Compare Beijing on Saturday with London during 1952's killer smog. Not just Beijing either; this is apparently happening all over the industrialized parts of East/Northeast China. It's a particularly cold winter, so maybe people are burning more coal. Xinhua and co are reporting this as fog, which sounds like one of those technically accurate statements intended to mislead.
The '52 event in London led directly to the Clean Air Act, which people of my parent's generation in Stoke still talk about as though it was a kind of miracle: there was a directive from above and lo, the skies cleared. Even so, I was in Beijing at this time in 1999 and the coal grit I could tatse in the air reminded me of growing up a couple of miles down the road from Shelton Bar steelworks. But we didn't have the black snow my grandma used to tell me about. Wonder if they have that in Beijing now.