Delhi's air quality significantly improved and was registered in 'poor' category on Monday due to "unusual increase" in the wind speed, authorities said.

According to the Central Pollution Control Board (CPCB) data, the overall air quality index (AQI) in the city was 226, which falls in the 'poor' category.

An AQI between 100 and 200 comes under the 'moderate' category, 201 and 300 is considered 'poor', 301 and 400 'very poor', while that between 401 and 500 'severe'.

The city's air quality was severe till Sunday but with improved wind speed which was blowing at 20 km/hour, the air quality significantly improved and was recorded in poor category, authorities said.

As many as 26 areas recorded 'poor' air quality and four 'moderate', the CPCB said.

In the National Capital Region (NCR), Ghaziabad, Faridabad, Noida and Greater Noida recorded 'poor' air quality, it said.

The overall PM2.5 level -- fine particulate matters in the air with a diameter of less than 2.5 micrometers -- in Delhi was at 109, while the PM10 level 192, it said.

The Centre-run System of Air Quality and Weather Forecasting (SAFAR) said the overall air quality index over Delhi is poor and it is likely to improve faster.

"The wind speed is unusually faster for this time of the year and improving the air quality rapidly. The AQI is predicted to remain in 'poor' category tomorrow and then to start increasing slowly from January 16 for next two days but to remain in the lower range of very poor," the SAFAR said.

It said the humidity is still high with colder conditions and continuing to work adversely but wind speed is a dominant factor.

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