India's capital, New Delhi, ranked as the most polluted mega-city in the world, and the 14th most polluted of all 3,000 cities and towns included in the ranking.

The country is also home to the second most polluted city in the world: Gwalior, in central India. The cities were ranked by the daily average concentration of PM2.5, particles in the air less than 2.5 micrometers in diameter, which are believed to pose the gravest health risks, because they can burrow deep in a person's lungs.

The U.S. city with the most serious air pollution - Visalia, Calif. - had an average reading of 18, too low to even qualify as one of the 1,000 dirtiest cities. Wood burning, emissions from cars and power plants, and dust kicked up by driving on unpaved roads are all sources of PM2.5 - and all of those pollutants are in abundance in India's fast-developing cities.

Although cities in India, China, Saudi Arabia and other developing countries fared worst on the list, the WHO found that more than half the people in cities it surveyed were living with PM2.5 levels that were 2.5 times or more higher than U.N. guidelines recommend.