In this paper we examine optimal emission standard policies under the assumption of an international situation of open economies. Our major results indicate that: (i) the welfare-maximizing emission standards would be looser than the environmentally optimal standards; (ii) given governments pursuing social welfare maximization, international cooperation on setting common emission standards could result in relatively tight standards; and (iii) given that governments maximize net environmental benefits and that pollution is a local (or regional) problem, international cooperation on setting common emission standards could result in relatively loose standards.