Over the past few years, India’s environmentalists have warned about the long-term effects of sustained exposure to air pollution levels that can reach the equivalent of smoking two packs of cigarettes a day. A recent report found that major causes of pollution in the capital and surrounding cities, a metropolis of more than 46 million people, were construction dust, vehicle emissions and burning of agricultural waste.

Dr. Arvind Kumar, a chest surgeon in the city, said that 90 percent of his lung cancer patients three decades ago were smokers. Today, he said, the ratio was one to one, with at least 10 percent of his clients only in their 30s.

“Fifty percent of the patients I operate on throughout the year are nonsmokers,” he said. “This kind of demographic change is shocking.”

The Indian government has tried to take action this year, including enforcing a Supreme Court ban on most types of fireworks ahead of Diwali and introducing new “green” designs. In October, the country’s health minister, Harsh Vardhan, said that the eco-friendly fireworks would “resolve the crisis of air pollution” and slash emissions by 30 percent.

But on Sunday evening, many families still celebrated the festival by climbing to their rooftops and setting off firecrackers.

By morning, the city was under a halo of fog. In parts of the city, levels of the most dangerous air particles, called PM 2.5, eventually climbed to around 600 micrograms per cubic meter, which is considered hazardous to breathe, according to data provided by the Delhi Pollution Control Committee. Scientists have linked that kind of air pollution to increased death rates.

Mohammad Islam, 43, a rickshaw driver who wore a mask on Friday, said he was worried for his job and his life. In recent years, he said, he has developed a persistent cough, forcing him to cut four crucial hours of work from his days.