THERE is an extensive literature on the photochemical reactions involved in the production of smog, which is reviewed in refs. 1. and 2. Early investigations of this problem3–5 revealed that at extremely low concentrations the oxidative constituents could damage the leaves of certain plants, and that differences in the leaf damage of plants at specified stages in their development could be used to characterize various types of smog oxidants6–8 Went9 contrasts the vulnerability of the photosynthetic cells in a leaf, which must operate by gaseous diffusion in very low carbon dioxide gradients between the free air and the cell contents, with the relatively protected position of the corresponding cells in the human respiratory system. He points out the advantages of exploiting the sensitivity of plant cells to give warning that the tolerance levels in pollution are being exceeded.