Salt Lake County, Weber County, and Utah County all suffer from inversions during cold weather months, which is much of the winter.

Inversions are where a layer of warm air traps cold, polluted air inside the valley and air pollution accumulates because it can't go anywhere. I grew up in Utah county and I remember the Geneva steel plant was a huge source of pollution. In my environmental studies class we calculated that on cold weather days when there was an inversion the amount of pollution each person breathed every day was equivalent to smoking 3 cigarettes.

Even before internal combustion engines and modern industry the campfires alone must have given Utah valleys the reputation as "smokey valley" by the native Americans. This is not a new phenomenon - the geography of high mountains with lots of population causing pollution in the valleys causes it naturally.