The air quality deteriorated to reach "very poor" level in Ghaziabad and Noida on Monday but remained "poor" in Greater Noida, Gurugram and Faridabad in the National Capital Region (NCR), according to a government agency.
Presence of pollutants 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 Monday was 365 in Ghaziabad, 301 in Noida, 286 in Greater Noida, 285 in Gurugram and 272 in Faridabad, according to CPCB's Sameer app.
On Sunday, it was 242 in Gurugram, 234 in Faridabad, 288 in Ghaziabad, 273 in Noida and 270 in Greater Noida, it showed.
The average AQI last Monday was 186 in Faridabad, 207 in Ghaziabad, 226 in Greater Noida, 243 in Noida and 246 in Gurugram, showing drastic improvement over the previous day due to a spell of rain in the NCR.
As per the CPCB, an AQI in the "poor" category may cause breathing discomfort to most people on prolonged exposure, while "very poor" may cause respiratory illness on prolonged exposure.
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.