For the third year in a row, massive dust storms from China have blown into South Korea, closing schools, canceling flights, and creating a run on facemasks and respiratory medication. The storms are the result of severe desertification in China, where the Gobi Desert grew by 20,000 square miles from 1994 to 1999; the desertification stems from overfarming, overgrazing, and deforestation, among other causes. In Seoul, 750 miles away, dust levels usually measure 70 micrograms per cubic meter of air; during last week’s dust storm, the reading was 2,070 micrograms, over twice the level deemed hazardous. And folks in South Korea aren’t inhaling mere dirt or sand: Dust from the expanding desert in China binds with airborne pollutants from the rapidly industrializing country, increasing the health hazards. Chinese dust has also traveled on the jet stream to San Francisco and Portland, Ore., heightening the beauty of sunsets but having little impact on health so far.