The air quality improved in Noida, Ghaziabad, Faridabad but stayed in the 'very poor' category on Thursday, while it was "poor" in Gurgaon and Greater Noida in the Delhi-National Capital Region, according to a government agency. Presence of pollutant PM 2.5 also remained high in the five immediate neighbours of Delhi, according to the air quality index (AQI) maintained by the Central Pollution Control Board (CPCB).
According to the index, an AQI between zero 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 average 24-hour AQI at 4 pm on Thursday was 301 in both Ghaziabad and Noida, 296 in Greater Noida, 312 in Faridabad and 299 in Gurgaon, according to CPCB's Sameer app.
On Wednesday, it was 444 in Ghaziabad, 414 in Noida, 394 in Greater Noida, 348 in Faridabad and 299 in Gurgaon.
It was 428 in Ghaziabad, 396 in Noida, 382 in Greater Noida, 360 in Faridabad and 296 in Gurgaon on Tuesday.
On Monday, it was 365 in Ghaziabad, 301 in Noida, 286 in Greater Noida, 285 in Gurgaon and 272 in Faridabad.
The average AQI on Sunday was 242 in Gurgaon, 234 in Faridabad, 288 in Ghaziabad, 273 in Noida and 270 in Greater Noida, it showed.
The CPCB states that an AQI in the 'poor' category may cause breathing discomfort to most people on prolonged exposure, while 'very poor' may cause respiratory illness on prolonged exposure.
The AQI for each city is based on the average value of all stations there. Noida, Faridabad, Ghaziabad have four stations each, while Gurgaon has three and Greater Noida two, according to the app.