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 Sierra County, California, and

Siskiyou County, California, give the percentage contributions from specific air pollution sources.  The Sierra County Seat is Downieville, California, and the Siskiyou County Seat is Yreka, California.