The Yellow Dust storms (or hwangsa, as they call it in Korea) are a phenomenon quite common in countries like Korea, Japan, or China. These storms are basically dust clouds coming from the Gobi Dessert that are brought to Korea by strong winds. This dust, on its way to Korea is mixed with sulfur, ash, carbon monoxide, and other pollutants released by Chinese factories, and passes over to the two Koreas, and Japan as a yellow fog which makes you not feeling like going out at all.
During days with hwangsa, it is very common to see people in the streets wearing these surgical masks to protect themselves from the dust, from children to the elders. These masks are usually sold in the street or in the subway stations, and there are a lot of different models to choose from (someday I will make a post about those too).
A child protecting himself from hwagsa with a mask (and from the cold with his bear hat.