According to the World Health Organization, the air in these 50 cities was found to be the most polluted in 2018.
Air-quality monitoring devices worldwide tracked levels of dangerous particulates. The most harmful kind, called PM2.5, contains tiny particles small enough to enter the bloodstream through the lungs—a problem that contributes to an estimated 7 million premature deaths every year.
We've ranked the cities here, from least to most polluted.
50. Linfen, China
Twelve years ago, Linfen was the most polluted city in the world, but even then, Beijing was making efforts to curb the city's coal-burning addiction.
Nearly 200 coal-fired boilers were decommissioned, and natural gas heaters replaced coal ones in local homes.
49. Yanbu, Saudi Arabia
Emissions from oil refineries and dust from the desert both contribute to the poor air quality in Yanbu.
48. Pingdingshan, China
This city in eastern China is one of the country's biggest coal producers.
47. Weinan, China
In 2018, the annual mean for fine particulate matter in Weinan was 76 micrograms per cubic meter, more than seven times the World Health Organization's target value.
46. Shangqiu, China
Shangqiu is 179 miles east of Pingdingshan in China's Henan province.
It is traditionally a farming city, but some modern industry, like this textile factory, has been developed in Shangqiu since the 1990s.
45. Luohe, China
This city is a manufacturing hub. Paper, leather, textiles, cigarettes and electronics are all produced here. The city is most famous for its massive ham-processing plant.
44. Zhengzhou, China
In 2016, a man sued the Zhengzhou local government, seeking reimbursement for the cost of pollution masks he bought while visiting the city.
43. Shouguang, China
Shouguang is an agricultural hub in China's Shandong province.
42. Al-Ahmadi, Kuwait
This photo shows smoke from a fire at a local oil refinery. Oil production and processing plants can release greenhouse gases and dangerous particulate pollutants.
41. Luoyang, China
Teams compete during the Dragon Boat Festival in Luoyang. The city's skyline is shrouded in smog.
40. Dongying, China
This photo shows a damaged oil rig off the coast of Dongying in Shandong province. In 2017, Beijing shut down 30 oil refineries in the province to decrease industrial air pollution.
39. Barisal, Bangladesh
The Health Effects Institutes' State of Global Air survey showed that the entire population of Bangladesh — around 168 million people — has been consistently exposed to unhealthy levels of fine particulates in the air since 1990.
38. Zaozhuang, China
A traffic policeman directs drivers on a heavy smog day in Zaozhuang.
37. Dezhou, China
A farmer rides a bike down a country road on a hazy day in Dezhou. In 2007, Himin Solar Energy Group began to develop Dezhou into a hub for renewable-energy tech companies.
36. Gazipur, Bangladesh
A train emerges from the haze during the annual Bishwa Ijterna gathering of Muslim devotees.
35. Xianyang, China
This aerial photo shows the weather in a residential area in nearby Xian.
34. Jining, China
Jining borders two other cities on this list, Zaozhuang and Heze.
33. Heze, China
This city in northeastern China has the Yellow River as its northern border.
32. Handan, China
These workers are testing a dust-suppression truck. This water cannon purports to suppress smog and promote clear, healthy air.
31. Nagpur, India
Nagpur, home to the Marbat festival, is the most polluted city in the Indian state of Maharashtra.
30. Xinxiang, China
The Xinxiang Environmental Protection Volunteers Association helped to close more than 100 factories in the area between 2001 and 2009. The effort was an attempt to clean up the air and water in central China.
29. Khulna, Bangladesh
The sun is barely visible through the smog in this photo of Bangladeshi laborers working at a brick factory in Khulna.
28. Jiaozuo, China
Workers from the Longfeng leather and fur factory cart material through the streets of Jiaozuo.
27. Liaocheng, China
An orange alert for heavy smog was issued on this hazy December day in Liaocheng.
26. Anyang, China
Onlookers watch as a plane performs tricks at the Anyang International Aviation Sports Tourism Festival.
25. Xingtai, China
In 2016, the industrial steel city of Xingtai ranked as China's most polluted.
24. Hengshui, China
This chemical factory in Hengshui might feed Hebei province's booming economy, but it also contributes to the region's air problems, according to pollution researchers.
23. Karachi, Pakistan
Karachi is home to a large percentage of Pakistan's manufacturing industry.
22. Shijiazhuang, China
Some schools have snow days, but schools in Shijiazhuang have the occasional smog day.
21. Jinan, China
Young students wear face masks on a red-alert smog day in Jinan.
20. Ulaanbaatar, Mongolia
As recently as 2019, children have been evacuated from Mongolia's capital due to the toxic air conditions.
Grandparents and other family members take vulnerable young people to rural Mongolia — or to nearby countries — to escape the negative health impacts of breathing the air in Ulaanbaatar.
19. Baoding, China
Chimneys cloud the skyline of Baoding. Coal is used to power the city and surrounding villages. It ranks as China's most polluted city.
18. Narayangong, Bangladesh
Bangladeshi travelers cross the Shitalakhya River. The banks in the distance are obscured by pollution from the bustling brick-manufacturing industry.
17. Jodhpur, India
The old Blue City of Jodhpur is a popular tourist destination. But a lack of rain contributes to its poor air quality.
16. Patiala, India
Laborers harvest wheat in this city in the Indian state of Punjab.
15. Kampala, Uganda
Ugandan military police patrol a street in Kampala in 2016. Pollution researchers have cited vehicle emissions as a leading cause of air pollution in Kampala.
14. Jaipur, India
Monkeys admire their reflections atop a motorcycle. The smog shrouds the city in the distance.
13. Rawalpindi, Pakistan
This area of Pakistan is home to steel-rolling mills, leather tanning facilities, marble factories and oil refineries. All of these industries can, under certain conditions, negatively impact air quality. In this picture, a laborer is seen working at a brick kiln.
12. Peshawar, Pakistan
Another black cloud rises from a brick factory.
11. Muzaffarpur, India
According to a 2017 study by the British medical journal Lancet, more than 33 percent of deaths in the Indian state of Bihar, where Muzaffarpur is located, were caused by pollution-related diseases during the previous calendar year.
10. Gurugram, India
Many Fortune 500 companies, including Google and Intel, have offices in Gurugram, just southwest of New Delhi.
Despite its position as a wealthy, white-collar business hub, the air quality in the city is still very poor. The annual mean for dangerous PM2.5 pollution is 12 times the World Health Organization's recommended levels.
9. Agra, India
Pollution haze is visible in this photo of the Taj Mahal. The particulate matter in Agra is caused primarily by the practice of burning of scrap tires to extract iron.
8. Bamenda, Cameroon
Unlike many of the world's most polluted cities, deforestation and changing weather patterns are the primary explanation for this city's poor air quality. It is the most polluted city outside of India.
7. Lucknow, India
In response to growing pollution from vehicle emissions, Lucknow built a metro rail service. This photo shows the fanfare of the train's inaugural run.
6. Delhi, India
The Indian capital was once the most polluted city in the world. The city still struggles with pollution from the burning of nearby rice paddies, burning municipal waste, and vehicle exhaust emissions.
5. Patna, India
Hindu devotees return from a ceremony during Chhath Puja, a fall Hindu festival thanking the Sun God for sustaining life on earth. Thick smog obscures the sunset.
4. Varanasi, India
Varanasi is considered to be the spiritual capital of India. Dust from construction contributes to the poor air quality in the city.
3. Gaya, India
A train shines a light through heavy smog at a railway station in Gaya, a holy city in the Indian state of Bihar.
2. Faridabad, India
The smog levels in Faridabad spike during winter, when cold temperatures trap pollutants close to the ground.
1. Kanpur, India
Workers pile buffalo hides to be made into leather at a tannery in Kanpur. Leather tanneries can release dangerous chromium into the air.