Since 1997, there have been a number of epidemiological studies in Europe and Japan concerning the incidence and persistence of respiratory diseases, primarily asthma, in urban site of known high pollution. Most of these studies focused on automobile-related pollutants in heavily traveled expressways and their relationship to asthma symptoms caused by seasonal allergens. They have generally concluded that high vehicle traffic is associated with asthma, cough, and wheeze in children with known allergic sensitization.