People exposed to high levels of air pollution are more likely to experience depression or commit suicide, according to a study that reviewed data from 16 countries. Published in the journal Environmental Health Perspectives, this is the first systematic review of evidence connecting air pollution and a range of mental health problems.
Researchers from the University College London (UCL) in the UK found that, if the relationship with depression reported in some of these studies is causal, then reducing global average exposure to fine particulate matter (PM2.5) from 44 microgrammes per cubic metre (?g/m3) to 25 ?g/m3 could result in a 15 per cent reduction in depression risk worldwide.
The World Health Organization (WHO) guidelines recommend that fine particulate matter pollution -- small airborne particles that can include dust and soot -- should be kept under 10 ?g/m3, the researchers said.
"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," said the study's lead author, Isobel Braithwaite from UCL.
"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," said Braithwaite.
The researchers analysed 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.
The team found that a 10 g/m3 increase in the average level of PM2.5 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," Braithwaite said.
Global city PM2.5 levels range from 114 and 97 in Delhi and Dhaka (Bangladesh), to 6 in Ottawa (Canada) and Wellington (New Zealand), the researchers said.
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.
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. The studies into short-term changes in suicide risk accounted for confounding factors such as weather changes, and day of the week.
The relationship is not affected by other neighbourhood or socioeconomic factors given that the comparisons being made are among the same individuals on days with different pollution levels.
The researchers said the evidence was particularly strong for the suicide risk link, but the effect was smaller than for depression -- an increase in suicide risk of 2 per cent for each 10 ?g/m3 increase in the average coarse particulate pollution level over a three-day period.
They cannot yet confirm whether air pollution directly causes mental ill health, but said there is evidence to suggest possible causal mechanisms. "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.
"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 the study's senior author, Joseph Hayes from UCL.
"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.