Since last week, several districts have reported a thick haze lasting from morning until dusk that has greatly reduced visibility on roads. 

Numerous air monitoring stations in 30 different locations in the southern metropolis have recorded heightened levels of pollutants including nitrogen dioxide, sulfur dioxide and carbon monoxide. The levels of fine particles (PM10, PM2.5) have also increased almost two-fold this month, according to a monitoring center under the municipal environment department.

Latest Air Quality Index (AQI) statistics by seven different monitoring stations in the city retrieved Thursday morning also showed an AQI level of 173 and PM2.5 level at 71.2 μg/m3, according to the Switzerland-based air quality monitoring facility IQAir AirVisual.

The AQI is a metric used by multiple governmental agencies to determine how polluted the air is. An AQI level above 100 is considered polluted or unhealthy for humans. Children, seniors and individuals with respiratory and heart diseases are recommended to avoid sustained and high-intensity outdoor exercises when AQI levels reach 150 or above.

PM2.5, also described as super fine particles, is a fraction of the width of a human hair, which is released from vehicles, industry and natural sources like dust. The World Health Organization (WHO) Air Quality Guideline recommends an annual mean exposure threshold of 10 μg/m3 to minimize health risks. Vietnam’s national PM2.5 threshold is 25 μg/m3.

HCMC’s AQI level made it the most polluted city in the world on Thursday morning, when Hanoi took the third position.

HCMC stands first and Hanoi in third place among the most polluted
cities in the world on Thursday morning, according to the Air Quality
Index recorded by IQAir AirVisual.

A combination of tropical convergence and cold air in the atmosphere produced cloudy sky in HCMC and high moisture levels, which caused air pollutants to condense into smog. As there was not enough sunlight to heat up the ground, temperature inversion kicked in and prevented the smog from being dispersed into the upper atmosphere, confining it close to the ground and making them thicker and longer-lasting, the center said. 

It denied that air pollution in HCMC was caused by forest fires in Indonesia as reported by some online media outlets, adding that similar pollution levels had been recorded in previous years during the months of September and October.

Meanwhile, local hospitals have reported that increasing numbers of HCMC residents are suffering from respiratory diseases as the city’s air quality plunges. Doctors are generally agreed that air pollution in HCMC in particular and the southern region in general has become a serious health risk now.

Inaccurate data

Hanoi is also faring badly on the air pollution front. The capital city recorded an AQI level of 204 and PM 2.5 level at 56.6 μg/m3 at 9 a.m. on Thursday, making it the most polluted city in the world, according to IQAir AirVisual.

But Ta Ngoc Son, a representative of the Hanoi Environmental Protection Agency denied the findings, adding that the statistics were neither comprehensive nor objective.

The AQI value was only collected from one monitoring station in the U.S. Embassy of Vietnam, in comparison to the agency’s 10 monitoring stations, Son said.

The embassy is next to Lang Ha Street, which has high traffic density and multiple construction sites, so it would be one of the city’s most polluted areas, he said.

To accurately determine air quality in the city as a whole, data must be collected from multiple stations at multiple times of the day, he added.

"As such, [that AQI level] was neither representative of the whole city nor accurate."

According to the AQI AirVisual website, its data was obtained from 12 monitoring stations in the capital city.