ABSTRACT
The principle of preventive action finds its origin in the rules governing state liability. In 1941, in the Trail Smelter dispute, Canada was held responsible for the damage caused to American farmers by emissions emanating from a plant on the Canadian side of the border. The arbitral tribunal said that a state is not allowed to use its territory in a way that can harm the territory of another state. In international law, states have a responsibility to prevent damages they can cause outside their territory, in spite of their sovereignty over their natural wealth and resources. With respect to the protection of the environment, a correlating duty has emerged imposing on states the responsibility to control and correctly manage activities based on their territory if they can impact on the territory of another state (Sands 1995; Paradell-Trius 2000; Trouwborst
2009). This obligation is limited to a conduct of “due care” or “due diligence”: a state will not be held liable for damages if it took reasonable measures to prevent them from happening (Crawford et al. 2010). That duty has been articulated during summit diplomacy in both Principle 21 of the Stockholm Declaration and Principle 2 of the Rio Declaration. It has found application in situations of transboundary harm and pollution and has been clearly stated as a customary rule of international law in several international litigations and dispute settlement mechanisms (Trail Smelter, Lac Lanoux, Gabcikovo-Nagymaros) (Sands 1995). Its precise normative content is provided for in bilateral and multilateral treaties such as the Convention on Environmental Impact Assessment and the transboundary air pollution regime, which formulate concrete measures of prevention applicable between states (assessments, consultation, cooperation, monitoring sources of potential transboundary harm). Moreover, in 2001, the International Law Commission codified the specific elements resulting from that due care obligation in the Draft Articles on Prevention of Transboundary Harm from Hazardous Activities (Barboza 2011).
