The world's most polluted cities

From left: Zabol, Gwalior, Allahabad, Riyadh and Raipur — all among the top ten most polluted cities. Photos: AP

In low- and middle-income nations, 98% of cities with populations exceeding 100,000 people do not meet the World Health Organization's air quality guidelines. Among high-income countries, the number falls to 56%.

The big picture: Together, the top ten most polluted cities have over 20 million residents. The 11th most polluted city in the world is Delhi, India, which has a population of 19 million and where spending one day outdoors is equivalent to smoking 44 cigarettes, per CNN.