India, Jan. 31 -- The city recorded an average Air Quality Index (AQI) of 127 on Friday-firmly in the "moderate" category-but remained shrouded in haze throughout the day, with poor visibility prompting residents to complain that they couldn't see the skyline.

These conditions, which have persisted for much of the month, have raised questions over why Mumbai appears so hazy even when official AQI readings do not indicate severe pollution. Over the past 30 days, the city's overall AQI has oscillated between "satisfactory" and "moderate", ranging from a low of 70 on January 28 to a high of 140 on January 4.

On 20 of these days, PM10 (particulate matter with a diameter of 10 micrometres or less, or roughly 1/5th to 1/7th the width of a hum...