Story highlights A large sandstorm has covered north east China in a thick smog

Levels of pollution reach beyond the index, residents warned to remain indoors

Beijing (CNN) Beijing's air quality reached hazardous levels Thursday, as a large sandstorm swept through the Chinese capital, turning the sky a murky yellow and forcing many residents to keep indoors.

Sandstorms are common across north east Asia during the spring, with prevailing winds carrying sand and dust particles from the Gobi Desert across the region.

But the speed with which the storm set in astounded locals -- between 3 a.m. and 6 a.m. Thursday, the city's air quality index (AQI) jumped from under 100 to over 500 -- from "moderate" to a rating of "beyond index."

By midday, the AQI level had hit a peak of 621, with PM2.5 levels of 684 micrograms per cubic meter

The WHO's recommended level of PM2.5 -- the smallest and most harmful pollution particles -- is 25 micrograms per cubic meter.

05-04-2017 12:00; PM2.5; 684.0; 621; Beyond Index (at 24-hour exposure at this level) — BeijingAir (@BeijingAir) May 4, 2017

Authorities in Beijing warned residents to avoid outdoor activity.