The air quality improved across Delhi-NCR a day after a spell of rain, with the index reaching "moderate" levels in Noida, Gurgaon and Faridabad and "poor" in Ghaziabad and Greater Noida, according to data issued by a government agency on Wednesday.
Pollutants PM 2.5 and PM 10 also remained high in the air of the five immediate neighbours of Delhi, as per 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 Wednesday was 221 in Ghaziabad, 239 in Greater Noida, 152 in Noida, 163 in Faridabad and 122 in Gurgaon, according to CPCB's Sameer app.
On Tuesday, it was 334 in Ghaziabad, 339 in Greater Noida, 293 in Noida, 277 in Faridabad and 272 in Gurgaon.
The CPCB states that an AQI in the "poor" category may lead to breathing discomfort to most people on prolonged exposure, while "moderate" may cause breathing discomfort to people with lungs, asthma and heart diseases.
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.