The air quality deteriorated to ''severe'' level in Ghaziabad, Noida, Greater Noida, Faridabad and Gurugram in the National Capital Region (NCR) on Sunday, according to a government agency.
Concentration of major air pollutants PM 2.5 and PM 10 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 Sunday was 448 in Ghaziabad, 441 in Noida, 417 in Greater Noida, 425 in Gurugram and 414 in Faridabad, according to the CPCB's Sameer app.
On Saturday, the average AQI was 456 in Ghaziabad, 425 in Noida, 394 in Greater Noida, 378 in Faridabad and 358 in Gurugram.
On Friday it was 382 in Ghaziabad, 337 in Noida, 336 in Greater Noida, 324 in Gurugram and 319 in Faridabad.
On Thursday it was 328 in Ghaziabad, 327 in Greater Noida, 305 in Noida, 304 in Faridabad and 293 in Gurugram.
On Sunday, PM 2.5 and PM 10 were the prominent pollutants in Noida, Greater Noida, Ghaziabad, Faridabad and Gurugram, according to the CPCB.
The CPCB states that an AQI in the "very poor" category may cause respiratory illness on prolonged exposure, while air quality in the ''severe'' zone 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 Gurugram has three and Greater Noida two, according to the app.