Why is Hanoi's air pollution worse than Ho Chi Minh City's, despite fewer vehicles?

On the morning of Nov. 30, many monitoring stations in Hanoi recorded a rapid surge in the Air Quality Index (AQI) to a "very unhealthy" level, marking the first severe episode of the 2025-2026 winter season. As one friend in Hanoi described it, "it's the suffocating season."

As of the morning of Dec. 2, Hanoi and many northern provinces recorded AQI indices exceeding 200 for the first time this winter. This level is a major public health warning, posing urgent harm to the lungs and cardiovascular system, often likened to the equivalent harm of smoking several cigarettes a day. Historically, significant air pollution in Hanoi begins around October and lasts until the following April.

Hanoi has identified five main sources of air pollution: road vehicles (including road dust), industry, population activities, biomass burning, and agriculture.

Heavy traffic in Hanoi. Photo by VnExpress/Hoang Giang

Looking at statistics as of the end of June 2025, Ho Chi Minh City (before being merged with Vung Tan and Binh Duong) managed over 9.6 million vehicles (over 1 million cars and nearly 8.6 million motorbikes), not accounting for the large volume of inter-provincial traffic.

Hanoi currently manages over 9.2 million vehicles (1.1 million cars and 6.9 million motorbikes), with the remainder being vehicles from other provinces.

Logically, Hanoi manages approximately 400,000 fewer vehicles than Ho Chi Minh City. Yet, on the same morning (Dec. 2), HCMC enjoyed cool wind, clear skies, and sunshine. This raises a critical question: If traffic volume is slightly lower, what makes Hanoi's air pollution cyclically worse than HCMC's?

The key difference, in my view, lies in the unique climate and terrain of the two regions.

Hanoi, along with the northern climate, frequently experiences temperature inversion during winter. This phenomenon traps a cold layer of air close to the ground, preventing the warmer layer above from rising. Consequently, fine dust and pollutants are "trapped" near the surface for hours, sometimes all day. This is why many neighborhoods in the morning often appear covered in a layer of dust and smoke, even when traffic volume is not at its peak.

In contrast, HCMC benefits from a tropical monsoon climate. The air is constantly diffused, and the wind speed is generally higher, allowing pollution to dilute and disperse much faster.

Another influential factor is the terrain. HCMC has an open, spacious structure with low and airy ground, facilitating better airflow.

Finally, dense urban development in Hanoi increases dust generated from ongoing construction and landfill activities, with heavy trucks constantly circulating. These factors -- climate, terrain, and construction activity -- contribute significantly to the cyclical alarming pollution levels observed every year.

Addressing Hanoi's air pollution requires a dual approach: reducing emission sources and increasing urban diffusion capacity.

First, the city must tighten management of construction projects, which represent a large but often poorly controlled source of dust. Regulations concerning site covering, vehicle washing, and limiting construction during peak hours must be strictly enforced.

Second, reorganizing traffic at congestion points is an urgent priority. When vehicles are forced to stand still for prolonged periods, emissions increase exponentially. Widening lanes, optimizing traffic lights, and implementing intelligent traffic flow systems can significantly reduce fine dust concentrations.

Hanoi also needs to prioritize opening more green spaces and wind corridors, as urban density makes dust dispersal difficult. Converting vacant lots, areas around lakes, and densely populated streets into green spaces is essential for absorbing dust. Only through a system of synchronous and resolute solutions can Hanoi successfully escape the pollution cycle that plagues it every winter.