Columbia subject headings:
Harvard subject headings:
Air--Pollution--United States
Automobiles--Motors--Exhaust gas--Environmental aspects--United States.
Fuel switching--Government policy--United States.
Pollution control industry--United States.
Synthetic fuels--Research--United States.
Princeton subject headings:
Air--Pollution--United States.
Automobiles--Motors--Exhaust gas--Environmental aspects--United States.
Fuel switching--Government policy--United States.
Pollution control industry--United States.
Synthetic fuels--Research--United States.