Smoke from burning wood can contribute a significant percentage of the total air pollution. The San Francisco Bay Area is considering a ban on wood burning on Spare the Air days during the winter, not all days. Similar to the existing ban on using charcoal BBQ's, lawn mowers, etc. on those bad air days during the summer months. Unfortunately, on those cold, foggy nights, the air pollution stays close to the ground and just accumulates.

I read a stat that a study in San Jose, California (a major metro area, not particularly a hotbed of wood burning activity, I would think) found that 40% of the particulate load in the winter there was attributable to wood burning. And as particulate load goes up, then health problems like asthma and heart disease-related problems are more common, so you see things like more ER visits for asthma. Plus there are assorted carcinogens emitted in wood smoke which are not found in vehicle emissions.