(This was cross-posted from WSJ's India Real Time blog.)

India's capital may have the worst air quality in the world on some days, but a new report shows that nationally, the air in the world's second-most-populous country is far less polluted than in China.

In fact, China's air is more than twice as dirty as India's, according to recently released estimates by the World Bank.

The bank's "Little Green Data Book" of environmental indicators, unveiled last week, included a new gauge of air pollution. To the standard measures of environmental health--including forest cover and carbon emissions--it added PM 2.5 levels, which measure airborne particles smaller than 2.5 microns.

These tiny pollutants are microscopic and can enter the lungs and even pollute a person’s blood stream. They are linked to severe health problems including lung cancer.