The disruptions follow smog problems that hit much of northern China starting in mid-December. At the time, officials closed legions of factories and restricted car travel.

As Reuters notes, smog alerts are relatively typical in northern China, where the winter cold has coal power plants working hard to meet demand. However, the pollution this time around was definitely bad -- concentrations of harmful PM2.5 particles were about 50 times higher than World Health Organization guidelines. China has been eager to reduce pollution in recent years, but it's clear that existing efforts haven't been enough to prevent smog from causing serious disruptions.