Transboundary Pollution

In one minute

Transboundary pollution is pollution that starts in one State and harms people, land or water in another State, or in areas that belong to no State. Smoke across a border, a poisoned river and a thinning ozone layer are all examples.

An Indian court cannot order a factory in another country to stop. So the answer lies in international law. Its oldest rule is the no-harm rule: a State must not let its territory be used to cause serious harm to a neighbour.

Legal provision

• Trail Smelter Arbitration (1938 and 1941): no State may use its territory, or let it be used, to cause serious injury by fumes in another State.
• The injury must be shown by clear and convincing evidence.
• Stockholm Declaration, 1972, Principle 21: States have the sovereign right to use their own resources. They also have a responsibility to ensure that activities within their control do not damage the environment of other States or of areas beyond national jurisdiction.
• Rio Declaration, 1992, Principle 2: repeats the same idea of State responsibility.
• Customary international law: the no-harm rule binds States even without a treaty.

Meaning and definition

Transboundary pollution is harm to the environment of one State, or of an area beyond any State, caused by a source in the territory or control of another State. It has two shapes.

• Point-to-point harm: the source and the victim can both be named and lie fairly close. Example: smoke from a border factory, or waste dumped into a river that flows into the next country.
• Global harm: the pollutant mixes into the air or the sea first, and the damage appears everywhere. Examples are ozone depletion and climate change.

1. 1. A source in State A releases fumes or waste.
2. 2. The pollution crosses into State B.
3. 3. State B shows serious harm with clear evidence.
4. 4. State B claims against State A under the no-harm rule.
5. 5. ResultTalks, a treaty or arbitration decide on stopping the harm and on compensation.

Detailed explanation

Step 1: why domestic law is not enough

The Water Act, the Air Act and the Environment (Protection) Act work inside India. They cannot bind a factory abroad. An Indian court's writ does not run beyond India's borders. A foreign victim also finds it hard to sue in the polluter's country, because of cost, language and unfamiliar procedure.

That is why these disputes have mostly been settled between States, by talks, treaties and arbitration, and not by the victims suing in court.

Step 2: the no-harm rule

The rule is a case of the old idea expressed in the maxim sic utere tuo ut alienum non laedas (use your own property so as not to harm another's). The Trail Smelter tribunal applied it between States. It is treated as customary international law (rules accepted as law by State practice), so no treaty is needed.

Stockholm Principle 21 balances two things. A State is free to use its own resources, but it must ensure that activities under its control do not damage other States. The no-harm principle is the working form of that balance.

Step 3: State responsibility and due diligence

The polluter is often a private company. The State is still answerable. Its own wrong is the failure to regulate and control an activity in its territory. This is State responsibility. The usual standard is due diligence: the State must take reasonable steps to prevent serious harm. Detailed study of this standard belongs to the note on State liability.

Step 4: remedies

A State that breaches the rule must stop the wrongful conduct and make good the injury. In the Trail Smelter case, the tribunal awarded compensation for past damage. It also fixed a regime of emission limits so that the smelter could keep working without fresh damage.

Step 5: the procedural side

The law also asks for process. In Pulp Mills on the River Uruguay (2010), the International Court of Justice found that Uruguay broke its procedural duties to inform and cooperate with Argentina before authorising two mills. The Court did not find that the mills had caused significant pollution. A State planning a risky project near a border should therefore give notice and assess the risk first.

Step 6: real events that show the problem

The 1986 Chernobyl accident spread radioactive fallout across borders. Acid rain from industry in one country has damaged forests and lakes in its neighbours. Such events led States to adopt treaties on specific problems, which later notes in this subject study.

Exceptions and special rules

Global harm is hard to litigate

For climate change, every State's emissions mix in one atmosphere. Showing by clear and convincing evidence that one State caused a particular injury in another State is very hard. Treaties are therefore the main response. The ICJ's advisory opinion of 23 July 2025 adds that the rules of State responsibility also apply to climate obligations.

The serious-harm threshold

The tribunal in Trail Smelter spoke of a case of serious consequence. Minor or trivial harm does not breach the rule.

Don't confuse: point-to-point and global harm

Point	Point-to-point harm	Global harm
Source	One identifiable source	Emissions from many States together
Victim	A named area or State	Every State
Proof of cause	Possible with clear evidence	Very hard to show for one State
Usual response	No-harm claim or bilateral talks	Treaty regime among all States

Common mistakes

• Writing that a private victim can sue the foreign polluter before an international court. Claims are usually State against State.
• Saying every border-crossing pollutant is unlawful. The rule needs serious, proved harm.
• Calling the no-harm rule a treaty rule. It is customary international law.
• Treating Stockholm Principle 21 as binding on its own. The Declaration is soft law, but the rule behind it is customary.

Apply it

Problem

A tannery cluster in State X, near its border with State Y, drains untreated chemical waste into a river that flows into State Y. Fishing villages in State Y lose their catch and their drinking water for months.

Issue

Can State Y hold State X responsible, and what can it ask for?

Rule

A State must not use or permit the use of its territory to cause serious injury to another State, shown by clear and convincing evidence. The standard is due diligence.

Application

The harm is serious and traceable to the tannery cluster. State X licensed the tanneries and did not control the discharge. State Y can show this with test results.

Conclusion

State Y can claim that State X must stop the discharge and make good the loss, by talks or arbitration. The villagers themselves cannot sue State X in an international court.

Leading case laws

• Trail Smelter Arbitration (United States v. Canada)

  Facts. Sulphur dioxide fumes from a Canadian lead-zinc smelter at Trail, British Columbia, damaged crops and forests across the border in Washington State.

  Principle. No State may use or permit the use of its territory to cause serious injury by fumes in another State, if the injury is proved by clear and convincing evidence.

  Why it matters. It is the founding authority for the no-harm rule and for State responsibility for private polluters.

How to write this answer in the exam

Key points to remember

• Transboundary pollution: harm in one State, or beyond any State, from a source in another State.
• Two shapes: point-to-point and global.
• No-harm rule: Trail Smelter (1938 and 1941); Stockholm Principle 21.
• The State answers for failing to control private polluters; the standard is due diligence.
• Global harm is handled by treaties, not by claims after the event.

Quick self-test

Related topics

• International Environmental Regime
• State Liability
• Customary International Law