Under the Convention on Transboundary Air Pollution the UN Economic Commission for Europe has produced a series of international agreements to reduce emissions of pollutants contributing to acidification, eutrophication and formation of tropospheric ozone. This includes the Second Sulphur Protocol signed in Oslo in 1994, which took into account maps of critical loads as those levels of annual deposition which are deemed sustainable without adverse effects. This paper describes scientific and technical work undertaken in the course of development of this protocol, and how the approach might be extended in future as attention turns to new protocols on NOx and VOCs.