## 1 Introduction  
Both asthma and allergic rhinoconjunctivitis, as well as other diseases of the exact etiology in general, are pathologies that, in recent decades, have been reaching increasing levels of prevalence worldwide. Pollen is considered as the primary aeroallergen inducing allergic hypersensitivity reactions. In addition, among people living in urban areas, there is a higher prevalence rate of these pathologies, which induce inadequate responses of the immune system to substances that, under normal conditions, are harmless [1]. In addition, the existence of atmospheric pollutants determines an interaction with aeroallergens capable of increasing their allergenic capacity. This interaction can occur according to different mechanisms. Thus, these pollutants can enhance the release of allergens present in pollen grains by weakening their cell walls, modifying their allergenic potential, acting as adjuvants for the stimulation of immunoglobulin E (IgE)-mediated responses, as well as acting as stressors, causing the exacerbation of the manifestation of some allergens present in pollen grains, and thus inducing immune responses in humans [1; 2; 3; 4]. The effects of atmospheric pollutants on pollen grains will differ depending on the pollen type involved. Thus, for example, in _Cupressaceae_, they can cause an increase in the fragility of the exine and, consequently, an increase in the allergenicity of the affected pollen grain, as well as an increase in the expression of the Cup a 3 allergen; or, in _Platanus_, they can cause an increase in the total protein content of the pollen grain, with a consequent increase in the levels of the major allergen of this pollen type, Pla a 1, after exposure of the pollen grain to gaseous pollutants [1; 5]. On the other hand, as shown in other studies, meteorological parameters also act synergistically on pollen grains (as well as on all types of aerobiological particles and pollutants) in the atmosphere. These parameters strongly influence--in a wide variety of ways and on different time scales--their presence and concentration in the atmosphere. This occurs both in the stages prior to flowering and during flowering itself, in the stages of emission, transport, dispersion, and/or deposition [6]. Thus, wind generally acts as a promoter of pollen grain transport if its intensity is within certain limits [6; 7; 8]. Solar radiation, with its accumulation over time, acts, like insolation, as a stimulator of biological activity and as an agent enhancing the increase in pollen concentrations in the atmosphere. However, ultraviolet radiation can lead to a stress modification of plant physiology and, together with an excessive increase in temperature and a variation in humidity in time and space, can cause excessive production of pollen grains and lead to a consequent increase in their allergenicity, with negative consequences for the health of citizens [9; 10; 11; 12]. If atmospheric pollutants bind to these pollen particles, an allergen with increased allergenic potential may be formed, leading to exacerbations in the manifestations of asthmatic disease or any other pollen allergic pathology. Likewise, pollen grains can release biologically active lipids, which can provoke the activation of immune cells. Thus, the morphological changes induced by atmospheric pollutants in pollen grains could be behind the increase in allergic diseases referred to above, which implies that in urban areas, the allergenicity of pollen may increase in comparison to rural areas. There is increasing evidence from studies indicating an association between urbanization and an increase in the severity and prevalence of allergic symptoms. Accordingly, it is necessary to manage air pollution and urban planning in terms of the selection of plants for green areas in cities, limiting as far as possible those with anemophilous pollination whose pollens have a higher allergenic potential [1; 13].