Some unregulated industries pour smoke into the atmosphere

The city is regularly cloaked in thick fog as an array of traffic and heavy industries throws 3,000 tons of pollutants into the air every day.

The warning comes as a series of reports points to the soaring costs and impact of environmental pollution in the sub-continent.

A World Bank report released on Wednesday estimates that environmental degradation costs India around $80bn a year in terms of public health.

Leading scientists have warned that the invisible costs to the health and well being of children could be far higher. One estimate is that nearly 8,000 people a year die prematurely in Delhi because of respiratory conditions caused or aggravated by pollution.

The United Nations says the city, along with Calcutta, is one of the five most polluted metropolitan areas in the world.

Dirty water

Many rivers are heavily polluted

The River Yamuna, which provides Delhi's water, contains high levels of raw sewage and industrial waste - and is getting worse every year.

But vehicles are the main problem. Traffic jams produce more concentrated pollution than roads where vehicles flow efficiently.

Delhi alone has 2.5 million vehicles. The congestion on its streets is legendary.

Old, inefficient cars are the main cause of urban pollution

The city is gradually phasing out older vehicles and has made unleaded petrol compulsory, but the rules are difficult to enforce and many vehicles run on subsidised diesel rather than cleaner burning petrol.

The subsidies are meant for farmers, but motor manufacturers build even small cars with diesel engines - spurred on by India's recent economic growth.

Many environmentalists are now warning that the emerging middle-classes of Delhi will have to scale down expectations and start curbing their own polluting activities to avoid the prospect of the city becoming impossible to live in.