A dense layer of smog enveloped Delhi and surrounding regions of Noida, Ghaziabad, Gurugram and Faridabad on Wednesday morning. Delhi's air quality index was in the 'Severe' category breaching the 400 mark, while that of Gurugram, Noida and Ghaziabad was in the 'Poor' category. Faridabad's AQI, at 188, was 'Moderate'.