The rapid industrialization across parts of Asia in recent years has been an economic boon. Unfortunately, industrialization and economic development are often at odds with environmental and public health concerns. In some parts of the world, a booming manufacturing sector and rapidly growing middle class have led to dangerous and potentially deadly levels of air pollution — often more than 10 times the safe standard set by the World Health Organization.

One specific type air pollution, commonly known as PM2.5 — shorthand for particulate matter 2.5 micrometers in diameter or smaller — is especially harmful. For perspective, the diameter of a human hair is 50-70 micrometers, so particles that are 2.5 micrometers or smaller are very fine and inhalable.

PM2.5 is typically generated by combustion — either from fires, automobiles, or factories — and this type of air pollution has been linked to a number of health issues, including asthma, lung infections, heart attacks, and premature death.

Heavy industrialization and a rising standard of living are the most common causes of high concentrations of air pollution in the cities on this list — though they are not the only culprits. In some of the most polluted cities, high concentrations of fine particulates in the air are the result of lacking or non-existent public works infrastructure, such as sewage systems.

24/7 Wall St. reviewed the average annual concentration of PM2.5 per cubic meter in nearly 3,000 cities around the globe to identify the world’s most polluted cities. The vast majority of cities with the highest concentrations of PM2.5 are in Asia, and a handful are in Africa. In each of the cities on this list, air pollution is at least 4.8 times worse than it is in the Visalia-Porterville, California, metro area, the most polluted metro area in the United States.

Click here to see the world’s most polluted cities.
Click here to see our detailed findings and methodology.