Twenty-one of the world's 30 most polluted cities are in India, according to IQAir's 2019 world air quality report. The company's ranking of the most polluted cities comes as President Trump is in India, and is working to potentially ink a new trade deal with Prime Minister Narendra Modi.

"The new dataset highlights elevated air pollution levels as a result of climate change events, such as sandstorms and wildfires, and pollution gains from the rapid urbanization of cities, in regions such as Southeast Asia," IQAir said in a press release.

Ghaziabad, in northern India, was listed as the most polluted city in the world. It was followed closely by Hotan, China, two cities in Pakistan, and Delhi, India, which neighbors Ghaziabad. Four more Indian cities were listed in the top 10.

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According to IQAir, 90% of the global population breathes "unsafe" air, making air pollution "one of the biggest threats to human health."

"While the new coronavirus is dominating international headlines, a silent killer is contributing to nearly 7 million more deaths a year: air pollution," IQAir CEO Frank Hammes said in the press release.

IQAir, a Swiss company that specializes in air purification, draws from thousands of air quality monitoring stations by aggregating air quality data "from governments, private individuals and non-governmental organizations" in order to create the annual ranking.

The ranking is based on levels of "fine particulate matter," known as PM2.5, which pose the greatest risk to human health. These tiny particles are 2.5 micrometers or smaller. For scale, the average human hair is about 70 micrometers, according to the Environmental Protection Agency.

The World Health Organization's guideline for annual mean exposure to fine particulate matter is 10 micrograms per cubic meter (μg/m3). According to IQAir, "cities in India, on average, exceed the WHO target for annual PM2.5 exposure by 500%."

The 2019 average of fine particulate matter in Ghaziabad was 110.2 (μg/m3). For comparison, the city of Los Angeles had an annual average of 12.7 (μg/m3), while New York City's was 7 (μg/m3), according to IQAir.

Particulate matter is categorized as either PM10 — 10 micrometers or less, or PM2.5 — 2.5 micrometers or less. The larger particles can "lodge deep inside the lungs" according to the World Health Organization. The WHO warn, however, that the microscopically smaller particles of PM2.5 are even more dangerous to human health because they have the ability to "penetrate the lung barrier and enter the blood system."

According to the World Health Organization: "Chronic exposure to particles contributes to the risk of developing cardiovascular and respiratory diseases, as well as of lung cancer."

While some cities in India on average exceed the World Health Organization's target for annual PM2.5 exposure, total air pollution in the country actually decreased by 20% from 2018 to 2019, IQAir reports. "These improvements are believed to be largely a result of economic slow-down," according to the company.