Strong scientific evidence has shown that ordinary peaks of outdoor air pollution worsen the symptoms and control of asthma. As for chronic exposure, elevated mean level of local, near-road air pollution may cause increased incidence of asthma among children, and probably also among adults. By contrast, while there is no doubt that air pollution worsens allergic inflammatory processes, it is not clearly established that it may increase allergic sensitization among the general population. In this regard, more research is needed, particularly on the effects of outdoor air pollution in the early periods of life.