The average air quality returned to the 'severe' category in Ghaziabad, Noida, Greater Noida and Faridabad after two days and was in the 'very poor' in Gurgaon, according to data issued by a government agency on Sunday.
The presence of PM2.5 and PM10 particles also remained high in the five satellite cities neighbouring Delhi, according to the air quality index (AQI) maintained by the Central Pollution Control Board (CPCB).
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 407 in Ghaziabad, 418 in Greater Noida, 405 in Noida, 404 in Faridabad and 359 in Gurgaon, according to CPCB's Sameer app.
According to the CPCB, AQI in the 'very poor' category may cause respiratory illness on prolonged exposure while that in the 'severe' category affects even healthy people and seriously impacts those with existing diseases. The average AQI on Saturday was 367 in Ghaziabad, 355 in Greater Noida, 344 in Noida, 300 in Faridabad and 269 in Gurgaon.
On Friday, it was 391 in Ghaziabad, 376 in Greater Noida, 386 in Noida, 328 in Faridabad and 302 in Gurgaon. On Thursday, the average AQI was 440 in Ghaziabad, 448 in Greater Noida, 441 in Noida, 408 in Faridabad and 361 in Gurgaon, according to the agency.
The AQI for each city is based on the average value of all air quality monitoring stations there. Noida, Faridabad, Ghaziabad have four stations each, while Gurgaon has three and Greater Noida two, according to the app.