After a severe attack last year that led to a four-day hospitalization, Ms. Kaur moved Mehtab into his parents’ bedroom and set her iPhone alarm for every two hours, so she could strap on his nebulizer mask at intervals throughout the night.
On the night of Diwali, the couple sat inside, listening to the neighbors celebrate. They could hear firecrackers going off outside, the expensive kind that sizzle and pop and burn for half an hour.
“We were feeling so disturbed,” Ms. Kaur said. “We knew what was going to happen.”
The two children, 20 miles apart and at different ends at the economic spectrum, got sick on the same night.
Thirty-six hours after Diwali, the pollution had pooled close to the ground. Delhi’s airport, Indira Gandhi International, reported visibility of around 1,000 feet, the worst conditions in 17 years.
After episodic smog events, it typically takes between one and three days for severe effects to emerge in children, according to Bhargav Krishna, who manages the Public Health Foundation of India’s environmental health system and is a co-founder of Care for Air.
The crisis typically comes in the form of a lower respiratory infection, like bronchitis or pneumonia, that can become dangerous, with fluid filling the lungs and plummeting levels of oxygen in the blood.
In the mosquito-ridden migrants’ settlement where her family lives, Vaishnavi coughed incessantly: Her face was red, and the tendons on her neck were popping out. Ravi stirred from a nap to hear his wife howling. He did not sleep again until morning, but instead rubbed the baby’s feet and hands, and listened for a heartbeat.
“There was not a sound coming from her,” said Bhanwari, her mother. “You hear a wheezing sound inside her. A rattling.”
Mehtab, too, was in trouble. His mother, weary of confining him, had finally allowed him to go to school, but he was sent home right away, sniffling. That much exposure to the air was enough. When night fell Ms. Kaur began administering the steroids every hour, something she had never done. He began to gasp, his rib cage heaving, 15 minutes after she removed the mask from his face.
“It was really bad,” Mr. Anand said. “I can’t explain it in words, actually.”
The decisions came quickly after that. Mr. Anand brought home an air purifier that cost 29,000 rupees — about $425 — and switched it on, peering at the display to see the concentration of PM 2.5. It was above 700 inside the house. Three days later, Ms. Kaur took Mehtab out of Delhi, boarding a train for her parents’ home, a few hours north.
What shocked her was how quickly his breathing eased. They arrived at night, and he slept so peacefully that she reduced the steroid treatments to every four hours. The next morning, she watched through the window as he played outside.
She called her husband, crying, and said it was time to leave Delhi.
Not so for Vaishnavi. This week, she sat on her mother’s lap, sucking on a lollipop, while her aunt cooked on a clay stove in the room, filling it with fumes.
The worst season here in Delhi has just begun. It will continue for three months, growing worse when the city’s vast homeless population begins setting nighttime fires for warmth, and when dropping temperatures push the emissions toward the ground.
Some emergency protective measures introduced during the week after Diwali, including a moratorium on construction, have been reversed.
The 43-year-old Badarpur coal-fired power plant will remain shuttered until Jan. 31. But it will then reopen, and new standards being imposed on coal plants next year will apply only to newly built plants, Mr. Krishna said.
“You stop being angry and start being cynical at some point,” he said. “Year after year, there are action plans issued with no follow-up. And every year, this kind of thing happens.”
With the tourists back on the street, Vaishnavi’s father, Ravi, was back at his corner, selling trinkets.
He had consulted a roadside doctor about protecting his family from air pollution, and gathered that they were supposed to eat cane sugar. He had also bought a pair of glasses to protect his eyes from the pollution, and removed several bricks from the wall of the small, airless room his family occupies, which he thought would improve air circulation.
Vaishnavi had improved with a course of antibiotics, but he felt no certainty that she would survive another week like the one that followed Diwali. Or, for that matter, that anyone would notice if she did not.
“Delhi people have no memory,” he said. “It would be one in a hundred who would ask me how she died.”
An earlier version of this article, using information from Bhargav Krishna of the Public Health Foundation, referred incorrectly to the shuttering of the Badarpur coal-fired power plant. The closing has been extended until Jan. 31, not “quietly reversed.”