Delhi's air quality would remain in the severe category till Saturday and the already toxic situation caused by smoke from fireworks is likely to aggravate further due to intensified stubble burning in neighbouring states, government-run agency SAFAR said Friday. Delhi's air quality showed "significant" improvement as compared to Thursday when it went off the charts to the "severe plus emergency" category. The city recorded the overall air quality index of 642, according to System of Air Quality and Weather Forecasting And Research (SAFAR). On Friday, the AQI was recorded in the severe category at 426. "As per satellite images, heavy fire counts were recorded in the last 24 hour and the latest SAFAR model results show a movement of cold front carrying heavy air mass towards Delhi region," SAFAR said. Elaborating on it, the weather forecast system said the air in the surrounding stubble-burning areas is already heavy due to increased moisture and pollutants.This heavy air will travel towards Delhi, where a similar situations exists, in the next 24 hours. This combined effect might add to the pollution woes. "There is a possibility that the wind may pick up at the upper level, late in the night and push the level of pollution and keep it in severe zone tomorrow with intensified fog which will further trap the pollutants," it said. The entry of heavy and medium goods vehicles into the national capital has already been banned for three days beginning 11 pm on Thursday night as a measure to check air pollution in Delhi.