sandstorm in China's capital Beijing drove the city's air quality index to dangerous levels on Wednesday prompting authorities to warn people to keep their windows closed and wear dust masks. A blue warning, the lowest in China's four tier system of air quality warnings was issued for residents in the city's north and surrounding provinces.
An air quality reading by the US State Department based on concentrations of breathable particles known as PM2.5 registered as very unhealthy. China's own official reading calculating six different pollutants, including PM2.5, registered a hazardous rating. Chinese authorities frequently blame Mongolia's Gobe Desert for the sand storms, which register on the index as larger particles known as PM10.
Beijing's air quality alert was due to last until Wednesday night local time.