Ghaziabad and Noida's air quality remained "severe" for the fifth day in a row while it stayed "very poor" in Faridabad and Gurgaon, according to a government agency data on Tuesday.
Presence of pollutant PM 2.5 and PM 10 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 Tuesday was 424 in Ghaziabad, 407 in Noida and 405 in Greater Noida, 382 in Faridabad and 347 in Gurgaon, according to CPCB's Sameer app.
On Monday, it was 438 in Ghaziabad, 422 in both Noida and Greater Noida, 388 in Faridabad and 357 in Gurgaon.
On Sunday, it was 428 in Ghaziabad, 402 in Noida, 396 in Greater Noida, 392 in Faridabad and 325 in Gurgaon.
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 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.