Columbia subject headings:
Air--Pollution--Economic aspects--United States.
Automobiles--Environmental aspects--United States.
Automobiles--Motors--Exhaust gas.
Environmental policy--United States.
Harvard subject headings:
Air--Pollution--Economic aspects--United States
Automobiles--Environmental aspects--United States.
Automobiles--Motors--Exhaust gas
Environmental policy--United States
Princeton subject headings:
Air--Pollution--Economic aspects--United States.
Automobiles--Environmental aspects--United States.
Automobiles--Motors--Exhaust gas.
Environmental policy--United States.