New Delhi:The falling level of air pollution in winter has again started rising in the Capital, thanks to the increase in number of diesel cars. So much so that the rising number of diesel cars are undermining pollution control efforts after introduction of CNG for public transport.

An assessment by Centre for Science and Environment of peak pollution trends of winter months show a rapid build up in the last two months. Until 2003, these levels were headed south and Delhi breathed easy in the past couple of winters.

Sustaining the pollution control efforts for further reduction in pollution levels is turning out to be an enormous challenge due to the explosive increase in the number of vehicles in Delhi. Over the last decade, the number of cars on Delhi roads has doubled. Most of these are diesel-run one, which emit higher levels of toxic respiratory particles and nitrogen oxides.

Emission from rising number of diesel cars is undermining the beneficial effect of converting to CNG buses. "Over a lakh diesel cars on city's roads is equivalent to adding particulate emissions from nearly 30,000 diesel buses," the NGO said in a release on Wednesday. Pollution from diesel is making a comeback from personal cars, even after buses, three-wheelers and taxis switched over to cleaner fuel. A large number of these vehicles, as well as jeeps and trucks, are day commuters into the city. Not only do diesel vehicles emit more particulates, but these also emit three times more notrogen oxide.

The NGO advocates that the first generation of pollution control norms are not enough to curb rising pollution levels. Implementation of conversion of public fleet to CNG, introduction of lead-free petrol, low-sulphur diesel, lowering of benzene levels and phasing out ageing vehicles are but the first steps. It calls for speedy build-up of public transport and lower number of cars.

The clean-air activists also call for more dedicated bus and railway services between Delhi and the surrounding satellite towns to discourage commuting on cars.