A new study shows that people who live with air pollution have higher rates of depression and suicide, giving even more credence to the World Health Organization’s declaration of dirty air as a “silent public health emergency.”

“We’ve shown that air pollution could be causing substantial harm to our mental health, making the case for cleaning up the air we breathe even more urgent,” Isobel Braithwate of University College London (UCL), one of the researchers who found the correlation, told The Guardian.

The particle pollution the researchers analyzed comes from burning fossil fuels for domestic, transportation, and commercial uses.

While the link between depression and air pollution isn’t rock-solid, meeting low pollution limits like those settled on by the EU could make a big difference. “You could prevent about 15 percent of depression, assuming there is a causal relationship,” Braithwaite said. “It would be a very large impact, because depression is a very common disease and it is increasing.”

Braithwaite explained that particulates from dirty air can reach the brain both through the respiratory system and the bloodstream. Air pollution has been linked to increased inflammation, damage to nerve cells, and changes in the production of stress hormones, all of which have been linked to poor mental health.

“The evidence is highly suggestive that air pollution itself increases the risk of adverse mental health outcomes,” said Joseph Hayes of UCL, a member of the research team.

The research, published in the journal Environmental Health Perspectives, pooled research data from 16 countries published up to 2017. The results revealed a strong link between air pollution and depression and suicide. Other, more recent research shows that toxic air is linked with “extremely high mortality” in people with mental illnesses, and with a 400% increase in the risk of depression in teenagers.

More than 90 percent of the world’s population lives with pollution levels above those recommended by the World Health Organization. “This is something everyone is exposed to, so at the population level it is potentially concerning,” Braithwaite said.

But the research team is wary of saying that air pollution is the direct cause of depression and suicide. There’s a strong correlation, but research that would prove causality is difficult because it would involve deliberately exposing people to harm, which ethical experiments cannot do. Although the study took into account many factors that could affect mental health, including income, education, smoking, and employment, it couldn’t separate the potential impact of noise. And, of course, every city in the world has some background level of noise, which is known to have harmful effects on mental health.

If further research were done to confirm the effects of air pollution on mental health, it could give policymakers one more reason to act to reduce pollution.

“We all need to do what we can to reduce our own contribution to air pollution,” Braithwaite said. “But we also need to be thinking about system change, meaning policies that help reduce overall air pollution levels.”

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