BEIJING — China’s northern region is being blanketed by heavy smog, causing hundreds of flights to be cancelled and highways to shut down.

According to Reuters, a total of 126 flights were cancelled in Beijing at the city’s main airport, while all buses from Beijing to neighbouring cities were suspended. In nearby Tianjin, the smog wasn’t as serious though visibility was much worse, with over 300 cancelled flights at Tianjin airport.

Heavy smog began last Friday and is expected to persist until Thursday. A total of 24 cities in China have issued red alerts for the latest bout of air pollution, the most severe warning in a four-tier system, prompting measures like limiting car usage and closing factories. Another 21 cities have issued orange alerts.

Average concentrations of small breathable particles known as PM2.5 were more than 500 micrograms per cubic metre in Beijing, which is 50x higher than World Health Organization recommendations, reports Reuters.

Chas Pope, a British engineering consultant, shot a timelapse video of incoming yellow smog from a Beijing bank on Monday. Taken over a period of just 20 minutes, the video has already been viewed close to 14,000 times.