Outdoor air pollution in urban areas and allergic respiratory diseases

Respiratory allergic diseases (rhinitis, rhinosinusitis, bronchial asthma and its equivalents) appear to be increasing in most countries, and subjects living in urban and industrialized areas are more likely to experience respiratory allergic symptoms than those living in rural areas. This increase has been linked, among various factors, to air pollution, which is now an important public health hazard. Laboratory studies confirm the epidemiological evidence that inhalation of some pollutants, either individually or in combination, adversely affect lung function in asthmatics. The most abundant air pollutants in urban areas with high levels of vehicle traffic are respirable particulate matter, nitrogen dioxide and ozone. While nitrogen dioxide does not exert consistent effects on lung function, ozone, respirable particulate matter and allergens impair lung function and lead to increased airway responsiveness and bronchial obstruction in predisposed subjects. However, besides acting as irritants, airborne pollutants can modulate the allergenicity of antigens carried by airborne particles. 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 after their allergenic potential. In addition, by inducing airway inflammation, which increases airway epithelial permeability, pollutants overcome the mucosal barrier and so facilitate the allergen-induced inflammatory responses. Moreover, air pollutants such as diesel exhaust emissions are thought to modulate the immune response by increasing immunoglobulin E synthesis, thus facilitating allergic sensitization in atopic subjects and the subsequent development of clinical respiratory symptoms.