I agree that it existed, but think it reasonable to assume that it became markedly worse and continues to "evolve" even now. The ultra-fine particulates are obvious suspects and are a little bit frightening once one takes a close look. They are "nanoparticles" and we are just now beginning to appreciate what a threat they may pose. For one thing they are so small that traditional barriers such as the BBB and the similar one lining the gut are pretty much ineffective. These guys are so small that a neuron looks like a super highway to them and they move along in both directions. They also are able to latch onto larger molecules and transport them across those barriers as well. As an irritant alone they are a constant source of inflammation plus they go deep into our lungs. They are, indeed, produced by burning wood but the smallest come from coal and diesel smoke. And they travel far with the wind. One area is called the "Alaskan Rectangle of Death" because global wind conentrates them north of Fairbanks. And remember that if the hypothyses (sp?) that I push is correct and it is about an inflammatory response to a number of possible triggers, that they would interact with most others to multiply their effects.
Rick