What are the most polluted cities in the world?

For a day or two during wildfire season, cities like New York, Chicago, San Francisco, Sacramento and Seattle earn the dubious title of worst air quality in the world, according to real-time data from air sensor company IQAir.

But most of these cities don't typically have such polluted air. So which cities regularly have the worst air quality, and why?

Air pollution is often measured in terms of the concentration of particulate matter (PM), or a mix of solid and liquid droplets suspended in the air. Hundreds of cities have air year-round that the U.S. EPA considers unhealthy for just one day, according to the World Health Organization's (WHO) most recent data on PM2.5 small particle pollution, or those inhalable particles smaller than 2.5 micrograms.

An IQAir report found that Lahore, Pakistan had the worst air quality in 2022. Hotan in China ranked number two, followed by a suburb of Delhi, India. In 2021 the top three most polluted cities were in India; among the world’s capital cities, Delhi ranked first, then Dhaka, Bangladesh and N'Djamena, Chad. These cities all averaged more than 90 micrograms per meter cubed of PM2.5 over the course of the year, or nearly 20 times the recommended level.

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Despite these rankings revealing some air pollution hotspots, it's impossible to determine which cities really have the worst air. The sensors used vary in quality, while many countries in Africa don't appear on the list because they don't report any data at all. The WHO uses only high-quality sensors, but it does not even try to rank cities: A spokesperson told Live Science that collection procedures vary too widely.

Even the metric being measured, PM2.5, has limitations. Inhaling small particles can cause health problems, and very few cities meet the WHO's most stringent PM2.5 guideline. But the chemical composition of these particles may also impact health. A WHO spokesperson said that research is not conclusive, but it is possible that cities with the same PM2.5 levels overall might have different health outcomes depending on the source of pollution.

There are several reasons why some cities have more PM2.5 than others. One is geography. Hotan is near the Taklamakan desert and experiences frequent dust storms. N'Djamena, likewise, is near the southern edge of the Sahara Desert. Mountains can impact air quality by making it harder for pollutants to disperse: Delhi, Lahore, Dhaka, and many other cities with unhealthy air lie just south of the Himalayas.

In Delhi, for instance, "just touch anything and you will have the dust, even if you clean every day," said Umesh Kulshrestha, the former dean of the School of Environmental Sciences at Jawaharlal Nehru University in New Delhi. Infrequent rain outside of the monsoon season helps dust accumulate — not only from far-off deserts, but also from unpaved roads and construction work in Delhi itself.

In addition, Delhi’s PM2.5 pollution also comes from fossil fuels. Fossil fuels harm air quality in cities worldwide. Indirectly, they increase global temperatures, which means more wildfires burn and more green spaces become deserts. Both those processes add PM2.5 to the air. Emissions from vehicles, power plants, and industrial sources also directly pollute cities locally. In a review in the journal Aerosol and Air Quality Research, Kulshrestha found that during the pandemic lockdowns in early 2020, Delhi's air quality improved, with its index score going down by 41%, thanks to reduced vehicle and industrial emissions.

Kulshrestha told Live Science that in South Asia, other common PM2.5 sources include brick kilns, crop burning, and the biofuels that many households still rely on for cooking and heating. He said that India is working to increase renewable energy sources and to replace biofuels with liquid natural gas — while this contributes to global warming, it can lower household PM2.5.

The U.S. remains the world's second-largest consumer of fossil fuels after China, contributing to poor air quality worldwide. Decades ago, though, persistent smog trapped in California's Los Angeles Basin prompted the Clean Air Act, which has since improved air pollution across the U.S.

Likewise, today's most polluted cities are also taking action to protect their skies. In Dhaka, bans on inefficient, highly-polluting two-stroke engines in three-wheeled vehicles helped keep PM2.5 levels from vehicle emissions steady as the city grew. Even though many Chinese cities are among the most polluted, there has been a dramatic decrease in pollution over the past decade since the country implemented clean air policies. In Delhi, Kulshrestha is pushing to create artificial lakes. More water should mean more evaporation, more rain and — Kulshrestha's research predicts — at least a little less dust.

Editor's Note: This story was updated on Sept. 8, 2023 to note that Umesh Kulshrestha is former, not current dean of the School of Environmental Sciences at Jawaharlal Nehru University in New Delhi.