A sandstorm sweeping across much of northern China left Beijing in an orange fog and helped push air quality levels in the capital to the worst since 2017.

Beijing’s government issued a yellow alert, the first sandstorm warning this year. The Air Quality Index surged to 500, well above health emergency levels, and a thick orange haze limited visibility to less than 3,280 feet (1,000 meters) Monday morning. Chinese social media was rife with pictures of the city’s iconic buildings enveloped by the dust, with many users saying it’s the worst sandstorm in years.

Levels of ultrafine particulates in the air in Beijing surged to as high as 680 micrograms per cubic meter, the highest concentration since May 2017, according to records kept by the U.S. Embassy there. Concentrations of slightly larger particles more commonly associated with sand surged to more than 2,000 per cubic meter at some monitoring stations.

Even before the storm, Beijing’s air quality had been worsening as the nation’s economy roared back from the pandemic, thanks to a heavy-industry led recovery that saw steel and cement production surge and a jump in fossil fuel consumption.

The storm, which will continue for a day in Beijing, originated in Mongolia and has also swept across the northern provinces of Shaanxi, Shanxi and Hebei. Inner Mongolia’s Baotou city canceled school classes because of the airborne dust. More than 400 flights were canceled at Beijing airports as of 9:30 a.m., Jiemian reported, citing the flight information app Flight Master.

Deforestation and drought are at least partly to blame for the sandstorms that often hit northern China. The government has launched massive tree-planting projects to try to curb the storms since the 1970s. The Three-North Shelter Forest Program, which protects regions affected by sandstorms sweeping out of the Gobi Desert, aims to grow new trees on 87 million acres (35 million hectares) by 2050.

The efforts seem to be having some success, with the annual number of sandy days in Beijing falling from 26 in the 1950s to around three after 2010, according to Xinhua.

Sandstorms typically occur in the spring and early summer, when the wind blows from the north. Last year, northern China experienced seven of them, with conditions lasting on average less than three days, according to the China Climate Bulletin released by the National Climate Center. That’s fewer storms for a shorter duration than usual, it said