Mice tests showed that, in the long term, dirty air could cause actual physical changes to the brain which resulted in negative effects, according to the Telegraph.

While other studies have looked at the impact of polluted air on heart and lungs, this is one of the first to look at the effect on the brain, said doctoral student Laura Fonken from the Ohio State University who led the study.

"The results suggest that prolonged exposure to polluted air can have visible, negative effects on the brain which can lead to a variety of healthproblems," she said, reports the journal Molecular Psychiatry.

"This could have important and troubling implications for people who live and work in polluted urban areas around the world," Fonken added.

Fonken and her colleagues at Ohio State exposed mice to either filtered air or polluted air six hours a day, five days a week for almost half their lifespan which was 10 months.

After 10 months of exposure, behavioural tests were carried out on the rodents, including a learning and memory test.

After five days of training they were placed on a brightly lit area and given two minutes to find the dark escape hole where they would be more comfortable.

The mice which breathed the polluted air took longer to learn where the escape hole was and at later tests they were more likely to forget where it was.

In another experiment, mice exposed to the polluted air showed more depressive-like and higher levels of anxiety-like behaviours in one test, but not in another.