A thick layer of toxic smog engulfed Delhi as the pollution level continued to remain in the ‘severe’ category for the fourth consecutive day on Friday. The high pollution can be attributed to low wind speed, which has led to stagnation of pollutants in the air and slowed down the dispersion process, authorities said.

The air quality index (AQI) in Delhi was in the severe category at 467 at 10 am. All the monitoring stations recorded severe air quality, according to the Central Pollution Control Board. Levels of PM 2.5 and PM 10 were nearly eight times higher than normal at several locations. Schools remained closed on Friday as the thick smog enveloped Delhi and neighbouring cities.

In the National Capital Region, Faridabad, Ghaziabad, Greater Noida and Gurgaon also recorded severe air quality, the CPCB said. Ghaziabad recorded the highest pollution level at 480, remaining points away from going off charts, the CPCB data showed. An AQI between 201 and 300 is considered ‘poor’, 301-400 ‘very poor’ and 401-500 ‘severe’.

The Ministry of Earth Sciences’ air quality monitor, SAFAR, said flushing of pollutants was almost insignificant due to stagnation and very low mixing height. Scattered rainfall is expected over Punjab due to the western disturbance. It is likely to lead to very low stubble burning incidents over the next two days, the System of Air Quality and Weather Forecasting And Research said. “Also, transport-level wind direction is not favourable for intrusion. Hence, stubble contribution is estimated to reduce significantly by tomorrow to around 5 per cent,” it said.

However, an increase in wind speed is forecasted by Saturday and it will likely to slightly improve the air quality, but it will still remain in the very poor category. “Further improvement is expected by Sunday,” the SAFAR said.