(Bloomberg) -- Beijing warned vulnerable residents to stay indoors Wednesday, as a sandstorm — the third this month — combined with regular industrial pollution to create a thick, unbreathable haze over the city, the worst in two years. 

The city’s Ecological and Environmental Monitoring Center set its highest pollution level and asked people to avoid outdoor activities because of sand and dust blowing in from the outskirts. 

After a few months of post-Covid bustle, sidewalks emptied again; almost everyone walking outside wore a mask and sometimes goggles. A layer of yellow dust thick enough to write on with a fingertip settled on anything that wasn’t moving. 

The gloomy skies aren’t out of place this month, which is on track to be the worst since 2017, according to a pollution meter at the US Embassy in Beijing. Levels of small pollutants typically associated with car and factory exhaust have been relatively high for the past few weeks. When the sandstorm kicked up larger particles of dust and dirt Wednesday, the air quality turned particularly vicious.

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Still, even this smog-choked March pales in comparison to the fetid skies of less than a decade ago, memorialized in the since-banned Chinese documentary Under the Dome. 

Since then, the city has made progress in cleaning the air by replacing coal with cleaner-burning natural gas, supporting electric vehicles and moving some heavy industry further away from population centers. 

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