The report by IQAir, a Swiss air technology company, said India's Byrnihat was the world's "most polluted metropolitan area of 2024".
The Indian capital New Delhi was the world's most polluted capital, closely followed by Chad's N'Djamena. The Bangladesh capital Dhaka came third, ahead of Kinshasa and Islambad.
Chad topped the rankings for the most polluted country overall, with readings 18 times higher than World Health Organisation safe levels for concentrations of particles less than 2.5 microns in diameter (PM2.5)
It was followed by Bangladesh, Pakistan, Democratic Republic of Congo and then India in fifth place. India was home to six of the world's nine most polluted cities.
Byrnihat, an industrial town on the border of Meghalaya and Assam states, had a PM2.5 reading of 128.2 micrograms per cubic metre on average in 2024, more than 25 times the WHO recommended level of five micrograms.
Concentrations across India were 50.6 micrograms per cubic metre, 10 times the WHO safe level, according to the report by IQAir, made with Greenpeace's support.
The overall level was seven percent down from 2023, but Indian cities are still suffering badly from concentrations of small particles, which come from vehicles, agricultural burning, garbage and industrial waste.
South Asia's pollution skyrockets in winter months, and highlighting India's struggle, the report said that Baddi in Himachal Pradesh state had an average reading of 165 microns in January -- 33 times the WHO safe level.
It said that five Pakistani cities saw levels rise above 200 microns during November.
The study was based on "more than 40,000 air quality monitoring stations across 8,954 locations in 138 countries, territories, and regions analyzed by IQAir's air quality scientists," it said.
"Oceania is the world's cleanest region, with 57% of regional cities meeting the WHO PM2.5 annual guideline value," the report said.
Bosnia was the worst polluted country in Europe, with PM2.5 levels more than five times over the WHO limit. It was followed by North Macedonia and Serbia. Serbia's Novi Pazar was the most polluted city, the report said.
Burkina Faso, 5th in 2023, as well as Iran and Afghanistan, were not included in the report due to insufficient data.
Only seven countries had concentrations below the WHO guidelines: Estonia, Iceland, Australia, New Zealand, the Bahamas, Grenada, and Barbados.
However, 17 percent of cities studied met the WHO standard in 2024, compared with just nine percent in 2023, the report said.
Air pollution -- atmospheric and domestic -- was the main environmental risk to health in 2021, responsible for 8.1 million premature deaths worldwide, according to estimates in the "State of Global Air 2024" report carried out by the Health Effects Institute and the Institute for Health Metrics and Evaluation.
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