Delhi's air quality turned "severe" on Thursday due to the prevailing "extremely unfavourable" conditions for dispersion of pollutants, government agencies said. The city's 24-hour air quality index (AQI) was 429. It was 354 on Wednesday, 293 on Tuesday and 243 on Monday.
This is the third time in January that the city's air quality stood in the "severe" category. The AQI was 441 on January 1 and 443 on January 2.
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". Kuldeep Srivastava, head of the India Meteorological Department's regional forecasting centre, said the wind speed has slowed down and the moisture in theairhas made the pollutants heavier.
The maximum wind speed was 10 kmph on Thursday. The central government's Air Quality Early Warning System for Delhi said low winds and ventilation conditions are "extremely unfavourable" for dispersion of pollutants. This will lead to further deterioration in air quality, the agency said.
The city's ventilation index — a product of mixing depth and average wind speed was 2,000 m2/s on Thursday. It is likely to be 1,000 m2/s on Friday. Mixing depth is the vertical height in which pollutants are suspended in theair. It reduces on cold days with calm wind speed.
A ventilation index lower than 6,000 sqm/second, with the average wind speed less than 10 kmph, is unfavourable for dispersal of pollutants.