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.

Zabol, Iran
  • Air quality: 217 micrograms of particles per cubic meter. The particles measured by this metric have diameters smaller than 2.5 millimeters and are small enough to enter the human respiratory system. The WHO says a concentration of under 35.4 is moderate and one under 12.0 is good.
  • Population: Approximately 150,000
Gwalior, India
  • Air quality: 176 micrograms of particles per cubic meter
  • Population: 1.069 million
Allahabad, India
  • Air quality: 170 micrograms of particles per cubic meter
  • Population: 1.117 million
Riyadh, Saudi Arabia
  • Air quality: 156 micrograms of particles per cubic meter
  • Population: 5.188 million
Al Jubail, Saudi Arabia
  • Air quality: 152 micrograms of particles per cubic meter
  • Population: Approximately 500,000
Patna, India
  • Air quality: 149 micrograms of particles per cubic meter
  • Population: 2.047 million
Raipur, India
  • Air quality: 144 micrograms of particles per cubic meter
  • Population: 1.01 million
Bamenda, Cameroon
  • Air quality: 132 micrograms of particles per cubic meter
  • Population: Approximately 500,000
Xingtai, China
  • Air quality: 128 micrograms of particles per cubic meter
  • Population: Approximately 7 million
Baoding, China
  • Air quality: 126 micrograms of particles per cubic meter
  • Population: 1.665 million

Source: World Health Organization's Urban Ambient Air Pollution Database

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