Indian cities top the world’s worst polluted list once again, according to a new report while several Chinese cities, including Beijing, have dramatically improved their air quality in recent years.

Twenty-one of the world's 30 cities with the worst air pollution are in India, according to data compiled in IQAir AirVisual's 2019 World Air Quality Report, with six in the top ten.

Despite new government policies meant to address the issue, New Delhi’s air quality has fallen from where it was five years ago, rising to the fifth-worst spot globally and making it by far the world’s most polluted major city, the report said.

The worst-ranked city -- Ghaziabad -- is a Delhi suburb, as are a number of others ranked separately in the top 20.

On the other hand, Beijing once infamous for its toxic haze has reduced smog levels and dropped down a list of the world’s most polluted cities, falling to 199 from 84 three years before, according to the 2019 World Air Quality Report.

India, China and other Asian countries remain disproportionately affected by toxic air as a result of factors ranging from crowded cities, vehicular exhaust, coal-fired power plants, agricultural burning and industrial emissions.

Although many Indian cities saw progress between 2018 and 2019, "unfortunately these improvements are not representative of the very recent, but promising National Clean Air Programme" and cleaner fuel standards, according to the AirVisual report.

Cities across Asia -- including Chiang Mai, Hanoi, Jakarta and Seoul -- saw sharp increases in PM 2.5 levels. Since 2017, Jakarta saw pollution increase by 66%, making it the worst in Southeast Asia.

In Thailand, Chiang Mai and Bangkok both saw a number of extremely smoggy days -- some of which led authorities in the capital to close schools -- resulting from construction, diesel fuel and crop fires in surrounding regions.

The problem is particularly challenging for South Asian countries. Using a weighted population average, Bangladesh was actually ranked the world's most polluted country, while its capital Dhaka was the second-worst after Delhi. Pakistan was the second-most-polluted country, while Afghanistan, India and Nepal were all in the top 10.

(Originally published in Bloomberg)