OBJECTIVE The relation between outdoor air pollution and asthma has always been a major focus of research. The evidence that current levels of air pollution in many countries result in increased morbidity and mortality is fairly consistent. With rapid urbanization in many communities, traffic exhausts have become the major source of pollution, and many recent research studies have attempted to investigate the detrimental effects of this type of pollution. This paper reviews the recent evidence of the possible detrimental effects of ambient air pollution on the inception and morbidity of asthma. RESULTS Traffic related pollution has been confirmed in both cross-sectional and longitudinal studies to be associated with increased asthma morbidity and cardiopulmonary mortality. There is also evidence that pollutants such as ozone and traffic exhausts may be responsible for new incident cases of asthma. Among the particulate pollution, research investigating the ultrafine particles and the bacterial components suggested that these particles may have important role in asthma morbidity. CONCLUSIONS More research studies are needed to reveal how various air pollutants may interact with the host systems, such as the immune system, leading to increased morbidity in susceptible individuals. Reduction of the current levels of ambient air pollution should be an integral part of the overall effort in minimizing asthma morbidity or mortality in the community.