These three Indian cities are among the world's 10 most polluted after Diwali

City of joy Kolkata and financial capital of Mumbai have joined New Delhi to be among the world's worst 10 for pollution on Monday morning, with smoke heavy in the air a day after revellers let loose with firecrackers for Diwali.

According to Reuters, citing Swiss group IQAir, New Delhi took, as it often does, the top spot with an air quality index (AQI) of 420, putting it in the 'hazardous' category.

The national capital was also joined in the top 10 by Kolkata, which came in fourth with an AQI of 196, while Mumbai was eighth with an AQI of 163.

A level of 400–500 on the AQI affects healthy individuals and poses a threat to those who already have illnesses, whereas a level of 150–200 causes pain for those who have heart, lung, and asthma issues. A score of 0 to 50 is regarded as good.

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A thick layer of smog had begun to circulate in New Delhi from Sunday night, sending its AQI to an alarming 680 a little after midnight.

Every year authorities impose bans on firecrackers in the capital, but only rarely do those bans appear to be enforced.

Air quality in India deteriorates every year ahead of winter, when cold air traps pollutants from vehicles, industry, construction dust and agricultural waste burning.

New Delhi's authorities postponed an earlier decision to restrict use of vehicles after a brief spell of rain on Friday brought some respite from a week-long exposure to toxic air.

The government plans to review the decision after Diwali.

With inputs from agencies