The air quality in Ghaziabad Tuesday deteriorated to the “severe” category after nine days, while it remained “very poor” in Noida, Greater Noida and Faridabad, and “poor” in Gurgaon, according to a government agency.

The presence of pollutants PM 2.5 and PM 10 remained high in the five National Capital Region (NCR) cities, 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 Tuesday was 428 in Ghaziabad, 396 in Noida, 382 in Greater Noida, 360 in Faridabad and 296 in Gurgaon, according to CPCB's Sameer app.

On Monday, it was 365 in Ghaziabad, 301 in Noida, 286 in Greater Noida, 285 in Gurgaon and 272 in Faridabad.

On Sunday, it was 288 in Ghaziabad, 273 in Noida, 270 in Greater Noida and 242 in Gurgaon, it showed.

Ghaziabad's average AQI had last slipped into the “severe” category on November 15, a day after Diwali, when the air in other districts had also reached similar low levels.

The CPCB states that 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 and “severe” could affect healthy people and seriously impact 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.