Chronic exposure of children to a complex mixture of air pollutants leads to recurrent episodes of upper and lower respiratory tract injury. An altered nasal mucociliary apparatus leaves the distal acinar airways more vulnerable to reactive gases and particulate matter. The heterogeneity of structure in the human lung can impart significant variability in the distribution of ozone dose and particle deposition; this, in turn, influences the extent of epithelial injury and repair in chronically exposed children. Cytokines are low-molecular-weight proteins that act as intercellular mediators in inflammatory reactions, including lung injury of various etiologies. Cytokines are involved in generating inflammatory responses that contribute to injury at the lung epithelial and endothelial barriers. Mexico City is a 20-million-person megacity with severe air pollution problems. Southwest Metropolitan Mexico City (SWMMC) atmosphere is characterized by a complex mixture of air pollutants, including ozone, particulate matter (PM), and aldehydes. There is radiological evidence that significant lower respiratory tract damage is taking place in clinically healthy children chronically and sequentially exposed to air pollutants while growing up in SWMMC. We hypothesize that there is an imbalanced and dysregulated cytokine network in SWMMC children with overproduction of proinflammatory cytokines and cytokines involved in lung tissue repair and fibrosis. The nature of the sustained imbalance among the different cytokines ultimately determines the final lung histopathology, which would include subchronic inflammation, emphysema, and fibrosis. Cytokines likely reach the systemic circulation and produce systemic effects. Individuals with an underlying respiratory or cardiovascular disease are less able to maintain equilibrium of the precarious cytokine networks. 2000 Harcourt Publishers Ltd.