In the present work, for the first time in the literature, the relationship between the degree of air pollution, the physiological state of the plants and the allergenic capacity of the pollen they produce has been studied. The physiological state of Lolium perenne plants growing in two cities with a high degree of traffic, but with different levels of air pollution, Madrid and Ciudad Real, have been explored. The photosynthetic efficiency of the plants through the emission of fluorescence of PSII, the degree of oxidative stress (enzymatic activities related to the ascorbate-glutathione cycle), the redox state (reduced and oxidized forms of ascorbate and glutathione) and the concentration of malondialdehyde have been evaluated. During the development period of the plants, Madrid had higher levels of NO2 and SO2 than Ciudad Real. The greater degree of air pollution suffered by Madrid plants was reflected on a lower photosynthetic efficiency and a greater degree of oxidative stress. In addition, NADPH oxidase activity and H2O2 levels in pollen from Madrid were significantly higher, suggesting a likely higher allergenic capacity of this pollen associated to a higher air pollution.