Patna, India, which 24/Wall St. recently identified as the world’s most polluted city, has struggled with air quality recently. The Times of India wrote that more people in the city now are suffering from health problems.

The Times quoted S.N. Jaiswal, a scientist at Bihar State Pollution Control Board, when he said:

Concentration of dust particles and harmful pollutants may further increase in winter season as the pollutants remain trapped in the lower level of atmosphere when the temperature drops.

24/7 authors recently wrote the following in its “25 Most Polluted Cities In The World” report:

Patna is one of many urban areas in India, and Asia as a whole, that face some of the worst air pollution on earth. The rapid industrialization across parts of the continent 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.