Our area had an emissions program until a couple years ago. Not being a tree hugger it's hard for me to admit testing actually helped. Spokane is in a hole surrounded by hills and mountains. Many days a year the area would get inversions and trap the air at the surface. The air got bad during those times.
They only went back 25 years so older cars were exempt, however if it did need testing and inspection, it better be clean, and every emission component better be on it. Those items included the air cleaner, hot air tubes, complete exhaust, EGR all of it.
Cars are cleaner today, and actually it was the EPA who requested the program be canceled.