Columbia subject headings:
Air--Pollution--Government policy--OECD countries.
Automobiles--Motors--Exhaust gas.
Harvard subject headings:
Air--Pollution--Government policy--European Economic Community countries.
Air--Pollution--Government policy.
Automobiles--Motors--Exhaust gas
Automobiles--Motors--Exhaust gas--Environmental aspects--European Economic Community countries.
Princeton subject headings:
Air--Pollution.
Motor vehicles--Motors--Exhaust gas--Environmental aspects.