The air quality index currently is in 'very poor' category, close to the 'severe' zone seen during Diwali - a level that triggers emergency measures, such as shutting down schools, in many countries. And unlike Diwali time, the contribution from farm fire in neighbouring states is very low.

Pollution has been rising since November 17, peaking to almost Diwali levels on November 21, 22 and 23, an analysis by SAFAR shows. These levels are worse than the same period last year and may remain high for a few more days.

While the PM 2.5 (fine, respirable pollution particles) levels were about 260 micrograms per cubic metre on Diwali day, pollution subsided because of moderate winds.

Now, relatively calm weather and dropping temperatures have brought down the boundary layer (lowest part of atmosphere), trapping pollutants near the surface. On November 21, the average PM 2.level was as high as 240 micrograms per cubic metre and on Monday it was about 220 micrograms per cubic metre, which is 3.6 times the national standard. Monday also saw a grey smog hanging over the city.

"There was no moisture in the air on November 21 and 22 which is why fog didn't form. But now, some cold winds have brought in moisture which helps accumulate particulate pollutants, which is making it very smoggy," said Gufran Beig, project director, SAFAR.

A study, commissioned to IIT Kanpur to identify local sources of pollution in Delhi, is yet to be made public. Ashwani Kumar, environment secretary told TOI that the department is likely to have a meeting with IIT scientists in December.

Kumar, however, acknowledged that vehicles are likely to be the major source of air pollution in winter followed by industry, thermal plants and others.

Delhi Pollution Control Committee's real time data showed PM 2.5 levels to be more than 300 micrograms per cubic metres at most monitoring stations such as Punjabi Bagh, Anand Vihar and R K Puram between 8am and 12.30pm on Monday. Nitrogen dioxide (NO2) levels were also more than 100 micrograms per cubic metres in most areas when the standard is only 80 micrograms, indicating the role of vehicles and traffic in the city.

Central Pollution Control Board's national air quality index bulletin showed "severe" air quality in Delhi for three consecutive days. In such cases an emergency is declared in Beijing, kindergartens and schools are closed. Industries are also directed to shut down temporarily.

Experts are calling for emergency measures in Delhi too. "We have noticed a very rapid build-up of air pollution in winter months because of inversion effect. We found between October 1 and November, PM 2.5 levels were not just several times the standard but also increased by seven times. We have to be very careful and this is the time to take preventive steps from the public health point of view," said Anumita Roychowdhury, head of CSE's clean air campaign.

Meanwhile, DPCC's monitoring of various pollutants in Dwarka during the car-free day on November 22 has shown it can have a huge impact on air quality. Nitrogen dioxide (NO2) levels, which can trigger immediate health impacts in people with asthma and other conditions, saw a reduction ranging from 20% to 55% in various parts of Dwarka.