Urban air pollution and respiratory allergy.

Evidence suggests that urbanization with its high levels of vehicle emissions, and westernized lifestyle are linked to the rising incidence of pollen-induced respiratory allergy seen in most industrialized countries. Moreover, the increase in respiratory allergy parallels an increase in outdoor and indoor air pollution. Although the role played by outdoor pollutants in allergic sensitization of airways has yet to be elucidated, it is well established that outdoor pollution exacerbates respiratory symptoms in atopic subjects. This review focuses on the evidence implicating outdoor pollution in pollen-related respiratory diseases. Acute and chronic exposure to such components of air pollution as sulphur dioxide, nitrogen dioxide, ozone and respirable particulate matter (isolated or in various combinations) enhances airway responsiveness to aeroallergens in atopic subjects. By attaching to the surface of pollen grains and of plant-derived paucimicronic particles, pollutants can modify the morphology of these antigen-carrying agents and alter their allergenic potential. In addition, by inducing airway inflammation, which increases airway epithelial permeability, pollutants overcome the mucosal barrier and so "prime" allergen-induced responses. Lastly, air pollutants such as diesel exhaust particulates can also facilitate the immunoglobulin (Ig)E response that leads to pollinosis symptoms in atopic individuals.