Former Sen. Jim Webb, D-Va., spent most of the first Democratic presidential debate in the shadow of frontrunners Hillary Clinton and Bernie Sanders. But he got in a few shots here and there.

During the debate, Webb discussed the issue of climate change, arguing that the United States can’t go it alone.

"We are not going to solve climate change simply with the laws here," Webb said. "We've done a good job in this country since 1970. If you look at China and India, they're the greatest polluters in the world. Fifteen out of the 20 most polluted cities in the world are in one of those two countries. We need to solve this in a global way. It's a global problem."

We decided to check whether Webb is right that "15 out of the 20 most polluted cities in the world" are in India and China, and whether his point is relevant for the climate change debate. (Webb’s staff did not respond to an inquiry.)

The top-20 list

There is indeed a credible top-20 list for most-polluted cities. It was assembled by the World Health Organization in 2014.

It compares cities based on how much fine particulate matter they have in the air. This measurement is called PM 2.5, standing for particulate matter that is 2.5 microns in width or smaller. (There are about 25,000 microns per inch, so these particles are very small.)

Such pollutants at the ground level pose such immediate health risks as eye, nose, throat and lung irritation, and they can elevate the risk for lung and heart ailments over the longer term. These small particles typically come from exhaust from cars, trucks, buses and other vehicles, though they can also come from natural sources such as forest and grass fires.

Here’s the list:

Rank CIty and country PM 2.5 pollution level 1 Delhi, India 153 2 Patna, India 149 3 Gwalior, India 144 4 Raipur, India 134 5 Karachi, Pakistan 117 6 Peshawar, Pakistan 111 7 Rawalpindi, Pakistan 107 8 Khormabad, Iran 102 9 Ahmedabad, India 100 10 Lucknow, India Featured Fact-check West Coast wildfires are due to “bad forest maintenance,” not climate change. 96 11 Firozabad, India 96 12 Doha, Qatar 93 13 Kanpur, India 93 14 Amritsar, India 92 15 Ludhiana, India 91 16 Idgir, Turkey 90 17 Narayonganj, Bangladesh 89 18 Allahbad, India 88 19 Agra, India 88 20 Khanna, India 88

So, 13 of the 20 cities on the most-polluted list are located in India (they’re highlighted in bold). That’s two fewer cities than Webb had said, and none of the 20 cities on the list are located in China, as he’d indicated.

Still, Webb’s not that far off, and he has a point that the list is dominated by developing countries, including many in south Asia.

The relevance of this finding for climate change

It’s important to note, however, that particulate pollution is not a key contributor to climate change. Rising global temperatures are more heavily tied to emissions of another substance -- carbon dioxide.

Webb’s comment "does seem to conflate two issues," said John Reilly, co-director of the Joint Program on the Science and Policy of Global Change at the MIT Sloan School of Management.

Looking at the U.S. experience over the past few decades can help clarify the difference between the two metrics.

"The U.S. cleaned up emission of particulates and sulfate aerosols by requiring cleaner, lower-sulfur coal and cleaner diesel fuel," Reilly said. "But cleaning up those emissions does nothing directly to reduce carbon dioxide emissions, since we continued to use coal and petroleum products."

Ironically, particulate emissions may actually have a cooling effect rather than a warming effect because they can reflect sunlight back into space, said Brian Soden, a professor in atmospheric sciences at the University of Miami's Rosenstiel School of Marine and Atmospheric Science. This reflectiveness may have made the earth warm more slowly than it might otherwise have.

That said, experts agreed that there’s likely a reasonably high correlation between cities that have high particulate counts and those that have high emission levels for carbon dioxide, particularly when you look beyond advanced industrialized nations like the United States.

While "the two lists will not necessarily line up perfectly, the 20 most polluted cities are certainly also top contributors of carbon emissions," Soden said.

And experts said that Webb does have a point about the need for international cooperation if carbon levels are to be cut. While the United States is a bigger carbon emitter right now, other nations like India are seeing their emissions rise more rapidly. Emissions are only going to fall worldwide if most major emitters reduce their output together.

"Yes, it is absolutely a global problem, but someone needs to lead," Soden said.

Our ruling

Webb said that "15 out of the 20 most polluted cities in the world" are in India and China.

He’s close -- 13 are in India, though none are in China. It’s worth pointing out, however, that the top-20 list measures cities with the greatest level of particulate pollution, which is not the same thing as emissions of carbon dioxide, the primary driver of climate change.

The statement is accurate but needs clarification and additional information, so we rate it Mostly True.

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