Noida: The average air quality slipped to "poor" category in Ghaziabad, Noida, Greater Noida, Faridabad and Gurgaon, according to data issued by a government agency on Sunday.

Pollutant PM 2.5 and PM 10 although remained in the air of 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 251 in Ghaziabad, 238 in Noida, 224 in Greater Noida, 218 in Faridabad and 209 in Gurgaon, according to CPCB's Sameer app.

On Saturday it was 218 in Ghaziabad, 167 in Noida, 180 in Greater Noida, 198 in Faridabad and 170 in Gurgaon.

On Friday it was 91 in Ghaziabad, 114 in Noida, 144 in Greater Noida, 105 in Faridabad and 160 in Gurgaon.

The CPCB states that an AQI in the "moderate" may lead to breathing discomfort to the people with asthma, lung and heart diseases while "poor" may cause breathing discomfort to most people on prolonged exposure.

The AQI for each city is based on the average value of all monitoring stations there. Ghaziabad, Gurgaon and Noida have four such stations while Greater Noida and Faridabad have two stations each, according to the app.