By Jyotsna Singh
BBC News, Delhi
India's capital, Delhi, faces a winter of smog, posing a heightened risk of respiratory illnesses, a prominent environmental group has warned.
The winter haze arrived early this year in Delhi
The Centre for Science and Environment (CSE) says urgent and tough measures are needed to clean up the air.
The government introduced green fuel for public transport seven years ago to control air pollution.
But, the CSE says, the city's air quality is now worse than it was before the measures were taken.
The rise in pollution is being blamed mainly on a staggering increase in the number of vehicles in the city, particularly diesel-fuelled cars.
India's economy is booming and so is its car market, with nearly 1,000 new private vehicles being added to Delhi's roads every day.
The number is expected soon to jump further when Tata, India's biggest car manufacturer, launches the world's cheapest car at an expected price of 100,000 rupees (about $2,500).
Ms Dixit has vowed to make the capital pollution-free by 2010
"We will have to take tough measures to control growing air pollution and fast," CSE director Sunita Narain told the BBC.
"Otherwise Delhi will find itself choked [as] in the toxic haze of the pre-CNG [Compressed Natural Gas] days, when diesel-driven buses and auto-rickshaws had made it one of the most polluted cities on earth."
She says sustaining the pollution control efforts is proving to be an enormous challenge due to the unbridled increase of vehicles on the city's roads.
Over the past seven years the authorities have introduced measures to cut pollution, including imposing green fuel for new smaller commercial vehicles and phasing out polluting commercial vehicles.
The measures have had a visible impact on bringing down pollution levels, but data analysed by the CSE shows a rise in pollution levels since 2003.
Experts say the threat from pollution levels is at its greatest during winter when cold temperatures stop dust and smoke particles from dispersing into the atmosphere.
The low, hanging haze impairs visibility and chokes people's lungs, they say.
Delhi's thick winter haze arrived a month early this year.
It has enveloped the city for the past two weeks, with many residents complaining of coughing and sneezing.
The CSE says the high pollution levels lead to greater risks of respiratory illnesses such as asthma and bronchitis and also heart disease.
At a conference last week, Delhi Chief Minister Sheila Dixit underlined her government's efforts to provide a healthier and more comfortable environment in the city.
The pollution haze can cause serious health risks
She said they were in the process of finalising a policy that would help 40,000 smaller commercial vehicles convert from diesel to CNG.
Ms Dixit has vowed to make the capital pollution-free by 2010 - the year the city is due to host the Commonwealth Games.
She also urged people "to speak up against diesel-run cars".
Delhi has more than four million registered vehicles and privately owned ones have risen more than 100% since 2000.
The Society for Indian Automobile Manufacturers says diesel cars are expected to continue to rise in number - by another 50% in the next three years.
That, says the CSE, threatens to undo efforts to check pollution because diesel is more toxic than other types of fuel.
Delhi's government also wants to bring on board their counterparts in the adjoining states to join efforts to control pollution.
"The pollution level in Delhi also goes up due to the entry of diesel vehicles from other states which are generally just transiting through the city," Ms Dixit says.
The Centre for Science and Environment wants a ban or higher taxes on diesel vehicles.
"Delhi leapfrogged once by introducing CNG, it can do so once again," says Sunita Narain.
Otherwise, she says, Delhi is set to face an air pollution disaster.