Ozone is a molecule comprising three oxygen atoms. Comparatively rare in the Earth's atmosphere, 90" is found in the stratospheric 'ozone layer', between 10 and 50 km above the Earth's surface. It absorbs all but a small fraction of the biologically active ultraviolet radiation emanating from the sun. Action in the US had proceeded somewhat further, spurred by earlier debates over the impact on the ozone layer of nitrogen oxide emissions from supersonic aircraft. Negotiations for an international convention on the ozone layer to regulate CFC production and use, started in 1981, proceeded slowly. At the heart of the Montreal Protocol lie the control measures it imposes on the production and consumption of ozone-depleting substances (ODS). Developing countries are treated differently from industrialized countries by the Montreal Protocol: a key feature of the agreement and one subject to much negotiation.