The World Health Organization sets the target concentration of PM2.5 at an average of 10 micrograms per cubic meter per year. Annual average PM2.5 concentrations in the world’s most polluted cities range from 86 micrograms per cubic meter in Zhengzhou, China to 149 micrograms in Patna, India. For comparison, the annual average PM2.5 concentration is 18 micrograms per cubic meter in the Visalia-Porterville metro area. In Los Angeles, the PM2.5 concentration is 11 micrograms per cubic meter.
We also included PM10 pollution, or concentration levels of larger inhalable particles that are 10 micrometer in diameter or smaller. While the finer PM2.5 particles can get deep into lungs and even into the bloodstream, the coarser PM10 particles are of less harmful, although they can irritate a person’s eyes, nose, and throat.
Of the 25 cities on this list, 10 are located in China. Polluted air in cities across China is largely due to the country’s rapid industrialization and growing middle class. Certain manufacturing sectors, steel in particular, can be especially harmful to air quality, and China accounts for about half of all steel production globally.
Some major steel-producing cities in China have been forced to temporarily halt or reduce manufacturing operations to improve air quality. Tangshan, the largest steel producer in the country and the world’s 13th most polluted city, was forced to enact emergency procedures that included reducing steel production in order to address a period of especially heavy air pollution in late 2017.
Similarly, the city of Handan, China ordered steel making operations to cut production by one-quarter from April until November 2018 in an effort to reduce pollution and improve air quality. Handan ranks as the seventh most polluted city in the world and the fourth most polluted in China.
Rapid economic growth and consequently improved standards of living are another factor in the high concentrations of pollution in Chinese cities. China’s annual GDP growth has been at least 6% since 1991, reaching as high as 14.2% in both 1992 and 2007. As the country becomes more affluent, personal automobiles are more affordable and more cars on the road in a country of 1.4 billion means worsening air quality.
For a handful of cities on this list outside of China, including some of those in Africa and Central Asia, pollution is largely attributable to underdevelopment. In Kabul, Afghanistan, for example, only about one in every 50 homes are connected to a sewage system, leaving most household waste to flow and often dry in the streets — much of it ultimately becomes harmful dust. Adding to the problem, many in the Kabul burn animal dung to heat their homes.
In Kaduna, Nigeria, in the absence of access to an electricity grid, many residents rely on generators, which spew out harmful fumes. The city is also without sound waste management infrastructure, resulting in many in the city simply burning their garbage.
To identify the most polluted cities in the world, 24/7 Wall St. ranked nearly 3,000 cities worldwide based on the annual average concentration of PM2.5 per square meter of air. Only cities for which PM2.5 concentration was directly measured and not imputed from other measures of air pollution were considered. Though it was not factored into the ranking, we also looked at PM10 concentration. Pollution data came from the World Health Organization. Population figures came from the United Nations’ report The World’s Cities in 2016. For cities not included in U.N. report, population estimates came from a range of sources.