For years, this sprawling city on the Yamuna River had the dirtiest air in the world, but few who lived here seemed conscious of the problem or worried about its consequences.

Now, suddenly, that has begun to change. Some among New Delhi’s Indian and foreign elites have started to wear the white surgical masks so common in Beijing. The United States Embassy purchased 1,800 high-end air purifiers in recent months for staff members’ homes, with many other major embassies following suit.

Some embassies, including Norway’s, have begun telling diplomats with children to reconsider moving to the city, and officials have quietly reported a surge in diplomats choosing to curtail their tours. Indian companies have begun ordering filtration systems for their office buildings.

“My business has just taken off,” said Barun Aggarwal, director of BreatheEasy, a Delhi-based air filtration company. “It started in the diplomatic community, but it’s spread to the high-level Indian community, too.”

The increased awareness of the depth of India’s air problems even led Indian diplomats, who had long expressed little interest in climate and pollution discussions with United States officials, to suddenly ask the Americans for help in cleaning India’s air late last year, according to participants in the talks. So when President Obama left Delhi after a visit last month, he could point to a series of pollution agreements, including one to bring the United States system for measuring pollution levels to many Indian cities and another to help study ways to reduce exhaust from trucks, a major source of urban pollution.

One driver for the change is a deluge of stories in Indian and international news outlets over the last year about Delhi’s air problems. Those articles, once rare, now appear almost daily, reporting such news as spikes in hospital visits for asthma and related illnesses. One article about Mr. Obama’s visit focused on how, by one scientist’s account, he might have lost six hours from his expected life span after spending three days in Delhi.

“We felt this was an issue we should take up, and we have taken it up,” said Arindam Sengupta, executive editor of The Times of India, whose campaign against air pollution has helped give prominence to the problem.

But Nicholas Dawes, a top editor at The Hindustan Times, said the media coverage was just one reason for the attitude shift. “I think the people of Delhi are increasingly unwilling to tolerate tough circumstances,” he said.

At least so far, that has not translated into much meaningful action by the government. In fact, the problem is likely to get worse as the government of Prime Minister Narendra Modi works to reboot the economy. His government recently promised to double its use of coal over the next five years.

But Dr. Joshua S. Apte, an assistant professor of environmental engineering at the University of Texas at Austin, who has studied Delhi’s air pollution since 2007, said recognition was a start. “The thing that gives me greatest hope is the huge increase in awareness that I’ve seen in Delhi just in the past year,” he said.

Delhi’s air is the world’s most toxic in part because of high concentrations of PM2.5, particulate matter less than 2.5 micrometers in diameter that is believed to pose the greatest health risk because it penetrates deeply into lungs. While Beijing’s air quality has generated more headlines worldwide, scientists say New Delhi’s air is often significantly worse, especially during the winter, when choking smog often settles over the sprawling city.

Four city monitors found an average PM2.5 level of 226 micrograms per cubic meter between Dec. 1 and Jan. 30 — a level the United States Environmental Protection Agency calls “very unhealthy” and during which children should avoid outdoor activity. The average in Beijing for the period was 95, according to the United States Embassy monitor there.

Indeed, there has not been a single 30-day period in Beijing over the past two years during which the average PM2.5 level was as bad as it was in December and January in Delhi.

Worse yet, the numbers tell only half the story because Delhi’s PM2.5 particles are far more dangerous than those from many other locales because of the widespread burning of garbage, coal and diesel fuel that results in high quantities of toxins such as sulfur, dioxins and other carcinogenic compounds, said Dr. Sarath Guttikunda, director of Urban Emissions, an independent research group based in Delhi.

“Delhi’s air is just incredibly toxic,” said Dr. Guttikunda, who recently moved to Goa to protect his two young children from Delhi’s air. “People in Delhi are increasingly aware that the air is bad, but they have no idea just how catastrophically bad it really is.”

Already, an estimated 1.5 million people die annually in India, about one-sixth of all Indian deaths, as a result of both outdoor and the indoor air pollution, a problem caused in part by the widespread use of cow dung as cooking fuel. The country has the world’s highest death rate from chronic respiratory diseases, and more deaths from asthma than any other nation, according to the World Health Organization. Air pollution also contributes to both chronic and acute heart disease, the leading cause of death in India.

Delhi residents attribute their longtime stoicism about the city’s pollution to a combination of fatalism, loyalty to their city and a sense of immunity.

Veena Dogra, 65, notices that family members who visit from abroad snuffle and sneeze, and she is aware that her usual black nasal discharge stops when she leaves India, and returns when she does. But she said she eventually forgets the contrast between Delhi and everywhere else, which is why she resists her daughters’ suggestions that she buy air purifiers or wear masks.

“Am I going to shut myself into just one room in my house?” Ms. Dogra asked. “You have to be tough to live in Delhi. If you’re not, you should leave. And I have too much family here to think about doing that.”

Dr. Anupama Hooda Nehra, director of medical oncology at Max Cancer Center in New Delhi, said she joined a morning cancer fund-raising walk this month even though she ended her own morning walks last year because she decided they were doing more harm than good and could encourage a cancer’s growth. “But I had to go to the fund-raiser because it was supported by my hospital,” she said. “And I worried that if I wore a mask, I would scare everyone since I’m an oncologist. It’s hard to know what to do.”

Dr. Nehra said her greatest worry is that her daughters, ages 14 and 8, will suffer lifelong effects from living in Delhi, a concern that has increasingly spooked Delhi’s expatriate community.

After Dr. Apte gave a presentation about Delhi’s air pollution to a hall packed with anxious parents recently at the American Embassy School, the administration invested in indoor air filters and increasingly restricts children’s outdoor activities when pollution levels are especially high. Even so, the sidelines during school soccer games are lined with players’ medicinal inhalers.

“I’m surprised kids there can even play soccer,” said Dr. James Gauderman, a professor of preventive medicine at the University of Southern California and an author of a landmark 2004 study on the effects of air pollution on children’s lungs.

In his study, Dr. Gauderman found that children raised in towns with PM2.5 levels of 30 had substantial reductions in lung function compared with those raised in towns with levels of 5. In the decade since his study was published, “we don’t see any evidence that functional loss is reversed,” Dr. Gauderman said. “The deficit appears to be permanent. I can’t imagine what that deficit would be with pollution levels almost 10 times higher. No one has studied that.”