All but one of the world’s top 20 most polluted cities last year were in Asia, a new study shows.

The majority of these cities – 13 – are in the world’s most populous country, India, where booming economic growth is fired largely by coal and where hundreds of millions live in traffic-clogged and congested megacities.

Another four are in neighboring Pakistan, with one in China and Kazakhstan respectively.

The only city outside of Asia featured on the list is N’Djamena, the capital of Chad in central Africa – which was named the country with the worst air pollution.

Meanwhile the cities with the worst pollution in North America were all in California.

The report by IQAir, a Swiss company that tracks global air quality, looked specifically at fine particulate matter, or PM2.5, one of the smallest but most dangerous pollutants.

PM2.5 comes from sources like the combustion of fossil fuels, dust storms and wildfires. It is so tiny – 1/20th of a width of a human hair – that it can travel past your body’s usual defenses into your lungs or bloodstream.

The particles cause irritation and inflammation and have been linked to respiratory problems and chronic kidney disease. Exposure can cause cancer, stroke or heart attacks and has been associated with a higher risk of depression and anxiety.

The World Health Organization (WHO) says average annual levels of PM2.5 should not exceed 5 micrograms per cubic meter.

Byrnihat, an industrial town in northeast India recorded a PM2.5 concentration of 128.2 last year – more than 25 times the WHO’s standard.

She blamed factories around the town and a booming construction industry and trees being felled as contributing to the toxic air.

“The pollution is particularly bad right now, visibility is not great, there is dust everywhere, my eyes also burn,” she said.

“I do not leave home without a mask.”

Twelve other cities in the top 20 are in India.

Its capital New Delhi featured as the world’s most polluted capital for the sixth consecutive year, with a PM2.5 concentration of 91.8. The report also listed six satellite cities – Faridabad, Loni, Delhi, Gurugram, Noida and Greater Noida – making the list.

Just last November, a throat-searing blanket of smog settled over Delhi, disrupting flights, blocking buildings from view and prompting the city’s chief minister to declare a “medical emergency.”

But overall, India – the world’s most populous nation with 1.4 billion people dropped from third to fifth place from the previous year, according to the report.

But the report said air pollution “remains a significant health burden… reducing life expectancy by an estimated 5.2 years.”

India’s neighbors Bangladesh and Pakistan – together home to some 400 million people – were second and third-most polluted countries globally in terms of PM2.5 molecules, according to the report.

China – which used to dominate global rankings of the world’s worst air – noted a small improvement, the report said.

Its national annual average PM2.5 concentration decreased from 32.5 micrograms per cubic meter to 31, with air quality improving in megacities like Beijing, Shanghai, Chengdu, Guangzhou and Shenzhen, the report said.

China is the world’s largest carbon dioxide emitter but in recent years has waged a campaign against air pollution, particularly in the cities that have fuelled its economic growth, and has pushed a massive expansion in solar and wind power.

But last month two clean-energy groups raised alarm over what they said were plans by China’s power industry to build nearly 100 gigawatts of new coal plant capacity last year, the most in nearly a decade.

All 20 of the world’s most polluted cities last year exceeded WHO PM2.5 guidelines by over 10 times, the IQ Air report showed.

Data gaps

“Air pollution remains a critical threat to both human health and environmental stability, yet vast populations remain unaware of their exposure levels,” said Frank Hammes, Global CEO of IQAir.

Iran and Afghanistan did not feature in this year’s report due to a lack of data availability.

Air quality monitoring in Southeast Asia is also a problem, with nearly all countries having “significant gaps in government-led initiatives,” the report found.

In 2024, 173 out of 392 cities in the region lacked government monitoring stations, while Cambodia had none, it said.

Those problems are likely to be exacerbated after the US announced earlier this month that it would stop sharing air quality data gathered from its embassies and consulates worldwide due to “funding constraints” the Associated Press reported.

“Air quality data saves lives,” said Hammes.

“It creates much needed awareness, informs policy decisions, guiding public health interventions, and empowers communities to take action to reduce air pollution and protect future generations.”

Worst cities in North America

Only 17% of 8,954 cities analyzed globally by IQAir recorded air quality which met WHO pollution guidelines, the report said.

The cities with the worst air pollution in North America were Ontario, Bloomington and Huntington Park – all in California, the report said.

Overall the United States saw a significant reduction in PM2.5 levels last year, with the annual average dropping 22% from 2023.

Northern America has long boasted vigorous air quality monitoring systems, contributing 56% of the total number of ground-based air quality monitoring stations included in the IQ Air report – helping scientists with their continued research on air quality and aiding policymakers to make decisions about public health.

Only 12 countries, regions, and territories recorded PM2.5 concentrations below the WHO guidelines, most of which were in the Latin America and Caribbean or Oceania region.

The report called on governments to dedicate funding for renewable energy projects and “strengthen emission limits for vehicles and industrial activities.”

Advice Suman wishes authorities in Byrnihat would take to save her city from appearing at the top of the most polluted list again next year.

“This is my birthplace. I am a local. I do not want to leave this area. We want the governments to do more, come together and work for us.”