The air quality remained "severe" in Ghaziabad, Noida, Greater Noida and Faridabad for the third day in a row on Thursday, while it was "very poor" in Gurgaon, according to a 24-hour data issued by a government agency.
Presence of pollutant PM 2.5 and PM 10 also remained high in the five immediate neighbours of Delhi, showed the air quality index (AQI) maintained by the Central Pollution Control Board (CPCB).
The average 24-hour AQI at 4 pm on Thursday was 440 in Ghaziabad, 448 in Greater Noida, 441 in Noida, 408 in Faridabad and 361 in Gurgaon, according to CPCB's Sameer app.
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 CPCB states that an AQI in the "very poor" category may cause respiratory illness on prolonged exposure, while ''severe'' affects even healthy people and seriously impacts those with existing diseases.
The average AQI on Wednesday was 472 in Ghaziabad, 476 in Greater Noida, 462 in Noida, 428 in Faridabad and 340 in Gurgaon.
On Tuesday it was 458 in Ghaziabad, 450 in Greater Noida, 437 in Noida, 407 in Faridabad and 377 in Gurgaon. On Monday, it was 391 in Ghaziabad, 366 in Greater Noida, 363 in Noida, 289 in Faridabad and 271 in Gurgaon.
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.