Transboundary Harm, and the Word 'Significant'

In one line

Not every harm that crosses a border engages responsibility, and the word that does the filtering is significant.

In exam wording: the threshold of the customary obligation of prevention is significant harm, meaning harm which is more than detectable but need not be serious or substantial, and which must be capable of being measured by factual and objective standards; the International Law Commission's Draft Articles on Prevention of Transboundary Harm from Hazardous Activities 2001 codify the obligation, requiring the State of origin to take all appropriate measures to prevent significant transboundary harm or at any event to minimise the risk thereof, to require prior authorisation, to assess the risk, to notify and consult, and to exchange information.

Why a threshold at all

Because every industrial economy emits something that crosses a border. Without a threshold every State would be in continuous breach of an obligation to every neighbour, and a rule that everybody breaches all the time governs nothing.

So the law admits a level of transboundary effect and prohibits what exceeds it. Fixing that level is what the word significant does.

The vocabulary, in ascending order

The threshold is significant, and it is deliberately lower than serious. The International Law Commission's commentary explains it as more than detectable but not necessarily serious or substantial, and requiring a real detrimental effect measurable by factual and objective standards. A candidate who says the threshold is "serious harm" has stated the Trail Smelter formulation rather than the modern one, and should say which is which.

The 2001 Draft Articles on Prevention

Adopted by the International Law Commission in 2001 and commended by the General Assembly. They are not a treaty, and they are the most systematic statement of the duty of prevention available.

Article 1: scope. They apply to activities not prohibited by international law which involve a risk of causing significant transboundary harm through their physical consequences.

That definition of scope contains the whole idea: these are lawful activities. Prevention is not about wrongdoing; it is about managing the risk created by activity that everybody accepts.

Article 2: use of terms. Risk of causing significant transboundary harm includes risks taking the form of a high probability of causing significant transboundary harm and a low probability of causing disastrous transboundary harm. Harm means harm caused to persons, property or the environment. Transboundary harm means harm caused in the territory of or in other places under the jurisdiction or control of a State other than the State of origin, whether or not the States concerned share a common border.