BEIJING — China's capital woke up to orange-tinted skies Saturday as the strongest sandstorm this year hit the country's north, delaying some flights at Beijing's airport and prompting a dust warning for Seoul.

The sky glowed and a thin dusting of sand covered Beijing, causing workers to muffle their faces in vast Tiananmen Square. The city's weather bureau gave air quality a rare hazardous ranking.

Air quality is "very bad for the health," China's national weather bureau warned. It said people should cover their mouths when outside and keep doors and windows closed.

China's expanding deserts now cover one-third of the country because of overgrazing, deforestation, urban sprawl and drought. The shifting sands have led to a sharp increase in sandstorms — the grit from which can travel as far as the western United States.

The Chinese Academy of Sciences has estimated that the number of sandstorms has jumped six-fold in the past 50 years to two dozen a year.

The latest sandstorm has also affected the regions of Xinjiang and Inner Mongolia and the provinces of Shanxi, Shaanxi and Hebei.