Two cities 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.

The national capital took, as it often does, the top spot. It had an air quality index (AQI) figure of 420, putting it the 'hazardous' category, according to Swiss group IQAir.

But it 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.

An AQI level of 400-500 impacts healthy people and is dangerous to those with existing diseases, while a level of 150-200 brings discomfort to people with asthma, lung and heart problems. Levels of 0-50 are considered good.

A thick layer of smog had begun to circulate in New Delhi from Sunday night. 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.