The fact that pollution is behind 1 in 8 deaths is “remarkable,” said Lalit Dandona, director of the India State Level Disease Burden Initiative, which conducted the study. “We’ve always thought of it as high, but to see it like that is quite a massive impact on health.”

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The study found that 77 percent of the country’s population is exposed to levels of harmful particulate matter that exceed the standard set by the Indian government. That threshold is already four times the limit recommended by the World Health Organization.

Air pollution in India is a complex phenomenon with sources that include car exhaust, industrial emissions, construction dust and the burning of crop residues. The use of wood, charcoal and dried dung for fuel and heating also creates harmful pollution within rural homes.

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The phenomenon is most intense in northern India and the country’s capital, New Delhi, during the winter months. That’s when temperatures drop and wind speeds fall, trapping pollutants and creating a regionwide haze.

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The study examined diseases for which it found firm evidence of causation by air pollution. Those included lower respiratory infections, chronic inflammatory lung disease, heart attacks, strokes, diabetes and lung cancer. It estimated that 1.24 million deaths last year — or 12.5 percent of the total — could be attributed to air pollution.

Just how deadly India’s air pollution has become is a subject of debate among scientists. A study published in August found that Indians on average lost 1.53 years of life expectancy because of ambient “PM2.5” pollution, the particulate matter considered most harmful to human health. Such particles measure less than 2.5 micrometers in diameter and can lodge deep within the lungs. That research also found three countries where the impact of air pollution on life expectancy was worse than in India: Bangladesh, Egypt and Pakistan.

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Another piece of research last month estimated that pollution had an even more dramatic impact on life expectancy in India, cutting 5.3 years from the average person’s life span.

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Dandona, the lead author of the Lancet study published Thursday, said that he saw a very thin silver lining in the otherwise dire figures. Indian policymakers have shown strong interest in the results, he said, which represents a marked shift from even a year ago, when such discussions of the health impact of air pollution were laced with skepticism.

“Now a much larger proportion of the public and policymakers are convinced that something should be done,” Dandona said. “Often you need data to push you forward, and we hope that this will be one of those things.”