Thick haze blankets Hà Nội’s Trần Duy Hưng District on Tuesday. — VNA/VNS Photo Trọng Đạt

HÀ NỘI — The capital city choked on unprecedentedly poisonous air throughout Tuesday with the air quality index (AQI) monitored at most sensor stations reaching hazardous levels of above 300 from early in the morning.

The AQI recorded at Long Biên District’s Nguyễn Văn Cừ Station, for example, hit 344 at 5am then dropped to 270 – very unhealthy – at 8am.

At other stations on Thanh Xuân District’s Nguyễn Chế Nghĩa Street and Cầu Giấy District’s Phạm Tuấn Tài Street, AQI was reported at nearly hazardous levels, with 299 and 295, respectively.

In the afternoon, air quality slightly improved but remained at an unhealthy level (from 151 to 200) with visible dense haze engulfing the entire city.

Independent air quality trackers, PAMAir and AirVisual, reported similar results. At 4pm, AirVisual ranked Hà Nội third worldwide for the highest AQI, following Pakistan’s Lahore and India’s Delhi.

Northern provinces including Phú Thọ, Tuyên Quang and Thái Nguyên suffered the same ordeal

According to Hoàng Dương Tùng, head of the Việt Nam Clean Air Partnership, this was the worst air quality to hit Hà Nội so far.

“The pollution is alarming. Hà Nội authorities should immediately warn citizens to protect themselves,” he said.

The city’s eight millions residents, regardless of their conditions, are susceptible to respiratory diseases caused by air pollution.

The Việt Nam Environment Administration blamed temperature inversion as the main cause of the severe pollution.

As Hà Nội move into the dry season, the gap between daytime and nighttime temperatures increases, leading to a big difference in surface temperatures and higher altitudes, triggering inversion.

Air pollutants, including fine particulate matter PM2.5, were trapped near ground level, worsening the air quality in Hà Nội and other northern provinces.

In the days ahead, air quality is forecast to remain at unhealthy to very unhealthy levels, especially at night and in the early morning. Citizens are recommended not to open windows or exercise outdoors at these times of day, and wear face masks when going out.

Tùng said Hà Nội's air quality had declined drastically recently but authorities seemed reluctant to take action to improve the situation. Temperature inversion has been long cited as the main cause of pollution in Hà Nội, but Tùng said he found the explanation hard to credit.

“There might be a bigger source of pollution in Hà Nội and neighbouring localities,” he said.

Hà Nội enjoyed a precious few good days in late October before suffering the extreme pollution. Rain forecast on Wednesday and Thursday under the impacts of the northeast monsoon is expected to partially wash away harmful pollutants. — VNS