The CPCB said seven areas in Delhi — Ashok Vihar, Anand Vihar, Jahangirpuri, Mundka, Rohini, Vivek Vihar and Wazirpur — recorded 'severe' air quality. Twenty areas in the national capital recorded 'very poor' air quality while it was 'poor' in six areas, it said.

New Delhi: Delhi's air quality remained in the 'very poor' category on Thursday due to "unfavourable meteorological conditions" that slowed down dispersion of pollutants, authorities said.

The overall air quality index (AQI) of Delhi was recorded at 353, according to Central Pollution Control Board (CPCB) data. An AQI between 0 and 50 is considered 'good', 51 and 100 'satisfactory', 101 and 200 'moderate', 201 and 300 'poor', 301 and 400 'very poor', and 401 and 500 'severe'.

The CPCB said seven areas in Delhi — Ashok Vihar, Anand Vihar, Jahangirpuri, Mundka, Rohini, Vivek Vihar and Wazirpur — recorded 'severe' air quality. Twenty areas in the national capital recorded 'very poor' air quality while it was 'poor' in six areas, it said.

The level of PM2.5 — particles in the air with a diameter of less than 2.5 micrometres — was recorded at 206 and the PM10 level was recorded at 360, it said. In NCR, Ghaziabad, Faridabad and Noida recorded 'very poor' air quality while Gurgaon fell into the 'poor' category, the CPCB data showed.

According to the Indian Institute of Tropical Meteorology, the wind speed and ventilation index are "extremely unfavourable" for dispersion of pollutants. Ventilation index determines how fast pollutants can get dispersed.

The ventilation index of around 6,000 sqm/second gets rid of pollutants, but it fell to 2,500 sqm/second on Thursday in the city. According to the Centre-run System of Air Quality and Weather Forecasting (SAFAR), the air quality is 'very poor' and likely to improve due to a slight increase in the wind speed. It will, however, remain in 'very poor' category for the next two days.

"Due to cold front up in the north, the wind speed has increased, so a decline (in pollution levels) is expected. However, at the time of withdrawal (of high wind speed) after two to three days, a lot of moisture may enter Delhi which is not favourable for the air quality. The contribution from stubble burning is nil," the SAFAR said.