Such statements within themselves are stupid and unscientific - chem prof or not.
Denver and its notorious "brown cloud" hung over the entire town. Obviously, car exhaust is found near roads. And property near roads are cheaper. So, where do poorer folks live? In the cheapest places. Secondly, "agriculture" is mostly cattle near Denver with most of the plow ground NE of town along the Platte River. Further the prevailing winds blow off the mountains and to the north and east, not the reverse direction. Therefore, any pollution from agriculture is extremely limited in Denver proper. The gas wells of the region and there were lots of them, are also a nothing burger composed mainly of some minor amounts of methane in addition to a lot of natural methane seeping from the ground. (without a long-winded explanation of natural methane, I'll briefly say that the geology of the Dakota Sands and other upturned rocks allows gas to escape at the surface.) So, a lot of the methane is natural and, of course, difficult to differentiate from leaking wells since wells tend to be drilled in areas with high natural gas seepages... In fact, was one way such gas fields were discovered.
As for the scope of the problem, the most gas seepage occurs in S. Russia and surrounding petroleum areas. As for wildfires, the incidence of wildfire is far below that it was 100 years ago when we peaked at about 10-15x the wildfires today. And "wildfire" is a misnomer since arson plays a significant part of today's fires. The majority of fires in 1920 were truly wild. No so today where most fires are deliberately set or a badly managed "controlled" fire. In fact, the homeless make this an even bigger problem by camping out in the woods and cooking over a fire.
You can go anywhere in the high plains and find dust in the air when the wind blows. While much of that is blown off fields, it is a product of dry climate and high winds. I've been in the Tularosa basin of New Mexico and trust me, it is a dirty sky blowing over the White Sands and has zero to do with agriculture or oil (because there is none.) As an old West Texan who came to Oklahoma City said, "You guys have wind but the air has no color to it..." (meaning the wind blows hard in both places, but the sky is blue down to the horizon but in West Texas the horizon is a brownish orange color)
