China's struggles with smog and air pollution have gotten a lot of attention over the years.

But the air quality in India now appears to be even worse — with one new study finding that excess pollution is reducing the life expectancy of 660 million Indians by 3.2 years, on average.

India's pollution problem is worse than China's

Some background: The World Health Organization recently published data on pollution around the world, focusing on airborne particles smaller than 2.5 micrometers (known as PM2.5). These particles come from coal plants and vehicles, and, at high levels, have been linked to serious respiratory problems.

On this score, 13 of the world's 20 most-polluted cities are all in India. And India's cities have much higher PM2.5 levels than China's:

Comparing air pollution in India's cities vs. China's vs. Europe vs. US

So how much harm is all this pollution causing? A new study in Economic & Political Weekly — a top journal in Mumbai — finds that 660 million people in India now live in areas where PM2.5 levels exceed the country's national air-quality standards. (And, as the chart shows, India's a standards are already looser than even China's.) That excess pollution alone, they estimate, reduces life expectancy at birth by 3.2 years, on average.

"The loss of more than two billion life years is a substantial price to pay for air pollution," Rohini Pande, director of Harvard Kennedy School's Evidence for Policy Design and a co-author of the study, said in a statement. "It is in India’s power to change this in cost effective ways that allow hundreds of millions of its citizens to live longer, healthier, and more productive lives."

How to calculate the death toll from air pollution

Researchers have long known that particle pollution in the air is terrible for people's health. But it's always been difficult to measure the precise impacts — especially from the high levels in China and India. After all, scientists can't just run tests where they expose people to heavy pollution at random.

A few years ago, however, economist Michael Greenstone and his colleagues found a way to conduct a quasi-natural experiment in China. They noticed that, back in the 1950s, the Chinese government started providing free winter heating via coal boilers to people living north of the Huai River. Meanwhile, those living south of the river didn't get the free boilers. This disparity gave the researchers a way to isolate the effects of air pollution.

Every 100 micrograms of 'total suspended particulates' per cubic meter cut life expectancy by three years

In a 2013 paper for the Proceedings of the National Academy of Sciences, Greenstone and his co-authors calculated that an extra 100 micrograms per cubic meter of "total suspended particulates" in the air was associated with a drop in life expectancy of about three years.

"I was surprised by the magnitude of the effect," Greenstone told me when that study came out. It meant that coal pollution in northern China had cut the lifespans of 500 million people by roughly 5.5 years, on average.

Now, this latest paper — which Greenstone co-authored with Janhavi Nilekani, Rohini Pande, Nicholas Ryan, Anant Sudarshan, and Anish Sugathan — applies those same results to India.

The authors first calculate that about 660 million people, mostly in north India, are living with annual levels of PM2.5 that exceed national air quality standards (which are currently set at 40 micrograms per cubic meter — weaker than those in China, Europe, or the United States):

Estimates of PM2.5 concentrations across India

Using the numbers from the China study, the authors could get a rough sense of how much harm this is doing — that's where they estimate that excess air pollution in these regions is reducing life expectancy by an average of 3.2 years.

Now, this is only an average: actual exposure can vary a lot from region to region and person to person. Policemen who are working in traffic all day get a higher exposure to air pollution. Wealthier families who can afford air purifiers in their homes get less. But the overall numbers are staggering.

The authors also find that air pollution levels in India's cities haven't shown any improvement in the last five years. If anything, things have gotten a bit worse.

Can India clean up its air pollution?

India and China, of course, didn't invent air pollution. Cities like London and Los Angeles also once had horrific smog and particulate problems. But as they got richer, they cleaned up — and there's every indication that developing countries will do the same. The question is when.

China, for its part, has already begun to crack down on its air pollution, through policies to limit coal burning in cities and curtail vehicle use.

So far, India has taken fewer steps in that direction. One reason for that is that India is still much, much poorer than China. (Among other things, India has 400 million people without electricity; China has virtually none.) So the country is still focusing heavily on economic growth — which often means expanding the use of fossil fuels. Energy analyst Mackay Miller summed this up on Twitter:

I asked why smog wasn't more of a political problem in India. The answer: "The list of problems is too long. " http://t.co/WnsstkWjXZ — mackaymiller (@mackaymiller) February 23, 2015

In this narrative, the very different political reaction to smog in China v India is a Maslow's hierarchy issue. China resolved the basics. — mackaymiller (@mackaymiller) February 23, 2015

Fossil fuels have unlocked growth, freeing countries of certain afflictions (grinding poverty), saddling them with new ones (lung disease) — mackaymiller (@mackaymiller) February 23, 2015

That said, the problem has gotten so bad of late that India's policymakers are beginning to take notice. In November 2014, Prime Minister Narendra Modi announced he would make air-quality data available to the public and pledged to set new emissions standards for power plants.

Then, in February, according to Gardiner Harris of The New York Times, Indian officials asked the Obama administration for help in both measuring pollution and in finding ways to reduce pollution from trucks. "One driver for the change," Harris reported, "is a deluge of stories in Indian and international news outlets over the last year about Delhi’s air problems."

In their paper, Greenstone and his colleagues recommend a number of additional steps, including better air quality monitoring (India's cities have far fewer monitors than China's do), civil penalties against polluters (India's pollution penalties are actually so severe that they're rarely enforced), and possibly an emissions trading system similar to the one that the United States set up for acid-rain pollution.