## Facts

- A smelter in Trail, British Columbia, operated by a Canadian private company, emitted sulphur dioxide (SO₂) fumes that crossed into Washington State and damaged crops, forests, and land.
- The smelter’s increased stack height in the mid-1920s contributed to wider dispersion of emissions down the Columbia River valley into the United States.
- After complaints by affected U.S. landowners and authorities, the matter was investigated by the International Joint Commission, which recommended compensation and control measures that did not resolve the dispute.
- The United States and Canada entered a 1935 convention submitting the dispute to an ad hoc arbitral tribunal to determine causation, compensation, and a forward-looking emissions control arrangement.
- The tribunal issued two awards: a 1938 award addressing past damage and indemnity, and a 1941 award addressing state responsibility, legal standards, proof, and a continuing control regime.

## Issues

1. Whether SO₂ emissions from the Trail smelter caused material damage in Washington State during the relevant period.
2. Whether Canada bears international responsibility for transboundary injury caused by a privately operated facility within Canadian territory.
3. What legal standard governs transboundary air pollution and what level of proof is required.
4. What remedies are appropriate for past injury and what ongoing measures are required to prevent future injury.

## Decision

- The tribunal found that the Trail smelter’s SO₂ fumes caused significant damage in Washington, particularly during the period from January 1, 1932, to October 1, 1937.
- Canada was held responsible in international law for permitting operations within its jurisdiction that caused proven, serious cross-border injury.
- The tribunal ordered a lump-sum payment of US$78,000 as final compensation for the specified period of past damage.
- For future matters, the tribunal required that injury be established by “clear and convincing evidence” and be of “serious consequence.”
- The tribunal imposed an injunction-like obligation that the smelter’s operation be controlled so as to prevent damage in the United States and established a continuing technical monitoring and control regime with procedures for review and future claims.

## Legal Principles

- A state may not use or permit the use of its territory in a manner that causes transboundary injury by fumes to another state’s territory or to persons or property there, when the consequence is serious and the injury is proven by clear and convincing evidence.
- A state can be internationally responsible for harmful transboundary effects of activities conducted by private actors within its jurisdiction when the state permits the activity and fails to prevent the prohibited harm.
- Monetary compensation is available for proven, non-speculative injury, while claims for indirect or remote damage may be denied as too uncertain.
- Transboundary pollution disputes may warrant prospective relief through a binding control and monitoring regime, not solely retrospective damages.

## Conclusion

The tribunal awarded compensation for proven cross-border SO₂ damage, held Canada responsible for permitting a private facility’s harmful emissions, articulated a no-harm rule conditioned on serious consequence and clear and convincing proof, and established an ongoing regulatory and claims framework to prevent future injury.


Trail Smelter Arbitration (U.S. v. Can.), 3 R.I.A.A. 1905 (Arb. Trib. 1941)