More specifically, many monitoring stations in Hà Nội showed the Air Quality Index (AQI) of above 300, which is seriously harmful to human health (AQI of above 100 is considered unhealthy for humans). In the early mornings from 2nd to 4th January, the sky was covered with fog, dust, and concentrated air. At 9 am on 4th January, the Airvisual stated an average AQI of 326 in Hà Nội. With this figure, Hà Nội was ranked as the leading air polluting city in the world on this day, followed by the city of New Delhi in India with an AQI of 259. Furthermore, the monitoring system of PamAir showed the air quality at many places like Tây Hồ, Hoàn Kiếm was higher the harmful level with AQI of over 300. Other inner districts of Đống Đa, Cầu Giấy, and Hai Bà Trưng have AQI readings ranging from 220 – 260. This can be considered the most air quality pollution event in Hà Nội since the beginning of autumn and winter. According to Hanoi’s Environmental Protection Department, the reason for this air pollution was that cold air weakened and changed its nature together with activities of construction and high frequency of inland traffic. In this condition, pollutants could not diffuse and combined with smog, causing a thick atmosphere and reduced visibility.
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Source & further information: “Chất lượng không khí Hà Nội suy giảm nghiêm trọng những ngày đầu năm 2022” at https://baotainguyenmoitruong.vn, accessed on 6 January 2022.