungs.
They speculate that tiny particles of air pollution could get past the lungs and into the blood and then travel on to the brain. There they could trigger inflammation and contribute to poor mental health.
Another theory is that the chemicals coating these particles could dissolve in the blood and be carried to the brain and again lead to inflammation.
The research team say that noise pollution could partially explain the link too - for example noisy traffic could disturb sleep and add to the stress of city life. But they were unable to measure this directly.
Although it is not always possible, experts suggest trying to use side roads and keeping away from the busiest roads when traffic is at its heaviest.
How sure can they be of the link?
Scientists estimated four outdoor pollutants; the gases nitrogen dioxide, nitrogen oxides, and particulate matter - PM2.5 and PM10 - at home addresses and two places the teenagers frequently spent time at - for example schools, work and shops.
And while the researchers say they took other factors like alcohol and drugs into account, they acknowledge it is impossible to be sure they factored in everything.
Dr Daniel Maughan, at the Royal College of Psychiatrists, said: "While the paper has not proved air pollution causes psychosis, the findings are concerning as they suggest that increased psychosis rates in urban areas are potentially linked to air pollution.
"We need a radical approach to air pollution as it is very likely damaging the mental health of young and older people alike."
Meanwhile Prof Andreas Meyer-Lindenberg, at the University of Heidelberg, said: "While this is a careful study of a large cohort, it should be borne in mind that researchers did not actually measure, but modelled, air quality and also could not know when participants were where.
"Also, most evidence points to the relevance of early childhood in psychosis risk, a period the study did not cover.
"While the authors looked at many potential factors that could influence air quality and psychosis risk, such as neighbourhood socio-economic status, the findings should be replicated in a setting where pollution and location are directly measured before firm conclusions can be drawn."
Psychosis or psychotic experiences?
Researchers say they looked specifically at adolescents, because their developing brains are more vulnerable to psychotic experiences. They consider these to be a milder form of the experiences that people have, who go on to develop disorders like schizophrenia in adulthood.
Teenagers who have them are more likely - though certainly not guaranteed - to develop psychotic disorders in later life. So scientists say identifying them at this stage could provide an opportunity to intervene ahead of time.
But they reassure parents that long-term psychotic disorders are rare and most teenagers who go through these experiences do not go on to develop them.
The NHS says that traumatic experiences, stress, alcohol and drug abuse can all trigger episodes of psychosis.
Researchers say anyone who is concerned about an adolescent's mental health should speak to their GP or seek urgent medical advice if the problem is immediate.