It varies, all over the world. In Ontario, Canada where I live and where no coal is burned for generating electricity, the single biggest source is residential heating at 39 percent, with transportation in second place at 19 percent. In Alaska. 73 percent comes from burning wood.
We have discussed this a number of times on TreeHugger, but as the dangers of PM2.5 become more clear, it is becoming also clear that as charming and beautiful as fireplaces and wood stoves are, we should not be burning wood at all.