People who live in highly-polluted areas may be more likely to battle depression or die by suicide, research suggests.
Scientists from University College London analysed nine studies that looked at the link between exposure to particulate matter (PM) and mental-health issues.
PM gets released from vehicle emissions and floats unseen in the atmosphere. When smaller than 2.5μm (PM2.5) - 400th of a millimetre - the particles are known to get “lodged” in the lungs.
The scientists found that for every 10 microgram increase in PM2.5 per metre cubed, the risk of depression rose by 10%.
They also looked at “coarser” PM of 10μm across, with suicides rising on days when the pollutant was higher.
PM2.5 comes about from the burning of coal and other fossil fuels for “electricity, transport and household heating”, according to the 2019 Lancet Countdown on health and climate change report.
On a global scale, exposure to PM2.5 is “the largest environmental risk factor for premature mortality”.
Inhaling the microscopic substances is said to have caused 2.9 million people worldwide to die too soon from cardiovascular or respiratory diseases in 2016 alone.
Air pollution’s impact on mental health, however, was less clear.
In the UK alone, nearly one in five (19.7%) of people aged 16 or over showed signs of depression or anxiety in 2014, according to the Mental Health Foundation.
Last year, 6,507 people died by suicide, Government statistics show.
After analysing studies from 16 countries, the scientists found a link between depression and PM2.5.
“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,” lead author Dr Isobel Braithwaite said.
“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.”
PM2.5 levels vary around the world, peaking at 114µg/m³ in Delhi, and as low as 6µg/m³ in Ottawa and Wellington.
UK cities tend to average at 12.8µg/m³, the scientists wrote in the journal Environmental Health Perspectives.
They estimate lowering levels to the World Health Organization’s recommended limit of 10µg/m³ would reduce a city dweller’s risk of depression by around 2.5%.
While it is unclear if air pollution directly impacts mental health, the scientists claim it is plausible.
“We know the finest particulates from dirty air can reach the brain via both the bloodstream and the nose,” Dr Braithwaite said.
“And air pollution has been implicated in increased neuroinflammation, damage to nerve cells and changes in stress hormone production, which have been linked to poor mental health.”
Study author Dr Joseph Hayes added: “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.
“A lot of what we can do to reduce air pollution can also benefit our mental health in other ways, such as enabling people to cycle or walk rather than drive - and enhancing access to parks - so this adds support to the promotion of active travel and urban green spaces.”
The scientists acknowledge none of the studies looked at green spaces, like parks, while many ignored the impact of noise on mental health.
They took into account other factors that may influence the risk of depression, like income, however, not all were considered.
“Why might a city with similar demographics have higher levels of PM2.5?,” Dr Katharina Janke, from Lancaster University, asked.
“Maybe it has a badly functioning public transport system, leading to a stressful commuting experience for residents. Such stressful experience might affect mental health.”
The scientists hope to look into the impact of indoor air pollution, such as from candles, on mental health.
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