Is it reasonable to combine so called “collar counties” that surround large municipalities in the same air quality management district as the large municipalities?

Is it reasonable to combine so called “collar counties” that surround large municipalities in the same air quality management district as the large municipalities? An example of this is in the Chicago air management district, which includes all of Cook County, and the surrounding counties of Kane (a rural farming area), Lake, DuPage, and others. Support your answers in details.