A State owes at all time a duty to protect other States against injurious acts by individuals from within its jurisdiction…. But the real difficult often arises rather when it comes to determine what … is deemed to constitute an injurious act.
… no case of air pollution dealt with by an international tribunal has been brought to the attention of the Tribunal nor does the Tribunal know of any such case. The nearest analogy is that of water pollution
There are, however, as regards both air pollution and water pollution, certain decision of the Supreme Court of the United States which may legitimately be taken as a guide in this field of international law, for it is reasonable to follow by analogy, in international cases, precedents established by that court in dealing with controversies between States of the Union…. where no contrary rule prevails in international law and no reason for rejecting such precedents can be adduced from the limitations of sovereignty inherent in the constitution of the United States
Considering the circumstances of the case, the Tribunal holds that the Dominion of Canada is responsible in international law for the conduct of the Trail Smelter … it is therefore, the duty of the government of the Dominion of Canada to see it that this conduct should be in conformity with the obligation of the Dominium under international law as herein determined