Delhi's air quality deteriorates to 'poor' zone

New Delhi: Delhi's air quality slipped back into the "poor" category on Saturday, a day after it had improved significantly due to favourable wind speed.

The city's air quality index (AQI) was 209 at 9 am.

The 24-hour average AQI was 137 on Friday, 302 on Thursday and 413 on Wednesday.

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".

Stronger winds and a drastic reduction in the contribution of stubble burning were two major reasons for cleaner skies over Delhi on Friday.

The winds slowed down at night allowing the pollutants to accumulate.