The constantly increasing pressure on the environment caused by the transboundary effects of human activities has posed the problem of defining the limits beyond which such transboundary deleterious effects become legally relevant under international law, giving rise to interstate claims of a compensatory, preventive or procedural nature. The abstract characterization of a certain intensity of transboundary environmental interference by using such attributes as ‘serious’, ‘appreciable’, ‘significant’, ‘considerable’, etc., has become an important method of describing that threshold. It is used in State practice mostly in rules of a general character – often in the absence of or in addition to specific environmental quality standards, limits or prohibitions. It plays a major role in defining the scope of the emerging rule of general international law prohibiting ‘significant’ transboundary environmental harm.