GRAP for the National Capital Region (NCR) is classified under four different stages of adverse air quality in Delhi: Stage I -- 'poor' (AQI 201-300); Stage II -- 'very poor' (AQI 301-400); Stage III -- 'severe' (AQI 401-450); and Stage IV -- 'severe plus' (AQI >450).

The air quality in Delhi reached its worst level today, with the AQI falling into the "severe" category. According to Safar data, the AQI in the national capital dropped to 434 at 6 am.

Every year, Delhi and northern states battle hazardous air during the winter months of October to January due to plummeting temperatures, smoke, dust, low wind speed, vehicular emissions and crop stubble burning. This time, experts say that the condition is likely to get worse in the coming days.

Air pollution in India's national capital increased to 'severe' on an official index on Wednesday for the first time this season as temperatures and wind speeds dropped, reducing visibility and affecting some flights.

The GRAP 3 curbs in Delhi include ban on demolition and construction activities, and entry of certain vehicles inside Delhi-NCR, among other curbs, to prevent further deterioration of air quality.

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Delhi overtook Pakistan ’s Lahore as the world’s most polluted city in Swiss group IQAir’s live rankings, with an air quality index (AQI) score of more than 1,000, which is considered “hazardous”, but India ’s pollution authority said the AQI was around 350.