The incidence of allergic diseases has increased dramatically over recent years in most industrialized countries of the world. Air pollution from motor vehicles has been implicated as one of the factors that are responsible for this increase. Epidemiological studies carried out in different geographical regions in the world have shown a significant and consistent association between ambient levels of pollutants and increased asthma and rhinitis symptoms. Recent human and animal exposure studies, as well as laboratory-based studies, have demonstrated that diesel particles, ozone and nitrogen dioxide induce an inflammatory response that involves various inflammatory cells, mediators and adhesion molecules, which could contribute to worsening of the allergic disorders. The present review describes our current understanding of the mechanisms by which pollutants such as diesel exhaust and ozone enhance the underlying allergic inflammatory response, and possibly explain the associations between pollutants and increasing allergic diseases noted in epidemiological studies.