The role of outdoor air pollution in causing chronic obstructive pulmonary
disease or asthma needs to be studied further in order to separate out
the effects of single pollutants from the combined effects of the complex
mixture of air pollutants in urban atmospheres (310). The impact of outdoor
air pollution appears to be smaller than that of cigarette smoke and indoor
pollution (in respect of chronic obstructive pulmonary disease) and that of
allergens (in respect of asthma) (107, 311–314). Outdoor air pollutants are of
particular concern in low and middle income countries (315).