Mumbai's Bandra Kurla Complex experienced a moderate smog layer with an Air Quality Index (AQI) of 123 on Sunday morning. This contrasts with Delhi, where the AQI reached a severe 391, with several areas exceeding 400. Authorities attribute the haze to cold weather, calm winds, and fog trapping pollutants. Measures like 'No PUC, No Fuel' and Stage IV GRAP actions are being enforced in Delhi to combat the worsening air quality.