The tropospheric level of the phytotoxic air pollutant ozone has increased considerably during the last century, and is expected to continue to rise. Long-term exposure of higher plants to low ozone concentrations affects biochemical processes prior to any visible symptoms of injury. The current critical level of ozone used to determine the threshold for damaging plants (biomass loss) is still based on the seasonal sum of the external concentration above 40†nl∑l22121 (AOT40). Taking into account stomatal conductance and the internal capacity of leaf defences, a more relevant concept should be based upon the 'effective ozone flux', the balance between the stomatal flux and the intensity of cellular detoxification. The large decrease in the Rubisco/PEPc ratio reflects photosynthetic damage from ozone, and a large increase in activity of cytosolic PEPc, which allows increased malate production. Although the direct detoxification of ozone (and ROS produced from its decomposition) is carried out primarily by cell wall ascorbate, the existing level of this antioxidant is not sufficient to indicate the degree of cell sensitivity. In order to regenerate ascorbate, NAD(P)H is needed as the primary supplier of reducing power. It is hypothesised that increased activity of the catabolic pathways and associated shunts (glucose-6-phosphate dehydrogenase, NADP-dependent glyceraldehyde-3-phosphate dehydrogenase, isocitrate dehydrogenase and malic enzyme) can provide sufficient NAD(P)H to maintain intracellular detoxification. Thus, measurement of the level of redox power would contribute to determination of the 'effective ozone dose', serving ultimately to improve the ozone risk index for higher plants.