Delhi's air quality has started deteriorating ahead of Deepavali, when pollution levels peak every year.

During the Hindu festival of lights, which is celebrated in India on Nov 7, firecrackers are set off, adding to the pollution.

The Air Quality Index showed that air quality was at the "hazardous'' level in one part of Delhi and at the "unhealthy'' level in other parts yesterday afternoon.

A haze has started engulfing Delhi and its surrounding areas, with Chief Minister Arvind Kejriwal warning that the city would become a "gas chamber soon", as the burning of crop stubble has begun in neighbouring states. Farmers in Haryana and Punjab burn their crop stubble from the end of this month to prepare their fields for the sowing of winter crops.

"The weather conditions are favourable for further deterioration in pollution levels. Unless there are strong winds in the category of at least 20kmh, this pollution won't go away," said Mr Mahesh Palawat, vice-president of meteorology and climate change at Skymet Weather Services. "The pollution from Deepavali firecrackers will add to it unless there is rain."

Delhi is one of the world's most polluted cities. This is due to several factors: Rapid growth in the number of vehicles, with many middle-class families owning more than one car, preference for cars running on diesel, construction in and around Delhi, and the burning of stubble, apart from open burning of waste.

Pollution also worsens during winter as falling temperatures trap pollutants in the atmosphere.

The environment ministry is implementing a Comprehensive Action Plan for the first time to combat air pollution in Delhi and its surrounding areas in the long run. It consists of measures such as stepping up air quality monitoring, as well as tackling emissions from vehicles, power plants, industries and construction works .

In April, Delhi switched to Bharat Stage VI, which is equivalent to the European Union's Euro VI vehicle emission standards adopted by Singapore last year.

"Delhi needs to reduce pollution by 74 per cent to meet clean air standards. The challenge is the capability and capacity of the government to do proper enforcement," said Ms Anumita Roy Chowdhury, executive director of the Centre for Science and Environment.