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 the air has 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 slow 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 the air. 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.
Ricky Ponting Pulls Big Surprise, Joins New IPL Franchise Ahead Of 2025 Season "I Want You Dead": Killed Gym Owner's Partner Details Lawrence Bishnoi Call Delhi High Court Permits Ashneer Grover, Wife To Travel To Doha And UK In Big Step Towards One Nation, One Election, Cabinet Clears Plan Explained: How Pagers Turned Bombs And Why Israel Is Being Blamed New XEC Covid Variant Spreads To 27 Countries, Here's What We Know So Far Veterinarian Warns Against Owning These 5 Dog Breeds Due To Health Concerns What Pager Attack On Hezbollah Could Mean For Volatile Middle-East Massive Asteroid's Near-Earth Approach In 2029. But There's A Bigger Danger Track Latest News Live on NDTV.com and get news updates from India and around the world.