London: People exposed to higher levels of air pollution are more likely to experience depression or commit suicide, researchers have found.
The systematic review and meta-analysis of evidence connecting air pollution and a range of mental health problems, published in the journal Environmental Health Perspectives, reviewed study data from 16 countries.
"Our findings correspond with other studies that have come out this year, with further evidence in young people and in other mental health conditions," said study's senior author, Joseph Hayes from the University College London.
"While we cannot yet say that this relationship is causal, the evidence is highly suggestive that air pollution itself increases the risk of adverse mental health outcomes," Hayes said.
The World Health Organization guidelines recommend that fine particulate matter pollution - small airborne particles that can include dust and soot - should be kept under 10µg/m3.
For the findings, the research team searched for studies that had investigated the association between particulate matter pollution and five different adverse mental health outcomes in adults.
They identified 25 studies that fitted their criteria, nine of which were included in the primary analyses.
Five studies looking at long-term particulate matter exposure and depression were included in one meta-analysis.
By pooling the results, they found that a 10µg/m3 (microgram per metre cubed) increase in the average level of fine particulate matter (PM2.5) air pollution people were exposed to over long periods was associated with an approximately 10 per cent increase in their odds of depression.
"We found quite consistent results across the studies we reviewed that analysed the relationship between long-term air pollution exposure and depression, even after adjustment for many other factors which could explain the association," said the study's lead author, Isobel Braithwaite from University College London.
"The association seems to be similar in magnitude to those that have been found for some physical health impacts of particulate matter, such as all-cause mortality," Braithwaite added.
Global city PM2.5 levels range from 114 and 97 in Delhi and Dhaka to 6 in Ottawa and Wellington.
In UK cities, the average particulate matter level that people are exposed to is 12.8µg/m3.
The researchers estimate that lowering average air pollution levels to the WHO recommended a limit of 10µg/m3 could reduce urban UK residents' depression risk by roughly 2.5 per cent.
The researchers also found evidence of a connection between short-term changes in coarse particulate air pollution (PM10) exposure and the number of suicides, from pooling the results of four different studies in a meta-analysis.
The risk of suicide appears to be measurably higher on days when PM10 levels have been high over a three-day period than after less polluted periods.
The researchers said that the evidence was particularly strong for the suicide risk link, but the effect was smaller than for depression (an increase in suicide risk of two per cent for each 10µg/m3 increase in the average coarse particulate pollution level over a three-day period).
"We know that the finest particulates from dirty air can reach the brain via both the bloodstream and the nose, and air pollution has been implicated in increased neuroinflammation, damage to nerve cells and to changes in stress hormone production, which have been linked to poor mental health," Braithwaite said.