The Hanoitimes - Air quality at many places in Hanoi was at hazardous level (purple) according to PAMAir.

Many areas in Hanoi's inner city had air quality at unhealthy level and were covered with a layer of thick smog on November 12 morning, indicating serious air pollution, according to the Vietnam Environment Administration under Ministry of Natural Resources and Environment (MONRE).

The air monitoring station at 556 Nguyen Van Cu street in Long Bien district issued a warning "brown" with air quality index (AQI) reaching 338 at 6:00am before dropping to 310 at 7:15am.

Besides, independent air quality analysis systems, PAMAir and AirVisual, also recorded similar results throughout the capital city, which were all at an unhealthy level (AQI above 100).

Airvisual's air quality measuring system at 9:00am recorded AQI of 317 in Tay Ho district, 245 at GreenID (Cau Giay district) and 211 at Hang Dau.

Air quality at many places in Hanoi was at hazardous level (purple) according to PAMAir: 156 in Bac Tu Liem district, 226 in Xuan Thuy and Mai Dich (Cau Giay district), 224 in Hang Bai (Hoan Kiem district), among others.

MONRE warns hazardous air pollution in Hanoi

"This is the first time I've seen such pollution," Hoang Duong Tung, president of Vietnam Clear Air Partnership, who has spent many years researching on air quality in Vietnam.

“With such levels, I think Hanoi should immediately notify its citizens so that they can protect themselves," Tung added.

The high level requires an emergency health warning to the entire population, the Vietnam Environment Administration told VnExpress.

The environment administration identified temperature inversion as the main cause of the pollution. The large gap between day and night temperature causes differences in temperature between the ground and higher altitudes, creating an inversion. It makes air pollutants, especially PM2.5 fine dust, stuck at the lower altitudes, slowly worsening the air quality and damaging the population.

Earlier, the Vietnam Environment Administration had reported that Hanoi's air quality was worsening in the first days of the month, with its PM2.5 levels rising. The PM2.5 levels consistently exceeded national standards (50 μg/m3) and exceeded 100 μg/m3 at times.

The environment administration recommended children, seniors and individuals with respiratory and heart diseases to avoid sustained and high-intensity outdoor exercises when AQI surpasses 150 or above. It advised citizens to refrain from exercising outside in the morning, keep windows closed and use air masks outside.