Cities like Lahore (Pakistan), Hotan (China) and Delhi (India) have consistently poor air quality, but it’s impossible to pinpoint the city with the worst air pollution in the world. A day or two after the start of wildfire season, cities like New York, Chicago, San Francisco, Sacramento, and Seattle earned the dubious title of having the worst air quality in the world, according to real-time data from weather sensor company IQAir.

But most of these cities don’t usually have such polluted air. So which cities have the worst air quality and why?

Air pollution is usually measured by the concentration of particulate matter (PM) or a mixture of solid and liquid droplets suspended in the air. According to the latest data from the World Health Organization (WHO) on PM2.5 pollution, or respirable particles smaller than 2.5 micrograms, hundreds of cities have air year-round that the U.S. Environmental Protection Agency describes as unhealthy for only one day.

An IQAir report revealed that Lahore, Pakistan, had the worst air quality in 2022. Hotan in China came second, followed by the Indian suburb of Delhi. The three most polluted cities reached India in 2021; Delhi took the first place among world capitals, followed by Bangladesh’s Dhaka and Chad’s N’Djamena. In all these cities, more than 90 micrograms of PM2.5 per cubic meter were detected on average throughout the year; this was almost 20 times the recommended level.

While these rankings do reveal some air pollution hotspots, it’s impossible to tell which cities actually have the worst weather. The quality of the sensors used varies and many African countries are not listed as they do not report any data. The WHO only uses high-quality sensors but doesn’t even try to rank the cities: A spokesperson told Live Science that collection procedures differ too much.

Even the measurable indicator PM2.5 has its limitations. Inhalation of fine particles can cause health problems, and very few cities meet WHO’s most stringent requirements for PM2.5. But the chemical composition of these particles can also affect health. A WHO representative said the research is not conclusive, but it is possible that cities with the same total PM2.5 levels could have different health outcomes depending on the source of the pollution.

There are several reasons why some cities have more PM2.5 than others. One is geography. Hotan is near the Taklamakan desert and dust storms are frequent. N’Djamena is also near the southern tip of the Sahara Desert. Mountains can affect air quality by making it difficult for pollutants to disperse: Delhi, Lahore, Dhaka and many other cities with unhealthy air are located in the south of the Himalayas.

For example, in Delhi, Umesh Kulshrestha, Dean of the Faculty of Environmental Sciences at Jawaharlal Nehru University in New Delhi, said, “When you touch anything, even if you clean it every day, you’re left in dust.” Infrequent rains outside of the monsoon season contribute to dust accumulation not only from the remote deserts but also from dirt roads and construction work in Delhi.

Also, Delhi’s PM2.5 pollution is caused by fossil fuels. Fossil fuels are harming air quality in cities around the world. It indirectly increases global temperatures, which means more forest fires and more green spaces turning into deserts. Both of these processes add PM2.5 to the air. Emissions from vehicles, power plants and industrial sources also directly pollute cities at the local level.

In a review in the journal Aerosol and Air Quality Research, Kulshrestha found that Delhi’s air quality improved and its index dropped 41%, thanks to reductions in vehicle and industrial emissions during the pandemic lockdown in early 2020.

Kulshrestha told Live Science that other common sources of PM2.5 in South Asia are brick ovens, crop burning and biofuels, with many households still using it for cooking and heating. He said India is working to increase the use of renewable energy sources and replace biofuels with liquefied natural gas, which could reduce PM2.5 in homes while contributing to global warming.

The US remains the second largest consumer of fossil fuels after China, contributing to the deterioration of air quality worldwide. However, several decades ago, persistent smog in the Los Angeles basin in California led to the passage of the Clean Air Act, which has since improved air pollution levels in the United States.

Similarly, today’s most polluted cities are taking steps to protect the skies. In Dhaka, banning inefficient, polluting two-stroke engines in three-wheelers has helped keep PM2.5 levels in vehicle emissions stable as the city grows. While many of China’s cities are among the most polluted, the past decade has seen a dramatic reduction in pollution since the country introduced clean air policies. In Delhi Kulshrestha insists on creating artificial lakes. More water should mean more evaporation, more rain, and at least slightly less dust, according to Kulshrestha’s research. Source