The Antarctic ozone 'hole' and a smaller deficit over the Arctic have been big news in recent years. So far, the fact that some depletion has also occurred in the stratospheric ozone right above us has received less attention. But it has occurred and, unfortunately, the chances are high that the ozone layer above Australia will continue to thin for some decades. Ozone in the upper atmosphere is an important gas because it absorbs UV-B. It so happens that chlorine and bromine, in reactive forms, can catalyse ozone breakdown. Synthetic compounds, such as the chlorofluorocarbons (CFCs), are currently responsible for 80 per cent of the stratosphere's chlorine. When these chemicals break down due to exposure to UV, their fluorine is generally not a problem for the ozone layer, but the chlorine is.