Why was the Trail Smelter in British Columbia restricted from harming Washington state with fumes?

The Trail Smelter arbitration (1941) restricted the smelter in British Columbia from causing damage to the U.S. state of Washington based on a fundamental principle of international responsibility.1

The tribunal held that no state has the right to use, or permit the use of, its territory in a manner that causes injury by fumes to the territory, properties, or persons of another state. This decision established a critical precedent in environmental law, affirming the ethical principle that nations must ensure activities within their jurisdiction do not inflict environmental harm upon their neighbors. This concept aligns with the broader prevention principle, which emphasizes that preventing environmental damage is essential for global stewardship and the protection of shared natural foundations.123