Delhi's air plummeted to 'very poor' on Tuesday after showing 'unusual improvement' in the last two days with the decline in wind speed, officials said.
The overall air quality index (AQI) increased from 262 (poor) to 349 (very poor) category, according to the Centre-run System of Air Quality and Weather Forecasting (SAFAR).
Anand Vihar recorded "severe" air quality. The AQI in 23 areas was registered in the "very poor" category, while the situation was "poor" in 12 areas.
The level of PM2.5 (particles in the air with a diameter of less than 2.5 micrometres) was recorded at 183, and the PM10 level was recorded at 326.
In the NCR region, Ghaziabad recorded the worst air quality with an AQI of 398. Greater Noida and Noida's air quality was registered in the "very poor" category. Faridabad and Gurgaon recorded moderate air quality, according to CPCB data.
An AQI between 0 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'.
According to SAFAR, Delhi's air quality will remain in the "poor" category, amid a gradual increase in the pollution level, for the next two days.
"The increase in pollution can be attributed to a decline in wind speed as compared to past two days. All other meteorological factors were already unfavourable," the SAFAR said in a report.
The air quality over the weekend showed an "unusual improvement" due to late night fast wind speed, SAFAR had said on Sunday.