My impression of Montana is tempered by the fact that visual images are often hazy due to particulates from dusty roads, agriculture, and wood burning - whether during wildfires or prescribed burns.

Consider a comment in a Missoula newspaper:

Controlled burns: Constant smoke is not healthy
http://missoulian.com/news/opinion/mailbag/article_092b9704-61bb-11df-87e0-001cc4c03286.html

Consider also a clean-air activist's summary:

Dealing with a Wood Smoke Problem
Dec 9, 2008 ...according to US EPA data, “wood smoke emits over 9600% more lead than natural gas!” It also emits arsenic, formaldehyde, mercury...
http://www.shirleybrandie.com/car_newsletter_jan2009.pdf

Or enjoy some of my smog photos:

Smog in Estes Park & Rocky Mountain National Park & other places
http://www.pbase.com/aspergerian/smog_in_colorado_rockies