New Delhi, Dec. 21 -- The Delhi government on Saturday threatened severe action against those violating curbs under the Stage-4 of the Graded Response Action Plan (Grap) to curb pollution, as Delhi once again was in the grip of "severe" air quality. The air quality index (AQI) was 398 ("very poor") at 4pm, according to the Central Pollution Control Board's (CPCB's) daily bulletin. By 6pm, it deteriorated to 405 ("severe") and then to 410 by 11pm. Delhi's environment minister Manjinder Singh Sirsa said, "Grap Stage-4 continues to be in place but there have been complaints of construction work being carried on in several parts of Delhi." He said that strict action will be taken against those who are not complying with curbs under Stage 4 of Grap and warned of the most stringent action possible against those violating the rules. "We have also done extensive survey on any industries causing pollution in Delhi and strict action will be taken against them as well from Sunday. We will not let anyone take the health of Delhi's people lightly," he said. The Municipal Corporation of Delhi (MCD) said it has levied fines amounting to Rs.33.95 lakh for violation of construction and demolition norms over the past week. Further, it has verified 1,792 sites and issued 771 challans to violators. In November, it levied over 900 challans, with fines amounting to Rs.1.5 crore. Stage-4 of Grap, the most stringent of the four stages, remains enforced across Delhi and the National Capital Region (NCR). Under it, non-Delhi registered vehicles with emission standards under BS-6-which was implemented in 2017-are not allowed to enter the Capital. Similarly, construction is already banned under Grap Stage-4, and schools have been instructed to conduct hybrid classes, except for classes 10 and 12. The Capital's air quality deteriorated throughout Saturday, and returned to the "severe" category after a brief spell of "very poor" air days. While the last few days could hardly be considered a respite, the situation is expected to worsen further with a sudden dip in temperature and unfavourable meteorological conditions compounding the pollution problem. Delhi clocked an AQI of 384 ("very poor") at 8am and 391 at 11 am, before it continued to fall to the "severe" category by the evening. So far, Delhi has already recorded six "severe" air days this season; three from December 13 to 15, and three from November 11 to 13. The Commission for Air Quality Management (CAQM) classifies an AQI above 450 as "severe plus" for the purposes of Grap implementation. According to forecasts by the Air Quality Early Warning System (AQEWS) for Delhi, the AQI is expected to be in "severe" category until Monday. "The air quality is likely to be in the 'severe' category from Sunday to Monday. The air quality is likely be in the very poor category on Tuesday. The outlook for the subsequent six days is that the air quality is likely to be in the 'very poor' to 'severe' category," the AQEWS bulletin said on Saturday evening. Sunil Dahiya, founder and lead analyst at Envirocatalysts, said, "While just a dip in temperature does not necessarily mean that it is going to cause a spike in pollution levels; however, if combined with low wind speed, that can lead to a build-up of pollution. The latter is precisely what is expected in the Capital in the next two days. The temperature is expected to remain low and wind conditions will remain calm, leading to a spike in pollution levels." Meanwhile, 22 of the working 40 air quality monitoring stations in Delhi clocked "severe" air on Saturday evening, while 17 recorded "very poor" air. The outlier was the AQI monitoring station on Mathura Road, which clocked "poor" air throughout the day, despite being located next to major traffic corridors. The AQI reading at 6pm, a peak traffic hour, was 265. The India Meteorological Department, which runs the station, did not respond to HT's queries on the accuracy of the reading....