Law
Customary International Law and Transboundary Air Pollution
Quick fact
The Trail Smelter Arbitration (1938/1941) is often cited as the first major case establishing a customary rule: a state has a duty not to cause significant transboundary environmental harm. This principle, now reflected in treaties and judicial decisions, is widely considered part of customary international law, binding all states regardless of formal agreement.
Why this is interesting
Imagine a country burns coal, and the resulting acid rain damages forests in a neighboring nation. No treaty exists between them—yet you might still say the polluting country has broken a legal rule. How can that be?