The strongest dust storm in 10 years hit northern China on Monday, illustrating what conservation groups have called an “ecological crisis.”

Li Shuo, policy director for Greenpeace China, told The New York Times that the storm, which grounded hundreds of flights, was “the result of land and ecological degradation in the north and west of Beijing.” Industrial pollutants in the vicinity of Beijing so far this year have already exceeded the last four years’ annual average, he added.

“Beijing is what an ecological crisis looks like. After two weeks of smog and static air, strong wind carries a sand storm in, sending [air quality index] off the chart,” Li said.

Beijing is what an ecological crisis looks like. After two weeks of smog and static air, strong wind carries a sand storm in, sending AQI off the chart. It’s hard to claim we are moving forward when you can’t see what’s in front. pic.twitter.com/m0wS5oYg6O— Li Shuo_Greenpeace (@LiShuo_GP) March 15, 2021

The weather system had its genesis in a winter storm that passed through Mongolia, where it killed at least nine and knocked out power in several regions of the country. In northern China, the air surpassed hazard levels for airborne particles.

The air quality readings reached 999 Monday after averages of around 80 through most of 2020.

China’s weather bureau declared a “yellow alert” due to the storm. The event extended from the northwestern Xinjiang and Gansu provinces to Inner Mongolia and Hebei province, according to NPR. Overall, the storm affected 12 provinces and cities, NPR reported, citing state media.

The weather has been compared to the “airpocalypse” events that struck the country in years past before Chinese Communist Party leaders took steps to slash pollution levels.

Beijing officials imposed a stay-at-home order for children and sick and elderly people as the resulting smog discolored the air. The effects are expected to persist through at least Tuesday morning, according to the Times.

President Xi Jinping has said a “green revolution” is needed in the country and that China will ramp up its efforts to cut carbon emissions, but this target has frequently been at odds with accelerated economic development. Heavy pollution in recent days has been tied to increased production of steel and cement, and China’s Ministry of Ecology and Environment recently told local officials that four steel mills in Hebei province have not adequately cut emissions, according to the Times.