When Vietnam’s capital city was, albeit momentarily, declared the most polluted one in the world Friday morning, officials scrambled to explain that it was a seasonal transition and a periodical episode.

The low quality of Hanoi’s air in the past days has happened because the city is in the transition from summer to fall and then winter. Therefore this is just a "periodical phenomenon" that happens every year, said officials of the Department of Natural Resources and Environment.

On Friday morning, Hanoi took the top spot for the city with worst air quality in the world in a ranking of more than 10,000 cities worldwide, when its air quality index (AQI) stood at 175. The previous evening, the index was over 200. An AQI level above 100 is considered polluted or unhealthy for humans. 

The city's level of PM2.5 or super fine particles, a fraction of the width of a human hair released from vehicles, industry and natural sources like dust, was 102.2 microgram per cubic meters (μg/m3) of air by Friday morning. The World Health Organization 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.

Hanoi stood first among the most polluted cities in the world on Friday morning,
according to the Air Quality Index recorded by IQAir AirVisual.

"The station at the embassy stands at the intersection of Lang Ha and De La Thanh streets, a major traffic point which is surrounded by many construction projects of large scale and that’s why the air condition there would be worse than in other places.Ta Ngoc Son, deputy head of general planning office at the Hanoi Environmental Protection Agency, said IQAir AirVisual has come up with the ranking based on data measured at the monitoring station at the U.S. embassy and that cannot tell the condition of the entire city.

"In addition, this is the period of seasonal transition so there has been no breeze or rain, leaving the city under a layer of fog, which has reduced the ability to disperse dust," he said.

Aside from the seasonal transition, the environment division also blamed the urbanization process with more construction projects and heavier traffic in the city, for the air pollution.

Le Thanh Hai, former deputy head of the Vietnam Meteorological and Hydrological Administration, told VnExpress earlier that Hanoi's current low air quality was the result temperature inversion. Thermal radiation is dispersed from the ground into the atmosphere, which causes fog at lower levels, he noted.

As a result, "air masses in the city are staying unchanged in fixed positions instead of spreading in various shapes and sizes," Hai said. He added that the situation will only improve when there was monsoon or rain.

Residents unimpressed

Many VnExpress readers were underwhelmed by the official explanations.

They said they were experiencing the air pollution themselves and it was high time authorities took action to deal with the real problem instead of trying to find excuses.

They also argued there is no need to justify where the air quality index is taken because the AQI app shows the indexes at all stations in the city and anyone can see what is happening.

"Now, it no longer matters where Hanoi stands in the ranking. What should be discussed now is that all monitoring stations have shown that the city’s air quality index exceeds the safe level," reader Huy Hoang wrote.

Another reader, Phong Le, dismissed the "seasonal" argument, saying Hanoi was not as polluted this time last year. "The index is increasing every year, clearly Hanoi is getting more polluted."

Some people blamed "too much construction" for the suffocating air in the city, while others pointed the fingers to factories and motorbikes.

"The authorities need to coordinate to improve the environment instead of dismissing the problem," said a man living near Lang Ha.

From mid this month until recently, the AQIs of Hanoi and several other parts in northern Vietnam have stayed in the 151-200 range.

Hanoi, which has eight million people and more than five million motorbikes and 550,000 cars, was ranked the second most polluted city in Southeast Asia by the World Air Quality report from the Switzerland-based air quality monitoring facility IQAir AirVisual earlier this year.

The organization measures Hanoi's air quality based on 10 monitoring stations run by Hanoi authorities and four by non-profit organizations.

Louise Watt, a spokeswoman of IQAir AirVisual, said it provides real-time air quality indices to help the public protect themselves and raise awareness over the matter.

When people are aware that the air quality is bad for their health, they will demand actions to fix the problem, as what has happened in other cities like Beijing, she said.