The city's high-rises could barely be seen from 500 meters away on Thursday morning. Vision was limited by the haze, especially at the My Thuy roundabout in District 2, which was the most polluted spot in the city according to several air monitoring results.

The air pollution has been visible to the naked eye since Thursday last week.

The Air Quality Index (AQI) recorded by IQAir AirVisual, a Switzerland-based air quality monitoring facility was an average 154 across different locations in HCMC on Thursday morning. AQI levels above 100 are considered unhealthy.

Weather expert Le Thi Xuan Lan said the southern part of Vietnam was entering the dry season with occasional rains in certain locations, so the humidity in the air was not high. The gray blanket covering the city over the past few days, therefore, was air pollution, she said.

"Haze is happening more and more frequently, although at different levels. This never happened in the past," Lan said.

She said the reason was that the atmosphere at about 100 meters height and lower has been polluted by dust and smoke from vehicles, factories and construction sites. The haze is thicker at places with many lakes and canals due to the humidity. For example, Landmark 81, Vietnam's tallest building, is often enveloped by thick haze because it stands near the Saigon River.

Air quality was at unhealthy levels at different spots in HCMC at 10:30 a.m.
on November 21, 2019, according to Air Visual.

The monitoring results at 30 locations in the city showed the dust index mostly exceeded safe levels set by the Ministry of Natural Resources and Environment in recent days, said Cao Tung Son, Director of the Center of Environmental Monitoring under HCMC Department of Natural Resources and Environment.

He said trucks and containers were active day and night, which is characteristic of the city, constantly releasing pollutants into the air. Additionally, the city is entering a season changing phase so there has been little wind and low temperatures – factors combining to prevent the dust from being dispersed into the atmosphere.

HCMC has stepped up manual monitoring of its air quality, analyzing samples per international standards. The accuracy rate is high, but it takes time. The collected data is used for strategic planning, scientific research and to inform and alert citizens.

"It is expected that in 2020 the city will build nine automatic air monitoring stations. We will have very fast indicators of air quality and pollution to notify people sooner," Son said.

A thick haze had engulfed the city in September, with AQI repeatedly reaching unhealthy highs.

Last month, city authorities said three major sources of air pollution have been choking Saigon - exhaust fumes from 10 million vehicles, smoke from 1,000 large factories and dust from numerous construction sites.

About 1,000 kilometers north, the capital city of Hanoi was also suffering from low air quality and a rise in PM2.5 levels earlier this month.

The government has asked both cities to devise clear and radical strategies to combat the worsening pollution and to keep it notified of steps taken.