At around 7:30 a.m., data retrieved by Vietnam Environment Administration under the Ministry of Natural Resources and Environment revealed Air Quality Index (AQI) at different stations in Bac Ninh Province to be between 221 and 243.

AQI levels above 100 are considered unhealthy. Children, seniors and individuals with respiratory and heart diseases are recommended to avoid sustained and high-intensity outdoor exercise when AQI levels reach 150 or above.

Other independent air quality analysis systems like PAMAir and AirVisual on Friday morning also recorded low air quality in several areas of northern Vietnam, some low enough to prove harmful to human health.

AirVisual for example revealed that all Hanoi areas where its air quality tracking stations were located suffered high air pollution levels, with localities like Ngoc Thuy Ward and To Ngoc Van Street having AQI levels up to 214, deemed to be very harmful to human health.

PAMAir recorded AQI levels as high as 289 in most of Hanoi and other northern locations like Thai Nguyen, Bac Ninh, Bac Giang, Hai Duong, Hung Yen, Ha Nam, Ninh Binh and Hai Phong.

As northern Vietnam enters winter, nocturnal and early morning fog and smog could prevent pollutants from being dissipated in higher altitudes, forcing them to remain at lower attitudes. That should explain the high AQI records in the morning, according to Vietnam Environment Administration, Vietnam News Agency reported.

Environment experts have advised people to refrain from outdoor activities, close windows and use masks while going out amid periods of low air quality.

By around 2:30 p.m., AQI levels in Hanoi and surrounding areas had dropped, being recorded between 33-140 on average, according to Vietnam Environment Administration data.

Air pollution issues are hardly new in Vietnam, especially in urban areas. Authorities have blamed low air quality on large-scale construction, the high rate of personal vehicles and heavy industry, including steelworks, cement factories and coal-fired plants.

Hazardous air quality in Vietnam amounted to losses between $10.82-13.63 billion a year, or 4-5 percent of Vietnam’s GDP, said Associate Professor Dinh Duc Truong, head of the National Economics University’s Department of Environment, Climate Change, and Urban Studies, at a January conference.