After Diwali, New Delhi, Mumbai, Kolkata among world's 10 most polluted cities

Two Indian cities joined New Delhi to become among the world's top 10 most polluted cities on Monday, with smoke heavy in the air a day after revellers let loose with firecrackers for Diwali, the annual Hindu festival of light.

The capital New Delhi took, as it often does, the top spot. It had an air quality index (AQI) figure of 407, putting it the "hazardous" category, according to Swiss group IQAir.

Financial capital Mumbai came in sixth with an AQI of 157, while Kolkata in the east was seventh with an AQI of 154.

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 started to circulate in New Delhi from Sunday night, sending its AQI to an alarming 680 a little after midnight.

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

Lawmaker Saket Gokhale posted a letter on X, formerly known as Twitter, in which he had asked the local police for details on number of cases of the use firecrackers and the action taken against the perpetrators.

The Delhi police spokesperson did not answer several calls made by Reuters asking for comment.

Air quality in northern 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 the use of vehicles after a brief spell of rain on Friday brought some respite from a week of toxic air.

The local government said it plans to maintain its ban on construction activities and to keep schools closed to protect people from the pollution.