“Going into this, my colleagues and I knew that pollution killed a lot of people. But we certainly did not have any idea of the total magnitude of the problem,” said Philip Landrigan, dean of global health at the Mount Sinai School of Medicine and co-chair of the commission behind the report. “I think all of us were really surprised when we saw this.”

The two-year project, which relied on data from researchers in more than 130 countries documenting the causes of disease and premature deaths in recent decades, found that poor air quality was the most significant pollution-related killer. That includes both outdoor pollution tainted by mercury, arsenic and other harmful particulates, and household air dirtied by the burning of wood, dung and other organic materials. The result: An estimated 6.5 million deaths in 2015 from heart disease, strokes, lung cancer and other respiratory problems.

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Water pollution, which includes everything from unsafe sanitation to contaminated drinking water, accounted for an additional 1.8 million annual deaths from gastrointestinal diseases and other infections, researchers found.

Pollution in the workplace also took a heavy toll on some of the world’s poorest workers. From bladder cancer in dye workers to the lung disease pneumoconiosis in coal miners, researchers found that occupational exposure to various carcinogens and toxins was linked to about 800,000 deaths annually.

In 2015, the largest number of deaths attributable to pollution occurred in India and China, with an estimated 2.5 million and 1.8 million deaths respectively. Other severely affected countries include Pakistan, Bangladesh and Kenya.

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Beyond the massive human toll, the authors of Thursday’s report also focused on the financial toll caused by pollution-related health problems.

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“Until now, people haven’t recognized what an incredible hit pollution makes on the economy of a country,” Landrigan said. “Pollution control can stimulate the economy because it reduces death and disease.”

They estimated the hit to national budgets at about 1.3 percent of gross domestic product in low-income countries, compared to about 0.5 percent in developed, high-income countries. In addition, nations facing crippling pollution tend to spend much more on health care to treat diseases related to the problem.

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“When you’re looking at developing countries, you really have to address this challenge if you want to move people out of poverty and into the middle class,” said Gina McCarthy, a former Environmental Protection Agency administrator who was not involved in the study but had studied its conclusions. “It is holding people back.”

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And the warming of the Earth’s climate is likely to fuel more deaths in the absence of international action, she said.

“Climate change is going to exacerbate the very problems that are identified in this article. There will be more contagious and infectious diseases. There will be more lives lost, more injuries, if we don’t identify a path that gets us out of the hole that we’re in,” McCarthy said. “What people don’t realize is the instability that results from poverty, the instability that results from migration as a result of climate change.”

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The startling conclusion that pollution accounts for 16 percent of deaths worldwide is, of course, an estimate. But the findings build on previous studies, including a 2016 report from the World Health Organization, detailing the extent to which pollution represents a public health crisis. “If countries do not take actions to make environments where people live and work healthy, millions will continue to become ill and die too young,” then-WHO Director-General Margaret Chan said last year.

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McCarthy said the Lancet study also is built on the most complete epidemiological data available.

“These are the best numbers that we have available to us. And even if they’re off by a factor of 10, you’re still talking about huge, huge impacts. But they’re not off by a factor of 10,” she said. “It’s very clear if you go to other countries and it’s clear if you go to some of our own communities that they are being held back because of the impact of pollution on their kids and their elderly. And we have to stop thinking that because we can’t see the pollution and it’s not as visible that it’s not there.”

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Landrigan said there is “an old wive’s tale” that developing countries inevitably suffer through troublesome pollution and disease on their way to becoming more prosperous.

Rather, he and the study’s other authors insisted that countries must do much to reduce pollution and improve the health of their citizens — and that they will reap economic benefits for doing so. In addition, he said developed nations can play a meaningful role in helping poorer countries slash pollution, and major nonprofit foundations that have largely steered clear of the problem must be convinced that it is a global priority.

“It doesn’t have to [get worse]. It’s not an inevitable outcome,” Landrigan said of the annual death toll. “Pollution control is a winnable battle.”