China’s dirty air often grabs global headlines with photos of its cities swathed in smog.

But it is India where air quality has been plummeting for years and is now the worst in the world. The cost of India’s economic growth is partly recorded in the fact that 13 of the 20 most polluted cities in the world are in India, according to the World Health Organization. New Dehli, India’s capital, is now number one on the list of the most polluted.

For many Indians, that news comes as a surprise. For years the Indian government has not shared data on air quality and has not initiated any serious public awareness about the rising problem. Debates on air pollution were largely dismissed as Western propaganda aimed at curtailing India's growth.

Few Indian cities have purchased or deployed the kind of monitoring equipment necessary to measure the levels of air quality, says Delhi-based environmentalist Sunita Narain. “We just don’t know how bad is the air we breathe,” he says.

Nor is the news getting better. A recently released joint study from the University of Chicago, Harvard, and Yale found that air pollution and attendant nasty chemical particulate matter, especially one called PM2.5, are significantly reducing life expectancies.

India recently crept into third place in the world for greenhouse gas emissions, as well. Yet unlike the deal forged this fall between President Obama and Chinese leader Xi Jinping on greenhouse gas reduction, Mr. Obama was unable to find the same kind of common ground with Indian Prime Minister Narendra Modi during his January visit to Delhi.

A nationwide problem

So grim is the current situation that Delhi has six times the level of airborne particulate matter identified by the US study as most likely to harm humans.

While Delhi may be newly awakening to its problems, the issue is a national one. Indian cities like Gwalior, Raipur, Lucknow, Firozabad, Kanpur, Amritsar, and Ludhiana are also being described as needing to more fully recognize the man-made conditions of bad air.

Delhi now records some 153 micrograms of PM2.5 per cubic meter. (The WHO's safe standard for PM2.5 is below 10 micrograms.)

But other Indian cities also record woeful figures. Patna has 149 micrograms. Gwalior has 144 and Raipur 134 micrograms. The other cities where the PM2.5 level is 10 times higher than WHO standard are Ahmedabad, Lucknow, Kanpur, Firozabad, Amritsar and Ludhiana. These figures are based on a spring 2014 WHO study and may not be the same today.

India hasn't made it simple to find out such information. Some 22 monitoring stations across the country check pollution in real time. But of these, only 12 are functional, and none check for PM2.5.

Factories and other industrial units in India are required to send air pollution data to state government agencies. But it is an open secret that the figures are fudged or hidden. Most states simply report data in terms of “average figures” rather than giving real-time data about air quality.

What's more is that state governments don't always update the figures. One official at the federal environment ministry points out that most data is more than two years old. The latest pollution data on the website of the state of Odisha is from 2006.

Clearing the air

Perhaps partly because of Mr. Obama’s evident displeasure at not getting a deal, and the witness of his delegation at the poor air, India’s air quality has received wider attention in the past month. The country's regulators appear emboldened after years of official denial.

Prakash Javedekar, India’s environment minister, says that air monitors have been installed on thousands of industrial units and in theory will send in accurate figures.

“We are facing a real crisis in as many cities and the world is seeing. I am not bothered about who describes me the baldest or worst,” Mr Javedekar said in an interview on Headline Today, an Indian news broadcast. “We are the worst.”

To improve air quality, India plans to initially track eight pollutants in 46 cities with populations exceeding a million people. After five years, the rest of the country will slowly be brought into the system.

But US Secretary of State John Kerry recently announced what some analysts took as a spur to the monitoring project. He said the US Embassy in Delhi would employ its own monitoring device and release daily data on a website, including figures on PM2.5.

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The US launched a similar program in Beijing, following complaints from American tourists and expatriates.

“We have tens of thousands of US government workers who are employed in some 150 posts around the world,” Mr. Kerry said last week. “In many of the cities where those posts are located, believe me, it can get hard to have regular access to reliable PM2.5 data.”