Thursday brought wintery showers to Beijing, but they were hardly as pure as driven snow.

In the morning, the city’s meteorological bureau warned residents three times that the snow falling at the time was “very dirty!!!” and advised them to stay indoors.

Beijing and many other cities in northern and central China are currently experiencing a prolonged spell of heavy air pollution. According to figures released by the Ministry of Environmental Protection, over the past 30 days the average air quality index (AQI) in Beijing was 195, with a peak at 470. For comparison, AQI levels up to 50 are considered healthy.

In two posts on its Weibo microblog, the meteorological bureau explained that the snow could pick up pollutants as it falls. “It’s not snowing, it’s poisoning,” commented one Weibo user.

Beginning in early December, thick smog has blanketed large parts of China for days at a time, leading many people to appeal to the government to make more of an effort to keep the air clean. Artists in the southwestern city of Chengdu organized a sit-in protest, parents in Beijing and Shanghai called on schools to install air purifiers, and residents of Shijiazhuang, a city in northern China where air pollution reached levels far beyond the AQI maximum of 500 in December, complained their children were still required to attend classes despite the heavy pollution.

In Linzhou City in central China, a principal was suspended after pupils at his middle school were forced to take an exam even though classes had been cancelled due to heavy pollution. Photos of the students seated at their desks on an outdoor sports field were widely circulated online; the back rows were barely visible through the smog.

According to the Ministry of Environmental Protection, northern China's current spell of heavy pollution is expected to last until Saturday.

(Header image: Two women eat Laba rice porridge on a snowy day in Beijing, Jan. 5, 2017. Pan Zhiwang/Beijing Times/VCG)