Abstract: The Environmental Protection Agency (EPA) compiles data on emission tonnages of air pollutants for US counties.  US Air Quality Gradebook ("AirGrades") source sheets such as this one for Hillsdale County, Michigan, and

Houghton County, Michigan, give the percentage contributions from specific air pollution sources.  The Hillsdale County Seat is Hillsdale, Michigan, and the Houghton County Seat is Houghton, Michigan.