NEW DELHI — A toxic, throat-burning cloud has settled over India’s capital, swallowing national monuments, sending people to emergency rooms and prompting officials Friday to declare a public health emergency and close schools for days.

Air quality in parts of New Delhi rose to levels around 20 times what the World Health Organization considers safe. By Friday afternoon, officials in the capital region had halted all construction projects, planned to limit the number of vehicles on roads, urged people to stay inside and shut several thousand primary schools until Tuesday.

“We are in trouble,” said Dr. G.C. Khilnani, a pulmonologist in the city.

Every winter, as wind speeds slow and farmers burn their crops to make room for a new harvest, dirty air settles over India’s cities, putting hundreds of millions at risk. Adding to it, pollution in New Delhi got even worse after weekend celebrations of Diwali, the Hindu festival of light, when families set off fireworks despite government warnings against it.

India has struggled to get in front of its pollution crisis. Reports have found that the country’s children may be facing permanent brain damage from poisonous air and that millions of Indians have already died from health problems connected to living in polluted cities.

The problem is not confined to the capital. Urban areas across the country, from Mumbai in the west to Varanasi in the east, are all struggling with filthy air, lending India the distinction of having 15 of the world’s 20 most-polluted cities, according to a recent study.

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But even as air pollution climbed to dangerous levels this week, turning the sun a murky white, some businesses in New Delhi kept their doors open, and patrons at higher-end restaurants chose to sit outside. Face masks were still a rare sight on streets, and many politicians, including Prime Minister Narendra Modi, refrained from publicly acknowledging the problem.

Jyoti Kumar, 47, the owner of a supermarket chain in New Delhi, said that many Indians had become fatalistic about the air. She said that more people were buying air purifiers if they could afford them, but that collective pressure on politicians to act was still not happening.

“I wish I could shut down my business and leave this city for good,” she said.

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% of his lung cancer patients three decades ago were smokers. Today, he said, the ratio was 1 to 1, with at least 10% 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.”

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The Indian government has tried to take action this year, including enforcing a Supreme Court ban on most types of fireworks before 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%.

But 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 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.

As the air grew worse this week, Islam said he began to wonder how much longer he could last.

“I have started to get shortness of breath, a suffocation I cannot explain,” he said. “It’s like someone is physically choking me.”