“It’s much worse in middle-income countries than ever before,” said Dr. Maria Neira, director of the Department of Public Health, Environment and Social Determinants of Health at the World Health Organization. “Fifty years ago, only a few cities had populations of more than two million. Today there are many.”

The highest numbers of deaths from outdoor air pollution are in China, India and Russia, according to the Institute of Health Metrics and Evaluation. That is in part because they have the most people. The countries with the highest mortality rates — deaths from air pollution per 100,000 total deaths — are in Eastern Europe: Bulgaria, Ukraine, Belarus, Georgia and Latvia. The causes vary. Some believe it could be related to a legacy of dirty Soviet industry and fleets of aging diesel cars.

The key ingredient in policy change is a strong desire for it on the part of the population, said Christine L. Corton, the author of “London Fog”. In England, that happened in 1952, when another heavy smog episode — this time from coal-burning fireplaces and cooking ranges — left as many as 12,000 dead.

“In the end, it has to come down to the people wanting it,” she said.

Pollution seems like something that must have always provoked outrage, but in Britain that was not always the case. The famous London smog, etched into history by writers like Dickens and Impressionist painters such as Monet, Turner and Whistler, was once a symbol of prosperity, Dr. Corton said. It signified home fires burning (in Dickens there are grim references to meager fireplaces with just a few lumps of coal) and thrumming factories.

“The 1952 smog was a real knock to the psyche,” she said. “People had been through so much — the war, the Blitz. People said we didn’t go through all those deprivations to die from coal smoke. They were fed up. They wanted a better quality of life.”

As for India, Professor Apte said he believed public opinion had shifted, and that there was a much broader recognition of air pollution as a problem. He hopes comprehensive health data from hospitals is collected from this recent episode. That could take the issue out of the realm of abstract statistics and make it real.