Mumbai experienced a cool morning on December 31, 2025, but the pleasant weather gave way to a thick smog, pushing the city's Air Quality Index (AQI) to 358, categorized as 'severe'. This deterioration in air quality, attributed to construction dust and fine particulate matter from ongoing infrastructure projects, poses significant health risks, particularly for vulnerable populations. Despite initial favorable conditions, the smog reduced visibility and created an acrid smell in many parts of the city.