The night before the biggest exam of his life, Sarthak Jain couldn't breathe. His limbs were listless, his breathing labored.

Jain's parents, both trained doctors, recognized the signs of a severe asthma attack. They put the 18-year-old in the family car and raced to a nearby hospital. Doctors determined that Jain was suffering from low levels of oxygen in his blood. His life was in danger. Jain was given injections and put on a nebulizer. His relatives, told to prepare for the worst, huddled together in the intensive care unit and cried.

Jain is one of 20 million Delhi residents forced to breathe the world's most polluted air. While Beijing grabs the headlines for poor air quality, scientists say the pollution here is far worse. In 2014, the World Health Organization released data on air quality in 1,600 cities, and Delhi was found to have the highest concentration of particulate matter smaller than 2.5 micrometers, also called PM 2.5.

The air in Delhi assaults the senses: On bad days, thick smog obscures the sun, reducing visibility to just a few hundred meters. The smog is often tinged with woodsmoke, and the scent clings to jackets and trousers like air from a smoky bar. Delhi's High Court has compared conditions in the city to "living in a gas chamber."

PM 2.5 particles are exceedingly small and can evade the body's normal defenses and penetrate deep into the lungs, causing chronic health problems. They have been linked to increased risk of asthma, heart disease, stroke and respiratory infections, as well as cancer of the trachea, lung, and bronchus.

"In the city of Delhi, exposure to the air is equal to smoking maybe 10 cigarettes a day," Rajesh Chawla, a respiratory physician at Indraprastha Apollo Hospital, told me. "Everybody is a smoker in this city."

Related: I live in the world's most polluted city

The same heavy use of fossil fuels that has helped turn India into the world's fastest-growing major economy is now choking its residents -- and raising questions about the government's blueprint for future development. Other cities have faced similar public health crises. Los Angeles used to be so polluted that, during World War II, residents mistook thick smog for a Japanese chemical munitions attack. Londoners suffered greatly during the industrial revolution. Polluting steel mills contributed to scores of deaths in the American Midwest.

But Delhi's leaders face a particularly fraught policy dilemma: Crack down too hard on the sources of pollution and risk damaging economic growth in a megacity where millions of people still live in abject poverty. Do nothing and sentence all of Delhi's children to a lifetime of breathing dirty air.

Jain, the young student, survived that night in the hospital. He made enough progress after doctors intervened to stagger out of the intensive care unit, and to his exam. Miraculously, he scored well, and now, two years later, is studying at a local university. Jain has changed, though: He wears a mask whenever he goes outside, and he sees a doctor regularly to keep his asthma in check. He still suffers from attacks -- some severe. He now works with other students to raise awareness about the dangers of dirty air, which he describes in the starkest of terms.

"I cannot just run away from it. I am dying a slow death, I know it," Jain said. "I live in a constant fear of what will happen. I am looking at a bleak future."

Delhi's battle against air pollution dates to at least 2001, when India's Supreme Court, appalled at conditions in the city, ordered public buses, taxis and autorickshaws to switch from diesel power to compressed natural gas. Residents still talk about the massive lines that resulted at petrol stations. The actions forced by the Supreme Court, combined with new fuel standards, greatly improved air quality. But now the trend has reversed. More often than not this winter, when I have checked on my phone, Apple's weather app has simply read "smoke."

The city is sprawling, and a lack of effective public transportation has fueled an explosion in car ownership. The city has 9 million vehicles, and new cars are being registered at a rate of 1,400 per day. India's fuel standards are roughly 10 years behind those of Europe, and low petrol prices incentivize driving. Cooking fuel, construction dust and coal-burning power plants add particles and toxic gasses to the mix. When the weather turns cold, the city's poor huddle together in slums and along busy roads, warming themselves around small wood or trash fires. This winter, PM 2.5 levels have frequently topped 500, a reading that is literally off the charts.

To find out why Delhi was again facing a major crisis, I took a trip to Gurgaon, a suburb of Delhi, to meet the first in a series of doctors, reformers, advocates, lawyers and entrepreneurs who are battling entrenched bureaucracy in an effort to clean the city's air.

The domination of cars over Delhi's transportation infrastructure is on full display during the drive south. I started in central Delhi -- the only part of the vast city that can be described as walkable. The streets feature the occasional crosswalk, and drivers will stop for pedestrians.

But as roads radiate from central Delhi, they become narrower and more chaotic. Crossing from one side to the other, on foot, can be a hair raising endeavor, and pedestrian bridges are rare. On busy streets, long, raised medians, which often include waist-high walls or fences, separate lanes of traffic, making passage impossible. Sidewalks vanish mid-block.

Delhi is an ocean of pavement. Traffic planners have responded to snarls and delays by widening key arteries to accommodate even more cars. At one point on the main route to Gurgaon, which is known for producing legendary traffic jams, the road briefly widens to 15 lanes. During rush hour, it resembles a parking lot. Making the trip on a bicycle would be impossible.

The angry doctor: 'The doom has come'

Medanta MediCity, a 1,250 bed hospital, is a gleaming monument to medicine -- all white walls and metal finishings, along with a massive atrium that resembles an upscale shopping mall. I met Naresh Trehan at his office in the emergency wing.

Trehan is not a typical doctor. In addition to being one of India's leading cardiovascular and cardiothoracic surgeons, he is the hospital's entrepreneurial chairman and managing director, and the kind of person who carries two cell phones. While we talk, he works through a stack of documents, signing page after page while speaking in complete paragraphs about the dangers of air pollution.

"Scientists have been screaming about this for many years," Trehan told me. "This is like Nostradamus predicting doom -- but the doom has come." To help raise awareness about the dangers of air pollution, Trehan released a set of photos that compared the lungs of two patients: one from Delhi, and one from a rural area with better air quality. The lung that had been breathing clean air was pink and fleshy, while the organ from Delhi was covered in a black substance that closely resembled tar. The Monday after our meeting, he released a video of three yoga exercises that can improve lung health.

Trehan has called on the government and courts to quickly implement new reforms. But he has also asked the public to make a collective sacrifice. "Before it was: 'I'll throw the garbage out on the street, because goddamnit it's out of my house.' Now it's coming back into your nostrils, into your lungs, into your children's lungs. We must drive that home," he told me while pounding a fist on his desk for emphasis.

The lawyer with a cause: Fighting for the 'right to life'

Much of the policy action on the crisis has taken place at India's Supreme Court, where judges have repeatedly ruled in favor of anti-pollution reformers. One of these advocates is Saurabh Bhasin, a lawyer at Trilegal who studied at Cardiff University and Cornell Law School, before returning to Delhi five years ago.

In late 2015, before India's festival season began in earnest, Bhasin and two other advocates filed a Supreme Court petition on behalf of their infant children, arguing that extreme air pollution in the city was violating their constitutionally guaranteed "right to life." The petition asked the court to grant an injunction against firecrackers, millions of which are exploded in Delhi to celebrate Diwali, the Hindu festival of lights. The mass use of these fireworks typically produces extreme pollution for days afterward.

"Three years ago, some of our friends actually picked up and moved [away from Delhi]," Bhasin said. "That's when we became aware of how bad the situation was ... we felt that if we went before the court with the interests of children, the court would look at that more favorably."

Bhasin indeed found a receptive audience. A short time later, the Supreme Court levied a new tax on commercial trucks entering the city. The anti-firecracker injunction that Bhasin requested, however, was not granted in time for Diwali. After the celebration, Delhi's pollution readings spiked to some of their highest levels of the year.

The lawyer blames India's politicians and legislators. "Frankly the government has never done anything about it," he said. "[The Supreme Court] has been the only body that has been open to bringing in reforms." His petition, which also asked the court to implement improved fuel standards, is still open before the court.

The air filter entrepreneur: 'We're in it to help you out'

I first met Jay Kannaiyan at a rooftop party in a leafy Delhi neighborhood as fall was giving way to winter. He was holding court near the bar, wowing guests with a small, black box that looked like a Geiger counter. The box was measuring the tiny pieces of carbon that make Delhi's air so dangerous. The levels were, predictably, off the charts.

Kannaiyan is the India head of a startup called Smart Air Filters. When he's not selling air filters, the slender Kannaiyan organizes motorcycle treks into the Himalaya Mountains. I visited him a few weeks later at the Smart Air office in Delhi, a modest warehouse-style space that features more laptops than lightbulbs.

Demand for air filters has skyrocketed in recent years in Delhi as people try to protect themselves from the dirty air, at least while they're indoors.

Smart Air was founded by expatriates living in China, and the company sells just one product in India. When Kannaiyan describes the filter as a fan with a filter strapped on top, he's not exaggerating. The purifier, which claims to do the same job as rivals that cost $100, $500 or even $1,000, is a very simple answer to a complicated problem.

In a little more than a month, the startup has received orders for roughly 500 filters from around India. Kannaiyan spends a lot of time dealing with logistics -- shipping and suppliers are major sources of frustration. Customers drop by the shop to pick up orders, many of them clearly relieved to be getting a filter for their home or office.

Price is a major selling point. A Smart Air Filter goes for 3,399 rupees ($50), which makes it affordable for most middle class families. "People like the fact that it's a bunch of young people who are not in it to make money," Kannaiyan said. "We're in it to help you out."

The science behind air filters is still being studied -- and there is no proof that they deliver better health outcomes. But most models from popular brands are able to dramatically reduce the number of PM 2.5 particles in a closed room.

The children: Living under coal's shadow

I left the Smart Air office and took an autorickshaw north toward Rajghat Power Station, one of the last coal-fired power plants in the city. The station, which had failed a series of environmental tests, was recently shut by the government. But its legacy of pollution has been preserved on Google Maps, where a satellite view of the area still shows a dark plume of smoke wafting skyward from the plant, darkening the landscape.

The plant's silent smokestack towers above its surroundings, which include the site where Mahatma Gandhi was cremated following his assassination in 1948.

At Gandhi's cremation site, hundreds of schoolchildren in matching maroon uniforms were waiting patiently for their turn to see the simple monument to India's liberator. None wore masks. A third of the city's children already have respiratory problems, and nearly 45% suffer from reduced lung function. Air pollution has even been linked to attention-deficit hyperactivity disorder.

Outside the gates of the power plant, three or four families have constructed a small slum consisting of flimsy tents. It's hard to ignore the role of coal in air pollution: India's coal has a high ash content, and it spews toxins and metals into the air when burned.

Yet Prime Minister Narendra Modi has pledged to double the country's coal production by 2020, to levels that will trail only China. During recent climate negotiations, India, already the world's third-largest emitter of greenhouse gases, asserted its "right to grow," an argument for continued coal production.

While Modi's government has also set ambitious solar and renewable energy targets, its supporters argue the country should continue to boost coal production until more of its citizens have access to basic services. An estimated 300 million Indians, for example, are still not connected to the electric grid.

The anti-pollution advocate: Delhi needs 'all of the above' policy

On New Year's Day, Delhi's city government launched a test of an "odd/even" program for drivers. For 15 days, cars with odd-number license plates would be allowed to operate only on odd days, while cars with even number places could be driven on even days. Private buses were pressed into service and additional traffic cops were put on the streets.

The aim: pull millions of cars off the roads.

Anumita Roychowdhury, the executive director of the Centre for Science and Environment, said a permanent program would force the government to improve mass transit, even if it didn't have much of an effect on air quality.

"They have to come up with the template, and that system should be sustained even beyond the 15-day program," Roychowdhury told me.

Roychowdhury is a soft-spoken veteran of the city's air pollution policy wars. She advocates an all-of-the-above approach to combating air pollution -- like better public transportation and more cycling. She takes issue with policies and tax incentives that keep parking and fuel costs low.

"You are actually making it cheaper for me to use my car than to take public transport," Roychowdhury said. Bus ridership has dropped from 60% in 2000 to 41% today. "The irony here today in Delhi is that cars are carrying about 14% of the travel demand, but 80% of transport investment in the city is tied to roads that facilitate car movement, not public transport."

Roychowdhury is a purist. During a recent meeting with her, I noticed there were no air filters in her office, and the front door was wide open. "We do not believe in masks; we do not believe in air filters," she said. "This is the great way for the rich and powerful to isolate themselves from the problem. It takes away the push you need to clean up air in this city."

The debate over Delhi's car restriction experiment began as soon as it ended.

To the surprise of many, drivers largely obeyed the "odd/even" rules. Researchers from Harvard and the University of Chicago, using data from taxi app Uber, determined that vehicular speeds had improved by 5.4% during "odd/even." That suggests traffic was improved, and fewer idling cars might mean cleaner air.

The researchers also compared air pollution levels in Delhi to those in the city's suburbs, which did not participate in the test. They found 18% fewer pollution particles in Delhi's air while the restrictions were in place. Still, the researchers were not convinced that "odd/even," implemented on a permanent basis, would solve Delhi's problems. Instead, they suggested the government experiment with congestion pricing, which would charge drivers for using their cars in crowded areas during peak traffic times.

"Air pollution is shortening lives in Delhi and too many other places in India and elsewhere," the Harvard-Chicago researchers wrote in The Indian Express. "The odd-even scheme has delivered over these two weeks, but may not over the long term."

But the Delhi government is counting the program as a major win. It has directed city agencies to find ways to improve it, and a second run of the plan has been scheduled for two weeks in April.

If nothing else, the car restrictions succeeded in raising awareness. "People are beginning to understand the problem," Roychowdhury said. But, she cautioned, cars are just one source of pollution. The government needs to improve public transportation, heighten fuel standards and crack down on industrial sources of dirty air. And it needs to act quickly. "The kind of levels we have today in Delhi's air, we have to act on each root cause," she said. "We just don't have a choice."