The air quality plunged to 'severe' level in Ghaziabad, while it remained 'very poor' across Noida, Greater Noida, Faridabad and Gurgaon, according to data provided by a government agency on Tuesday. The presence of pollutants PM2.5 and PM10 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, 388 in Greater Noida, 387 in Noida, 335 in Faridabad and 311 in Gurgaon, according to CPCB's Sameer app.
On Monday, it was 298 in Faridabad, 291 in Gurgaon, 346 in Noida, 349 in Greater Noida and 377 in Ghaziabad.
On Sunday, it was 236 in Faridabad, 242 in Gurgaon, 268 in Noida, 273 in Greater Noida and 300 in Ghaziabad.
The CPCB states that an AQI in the 'very poor' category may cause respiratory illness on prolonged exposure, while that 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 Gurgaon has three and Greater Noida two, according to the app.