The air quality in the national capital turned "severe" on Friday, the first time in 20 days, with cold weather and calm winds allowing accumulation of pollutants.
The city recorded its overall air quality index at 404 at 4 pm, as a cloud of toxic smog continued to linger over it for the second consecutive day. The last time Delhi''s AQI was recorded in the ''severe'' zone was on November 15.
The air quality in the suburbs of Faridabad (450), Ghaziabad (471), Greater Noida (442), Noida (469) and Gurgaon (460) dropped further.
An AQI between 0-50 is considered ''good'', 51-100 ''satisfactory'', 101-200 ''moderate'', 201-300 ''poor'', 301-400 ''very poor'' and 401-500 ''severe''. An AQI above 500 falls in the ''severe plus'' category.
The levels of PM2.5 -- particulate matter so small that they can enter the lungs and even the bloodstream -- shot up to 230 micrograms per cubic metre, around four times the safe limit of 0-60, by 6 pm.
Weather experts said the period of stubble burning had almost ended, and the smog that continue to shroud the city was mostly local pollutants and moisture.
Falling temperatures and calm winds were allowing accumulation of pollutants and the situation may persist for another two to three days. Any significant improvement is likely only after December 10, they said.
The government's air quality monitoring and forecasting centre, System of Air Quality and Weather Forecasting And Research (SAFAR), said smoke from stubble burning accounted for three per cent of Delhi's PM 2.5 pollution on Friday and it is likely to drop to two per cent on Saturday.
The AQI is predicted to oscillate between "very poor" and "severe" categories until December 7.
"A fresh western disturbance is likely to affect northwest India by December 11 and improve ventilation. Hence, there is no significant relief till December 10," SAFAR said.
After the first episode of smog in November, the Supreme Court banned construction activities in the region till further orders. A CPCB official said the ban imposed on hot mix plants, stone crushers and diesel generator sets by the Environment Pollution (Prevention and Control) Authority also continues.
The EPCA had, however, lifted the restrictions on the coal-based industries in NCR and non-PNG one in Delhi on November 16.
A CPCB-led task force had on Thursday asked all agencies in Delhi-NCR to remain on high alert and to take stringent measures to control air pollution. It asked them to intensify enforcement activities in hot spots and industrial areas and recommended people to minimise use of private vehicles.