Researchers from University College London reviewed nine studies analysing the link between emissions and mental health.

They found that being exposed to small airborne pollutants known as PM2.5 was associated with a greater chance of depression, while exposure to slightly larger particulate matter known as PM10 was linked to an increased risk of people taking their own lives.

"We already know that air pollution is bad for people's health, with numerous physical health risks ranging from heart and lung disease to stroke and a higher risk of dementia," Dr Isobel Braithwaite, of UCL Psychiatry and UCL Institute of Health Informatics and the study's lead author said.

"Here, we're showing that air pollution could be causing substantial harm to our mental health as well, making the case for cleaning up the air we breathe even more urgent."

The World Health Organisation guidelines recommend that PM2.5 should be kept under 10 micrograms per cubic metre.

The researchers found an increase of 10 micrograms per cubic metre of PM2.5 was associated with an approximately 10 per cent increase in the odds of depression.

They estimate that lowering average air pollution levels to the WHO recommended limit could reduce the risk of depression among those living in the cities by roughly 2.5 per cent.

The findings, published in Environmental Health Perspectives, also show evidence of an association between short-term changes in PM10 exposure and the number of suicides.

The scientists said risk of suicide appeared to be "measurably higher" on days when PM10 levels were high over a three-day period, with figures showing a two per cent increase in suicide risk for each 10 micrograms per cubic metre rise in the average pollution level.

However, the researchers say they cannot yet confirm whether air pollution directly causes mental ill health, but say there are "a number of biologically plausible mechanisms that may underlie such a link".

Dr Braithwaite told the PA news agency: "These (mechanisms) include the fact that exposure to air pollution increases the levels of inflammation within the brain, which has been linked with depression and other mental health problems through impacts on brain development and, potentially, through impacts on stress hormone production.

"We also can't rule out the possibility that some or all of the relationship we observed is actually due to factors associated with both local air pollution levels and depression risk but which were not fully accounted for by the individual studies, such as noise pollution or access to green space."

Meanwhile, in a separate study by the National Institute of Environmental Health Sciences in the US, scientists found PM2.5 increased a pregnant woman's risk of high blood pressure.

The researchers found that exposure to PM2.5 from traffic emissions was associated with development of hypertensive disorders in pregnant women, increasing the likelihood of developing preeclampsia - a complication that puts both mother and baby at risk - by about 50 per cent.

Lifeline 13 11 14

beyondblue 1300 22 4636

Australian Associated Press