The first time I heard the phrase “pollution refugee” was more than two decades ago, when I lived in Mexico City. My home office sat in a garden of roses and bougainvillea, but it was shrouded in unremitting haze. The city’s air in those days was so contaminated by lead, ozone and other chemicals that birds dropped dead in the smog — and I developed a wheezing cough after games of basketball. On a rare blue-sky day more than a year after I arrived, I gazed, for the first time, at one of the snow-capped volcanoes that rise above the city. Hard to believe: 17,694-foot Popocatépetl had loomed there all along. The next day, the smog returned. I would see the volcano only once more before I moved away.

Around this time, in the 1990s, officials in another heaving capital, New Delhi, began to anticipate an air-pollution crisis of their own, a looming threat to public health that could have grave economic consequences as well. A 1997 government white paper identified the main culprits (vehicle exhaust and industrial emissions) and outlined urgent actions to avert the smog-choked fate of Mexico City, among the world’s most polluted metropolises at the time. But the Mexican capital led the way. Over the next two decades, the city government enacted a series of measures (limiting traffic, shutting down coal-fired plants, mandating cleaner fuels and catalytic converters) that have cut pollution by more than 50 percent. These days, it’s no longer a surprise to see Popocatépetl.

The Delhi action plan urged similar long-term policies, but few of them were carried out. More than 20 years later, the toxic morass hanging over the capital this winter has been so thick that even Delhi’s chief minister, Arvind Kejriwal, criticized it, calling his region “a gas chamber.” He blamed the hazardous haze on the seasonal burning of crop stubble in neighboring states, while others condemned the 13 thermal power plants around the overcrowded capital. At one point in November, when pollution levels were 20 times the World Health Organization’s safe level, the city’s plan to use smog-dispersing helicopters was grounded — by the smog itself. The government undertook some stopgap measures, closing schools, shutting kilns and factories and banning trucks. But a statue of Mohandas Gandhi in a Delhi park seemed girded for the struggle ahead: Antipollution campaigners had fitted the mahatma with a respirator mask.

Nothing captured more attention in a nation of cricket fanatics, however, than the sight of a smog-sickened Sri Lankan player doubled over and vomiting on the grass during a nationally televised test match in Delhi in early December. Though India outperformed its rival that day, the thrice-delayed match was overshadowed, quite literally, by pollution. Santosh Harish, a Delhi-based researcher with the Energy Policy Institute at..