According to IQAir, a platform aggregating data from over 30,000 air quality monitoring stations worldwide, Hanoi ranked as the most polluted city globally at 7.41 am on March 21, 2025, leading a list of 126 cities.

With an Air Quality Index (AQI) of 207, Hanoi's air pollution was classified as "very unhealthy" (purple level).

Within the city, the monitoring station in Tay Ho District recorded the highest AQI of 354, reaching the "hazardous" (brown) level.

These figures fluctuate depending on the time of day and time zones as other global cities enter peak hours with increased traffic and industrial activity.

By 11 am on March 21, Hanoi had moved to the second spot in the global pollution ranking, with an AQI of 199.

AccuWeather also reported that Hanoi's air quality this morning and midday remained "very unhealthy".

The Hanoi Department of Natural Resources and Environment's air monitoring system recorded poor and unhealthy air quality across the capital, which poses health risks.

Hanoi's pollution is primarily caused by fine particulate matter (PM2.5), which can penetrate deep into the lungs and bloodstream, leading to respiratory and cardiovascular diseases. According to World Bank data, approximately 5,800 Hanoi residents die annually from PM2.5-related illnesses.

Experts have identified four significant sources of pollution in Hanoi: traffic, construction, industrial production, and residential activities such as waste and straw burning. Traffic emissions are the largest contributors.

Health experts advise residents to limit outdoor activities due to hazardous air quality. Those in sensitive groups, including individuals with chronic respiratory or cardiovascular conditions, should avoid strenuous activities and reduce time spent outdoors.

Besides Hanoi, several other provinces, including Thai Nguyen, Bac Giang, Bac Ninh, Thai Binh, Ha Nam, Hung Yen, and Hai Duong, have also reported severe air pollution.