The air quality was recorded in the "poor" category in Ghaziabad, Noida, Greater Noida, Faridabad and Gurgaon, according to data issued by a government agency on Saturday.
Pollutants PM 2.5 and PM 10 also remained high 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 Saturday was 249 in Ghaziabad, 259 in Greater Noida, 227 in Noida, 296 in Faridabad and 223 in Gurgaon, according to CPCB's Sameer app.
On Friday, it was 340 in Ghaziabad, 359 in Greater Noida, 302 in Noida, 350 in Faridabad and 272 in Gurgaon.
On Thursday, it was 373 in Ghaziabad, 389 in Greater Noida, 336 in Noida, 340 in Faridabad and 307 in Gurgaon.
On Wednesday, it was 364 in Ghaziabad, 369 in Greater Noida, 328 in Noida, 314 in Faridabad and 309 in Gurgaon.
The CPCB states that an AQI in the "very poor" category may lead to respiratory illness on prolonged exposure, while air in the "poor" zone 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.