This is one of those things that not too many people know about. it is usually mandated by the EPA, with little or no notification to the general public. States can decide to put tougher restrictions, but most don't.
Back in the early 90's I had a manufacturing company and we did painting. I got all of the necessary permits from the local building department and Fire Marshall. I thought I was good to go but a few months later, this EPA inspector comes by and starts right off with a $20,000 fine and an order to shut down my spray booth. It seems that the county I lived in was a non-attainment county and I was required to get an EPA permit and use low VOC paints and log everything. Three miles up the road, in another county, no EPA permit was required, because they were considered to meet the EPA requirements for air quality. Basically I could have moved my operation 3 miles and polluted the environment as much as I wanted to and this was OK with the EPA. Even in my county, if I had a body shop and was doing "refinish" painting, I could use any kind of paint I wanted. Since I was building new products, I had to meet the EPA restrictions.
In any case, this is why you have smog inspections/restrictions in some counties within a State, and not others.
Here is a list of the non-attainment counties per State and it will give you some idea of how the EPA works when it comes to pollution and, in the case of diesels, particulate matter. As you can see, the EPA considers Phoenix to have a "serious" problem but if you go to Lake Havasu City, you can belch great clouds of stuff and they don't care.
EPA rules
