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.

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

176 micrograms of particles per cubic meter Population: 1.069 million

Allahabad, India

Air quality: 170 micrograms of particles per cubic meter

170 micrograms of particles per cubic meter Population: 1.117 million

Riyadh, Saudi Arabia

Air quality: 156 micrograms of particles per cubic meter

156 micrograms of particles per cubic meter Population: 5.188 million

Al Jubail, Saudi Arabia

Air quality: 152 micrograms of particles per cubic meter

152 micrograms of particles per cubic meter Population: Approximately 500,000

Patna, India

Air quality: 149 micrograms of particles per cubic meter

149 micrograms of particles per cubic meter Population: 2.047 million

Raipur, India

Air quality: 144 micrograms of particles per cubic meter

144 micrograms of particles per cubic meter Population: 1.01 million

Bamenda, Cameroon

Air quality: 132 micrograms of particles per cubic meter

132 micrograms of particles per cubic meter Population: Approximately 500,000

Xingtai, China

Air quality: 128 micrograms of particles per cubic meter

128 micrograms of particles per cubic meter Population: Approximately 7 million

Baoding, China

Air quality: 126 micrograms of particles per cubic meter

126 micrograms of particles per cubic meter Population: 1.665 million

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