Data from air quality monitoring app IQAir showed Hanoi’s Air Quality Index (AQI) in the red zone. At this level, even healthy people may begin to experience adverse effects, while sensitive groups face a greater risk of serious respiratory problems.
Several areas, including Ba Dinh and Tay Ho, recorded pollution levels in the purple zone, indicating even worse air quality.
The pollution is being driven by a combination of emission sources and unfavourable weather conditions.
According to the Environment Department under the Ministry of Agriculture and Environment, the sources of fine particulate matter (PM2.5) in major cities such as Hanoi include secondary particles formed through reactions between pollutant gases, accounting for 27 per cent; road transport, 23 per cent; construction and road dust, 20 per cent; open burning of waste and agricultural residues, 12 per cent; industrial production, 10 per cent; and other sources, 8 per cent.
A large difference between daytime and night-time temperatures has also contributed to radiative temperature inversion. Cold air near the ground becomes trapped beneath a layer of warmer air, forming a lid that prevents PM2.5 particles from dispersing upwards. The pollutants remain suspended close to the ground, creating a layer of polluted haze.
With forecasts indicating continued dry, sunny weather and large day-to-night temperature swings, the hazy conditions are expected to persist for several days.
Air monitoring data for the first nine months of 2026 showed some improvement in Hanoi, with a significant reduction in the number of days when pollution exceeded regulatory limits. Air pollution in September was also less severe than in previous years.
However, Hanoi’s peak pollution season typically begins in October and lasts until the end of March. Associate Professor Hoang Anh Le of the Environmental Management Division at the University of Science, Vietnam National University, Hanoi, warned that residents should not become complacent as a new pollution season was beginning.
Residents are advised to take precautions to protect their health. When the AQI reaches hazardous levels, they should avoid going outdoors as much as possible, suspend outdoor physical activities and keep windows closed to limit exposure to airborne pollutants.
Those who must travel outside should wear masks designed to filter PM2.5 particles. More stringent precautions are recommended for vulnerable groups, including older people, young children and those with underlying health conditions. They should remain indoors as much as possible, ensure adequate and safe ventilation, and use air purifiers to protect their respiratory health.