The ministry had filed an affidavit this week that claimed that vehicles contribute only 6.6 per cent of particulate matter (PM), tiny soot-like particles that can penetrate the lungs and at excess levels cause respiratory distress.

The environment ministry affidavit has attributed air pollution to dust from roads and construction activities, claiming that industry and power plants are responsible for 78 per cent of nitrogen oxides and 95 per cent of sulphur dioxide in the capital's air.

The Centre for Science and Environment (CSE) has questioned these claims. "We are deeply shocked at the callous and indifferent attitude of the ministry towards one of the most serious health crises looming in Delhi and other cities of India," Sunita Narain, the CSE director general, said in a media release.

The environmental NGO that has long been campaigning for government measures to curb the growth of vehicular traffic, particularly private vehicles, has said the ministry's affidavit appears intended to play down the role of vehicles.

"This (affidavit) protects the automobile industry and car users as combating vehicular pollution requires tough measures to restrain cars, encourage public transport and leapfrog vehicle technology," said Anumita Roychowdhury, the CSE executive director and head of the air pollution campaign. The CSE says there is abundant evidence, including Central Pollution Control Board observations, to link traffic with air pollution.

The CSE has said the ministry has assessed only the inventory of PM10 (particles less than 10 micrometres in size) and ignored PM2.5 (particles less than 2.5 micrometres in size) that can penetrate deep into the lungs and are a greater danger to human health.

The ministry filed the affidavit in response to a notice issued by the Supreme Court asking for government action to curb air pollution. An analysis by the CSE has shown that pollution levels are lower when traffic density on the roads is lower.

The CSE has pointed out that the ministry had in 2009 conducted its own study carried out by the School of Environmental Sciences at the Jawaharlal Nehru University, New Delhi, which had said vehicles contributed at least 60 per cent of PM at at least five busy sites in the capital.

The ministry has used another study by the National Environmental Engineering Research Institute, Nagpur, to claim vehicles do not contribute much to air pollution. But Roychowdhury claims that study was based on a flawed methodology.

The CSE has cited air pollution data from Delhi and Bangalore as evidence to support its argument that pollution levels drop on holidays when traffic density is lower than on working days. The PM2.5 levels, for example, on January 26, 2013, was 140 microgrammes per cubic metre, but jumped to 187 on January 27 and 216 on January 29.