Researchers at the University of California, Santa Barbara, estimate that China's efforts to reduce air pollution have prevented 46,000 suicide deaths in the country over just five years. The team used weather conditions to tease apart confounding factors affecting pollution and suicide rates, arriving at what they consider to be a truly causal connection.

The results, published in the journal Nature Sustainability, unearth air quality as a key factor influencing mental health. Issues like air pollution are often framed as a physical health problem leading to a spectrum of acute and chronic illnesses such as asthma, cardiovascular disease and lung cancer, the researchers said.

The research team had previously studied the effect of temperature on suicide rates in India, finding that excessive heat drives those rates up. The team was curious to notice that the suicide rate in China dropped far faster than its decline in the rest of the world.

In 2000, the country's per-capita suicide rate was higher than global average but two decades later it has fallen below that average, which itself is declining, the researchers said. At the same time, air pollution levels were plummeting, they said.

"It's very clear that the war on pollution in the last seven to eight years has led to unprecedented declines in pollution at a speed that we really have not seen anywhere else," said study co-lead author Tamma Carleton, an assistant professor at UC Santa Barbara.

Carleton and co-lead author Peng Zhang, a former UC Santa Barbara doctoral student, teamed up with researchers in Xanghai and Beijing to examine the effects of China's recent crackdown on air pollution on suicide rates across the country. They gathered demographic data from 2013 through 2017 from the Chinese Center for Disease Control and Prevention and meteorological data from the China Meteorological Data Service Center.

The team compared suicide numbers across 600 counties between weeks with inversions and those with more typical weather, running the data through a statistical model. "Suicide rates increase substantially when air pollution rises," the researchers found.

The effect was particularly strong for elderly people, with older women 2.5 times more vulnerable than other groups. The researchers are not certain why older women are especially vulnerable to this effect, though it may be partly cultural.

Previous research suggests that most suicides by women in China are driven by acute crises. If pollution has an acute effect on mental health, it could disproportionately impact older women, they said. The phenomenon does appear to happen relatively quickly. Rates increase within the first week of exposure, and then abruptly decline once conditions improve, according to the researchers.

This suggests that pollution may have a direct neurologic effect, rather than creating chronic health issues that drive suicide rates up later on, they said. Carleton hopes the findings can reframe how society approaches suicide prevention.

"Public policy about air pollution -- something you can't control, what's outside your window -- is affecting the likelihood that you take your own life. And I think that puts a different lens on the solutions we should be thinking about," she added.