Evidence suggests that allergic respiratory diseases have become more common in the last three decades, and, in particular, asthma morbidity appears to be increasing in industrialized countries all over the world. Moreover, it is a common consideration that there is a higher frequency of allergic respiratory symptoms among subjects residing in industrial and residential areas than among those living in rural ones. There is also evidence that at the same time as atopic respiratory diseases are increasing, there is an increasing atmospheric concentration of various pollutants, such as gases or particulate matter. It is known that three main factors are responsible for clinical outbreaks of atopic respiratory allergy: a sensitizing agent, a genetically committed individual, and a certain degree of environmental pollution. The interplay of genetic and environmental factors in the development of bronchial asthma remains a subject of investigation. Since bronchial asthma is a multifactorial condition associated with, and affected by, a large variety of environmental and host factors, it may be difficult to pinpoint a single factor that will account for the rise. Data from various investigators in different parts of the world suggests that air pollution contributes to allergic sensitization of the airways in predisposed subjects and, consequently, putative cofactors in increasing the risk of respiratory allergy are believed to be the increasing number of people moving from the country to live in urban districts and the increasing atmospheric pollution. In the atmosphere of urban areas pollen allergens can interact with components of air pollution, which could modify the allergenic potency of these particles. In addition, diesel exhaust emissions are thought to stimulate IgE synthesis, thereby facilitating allergic sensitization in predisposed subjects and the subsequent development of atopic respiratory diseases.