The higher one’s exposure to air pollution, the more likely one is to experience depression or die by suicide, concludes a new, global study. With data coming from 16 countries and spanning 40 years, it’s the first large-scale review of research related to air pollution and mental health.

The finding bolsters the credibility of earlier studies — smaller, less rigorous, and therefore less conclusive — that have suggested links between air pollution and psychiatric disorders, including depression. It also has serious implications for India, where hundreds of millions live in 22 of the world’s 30 most polluted cities and where mental health care is both nascent and stigmatized.

The researchers concluded that for every increase of 10 micrograms of fine particulate matter (PM2.5, the most dangerous kind of air pollution) per cubic meter (ug/m3), a year’s exposure increased the risk of depression by 10%. 12 ug/m3 is the level considered safe for daily breathing over the course of a year, per the U.S. Environmental Protection Agency. In November, Delhi’s PM2.5 count hit 475 ug/m3.

The study, published in the journal Environmental Health Perspectives, also found a relationship between air pollution and several other mental health problems, including anxiety. The researchers did not establish air pollution as a cause of depression, suicide, or any other mental health disorder, but one told The Guardian, “The evidence is highly suggestive that air pollution itself increases the risk of adverse mental health outcomes.”

If so, they estimated, then cutting the global average exposure to fine particulate matter (known as PM2.5), the most dangerous type of air pollution, by less than half could drop depression risk worldwide by 15%. However, even this reduction would still leave global average exposure to fine particulate matter well above World Health Organization recommendations.

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“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,” the study’s lead author, psychiatrist and professor Dr. Isobel Braithwaite, of University College London’s Institute of Health Informatics, said in a statement. “Here, we’re showing 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.”

Braithwaite’s study is the latest in a fast-growing mountain of evidence detailing the negative effects of air pollution on human health. The only glimmer of hope here is that efforts to curb air pollution can also reinforce better mental health, creating an urgently needed cycle of positive reinforcement for both the environment and our minds.

“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 study’s senior author, psychiatrist and professor Dr. Joseph Hayes of University College London, added in the statement.