In China, tens of thousands of residents have fled their homes amid an air quality “red alert,” as a thick blanket of toxic smog settled in over nearly a half-billion people. Since Friday, authorities have warned of off-the-charts air pollution levels in at least 23 cities, mostly in northern China. Visibility from the smog was so bad in places that officials closed highways and grounded airplanes. China is the world’s largest producer and consumer of coal, which is largely responsible for the smog.