Air pollution caused more than 5.5 million premature deaths in 2013, with more than half of those occurring in China and India, according to a new study, and pollution mortalities are expected to continue rising.

Researchers found that poor air quality in China and India — the world's fastest growing economies — killed 1.6 million and 1.4 million people in those nations, respectively. Emissions from power plants, automobiles, industrial manufacturing, and inefficient heating systems and stoves have become a leading cause of death and disease worldwide, even as air pollution has decreased in high-income countries.

"Air pollution ranks fourth globally as a risk factor for death in the world," Michael Brauer, a public health researcher at the University of British Columbia, said. "It's one of the big ones."

Brauer worked with researchers from China, India, and the United States as part of the Global Burden of Diseases project to estimate health risk factors worldwide. The team found air pollution contributes to heart disease, stroke, lung cancer, and other maladies "on the same level" as leading killers like smoking, high blood pressure, and poor diet. The team presented their research at the annual meeting of the American Association for the Advancement of Science.

Vehicles and cyclists cross a main intersection in Beijing, China on December 22, 2015. (Photo by Wu Hong/EPA)

In the United States, the White House is wrestling with a federal appeals court and state governments over the implementation of air pollution standards designed to kick the country's coal habit. But in the large developing economies of Asia, coal remains a major source of power — and pollution.

Qiao Ma, a PhD student at Tsinghua University and member of the Global Burden of Diseases team, found burning coal to be the largest contributor to air pollution in China. In 2013, Ma estimates, coal pollution alone caused 366,000 deaths. And, even if the country meets its present emissions reduction targets, she projects the number of premature deaths caused by coal pollution will reach between 990,000 and 1.3 million by 2030.

"Our study highlights the urgent need for even more aggressive strategies to reduce emissions from coal and from other sectors," Ma said.

Beijing, home to Ma's Tsinghua University, went on "red alert" for poor air quality in December, days after the landmark United Nations climate agreement was reached in Paris. The city's heavy smog often exceeds the levels of pollution the World Health Organization deems safe.

New Delhi, the most polluted big city in the world according to the World Health Organization, and Beijing have introduced vehicle restrictions in an effort to cut down on air pollution. And China has pledged to drastically cut back on coal burning in coming years in an effort to reign in its greenhouse gas emissions.

But the Global Burden of Diseases project found low-tech combustion is a major driver of dangerous pollution in India. In the poor, rural parts of the country, millions of families burn wood, dung,, and other substances to cook and heat their homes. An estimated 920,000 premature deaths resulted from household pollution created by these burning fuels, while outdoor pollution caused another 590,000.

The high death tolls from pollution are part of the puzzle developing countries face as they try to lift their populations out of poverty, even as efforts to fight climate change discourage the use of the dirty energy sources that drove the Industrial Revolution.

Along with India, Brauer pointed to Bangladesh and Pakistan as countries with dangerously high pollution levels. This, he said, is part of a common pattern that sees pollution rise and eventually peak as countries develop. The United States, Western Europe, and Japan began the regulation process that starts leveling off pollution in the 1970s, and Brauer suggested China is now beginning the same process.

Naga women in northeastern India sell coal and locally made coal stoves on a cold winter day. (Photo via EPA)

A major breakthrough in international climate negotiations over the past several years is the establishment of the Green Climate Fund, which, if sufficiently funded, could provide $100 billion a year in clean energy investments by 2020.

Brauer called the Green Climate Fund a "watershed" and said that offering this type of support is part of the responsibility of richer countries. He particularly noted that the program could help countries cut back on easily avoidable pollution, like that from burning wood for cooking and heating, which usually goes neglected because it primarily affects poor, rural populations.