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The prevalence of asthma-a chronic inflammatory respiratory disease-is increasing worldwide. One hypothesis suggests that this trend is related to enhanced exposure to air pollutants. Chronic inflammation generates oxidative stress, and cells involved in an allergic reaction are capable of producing reactive oxygen species that may predispose asthmatics to increased deoxyribonucleic acid (DNA) damage. The authors estimated DNA strand breaks by use of single-cell gel electrophoresis assay on 2 different cell types (i.e., nasal epithelial cells and leukocytes) sampled from asthmatic and nonasthmatic medical students in Mexico City. The authors found that asthmatic subjects had more DNA breaks in their nasal epithelial cells than did their nonasthmatic counterparts. In contrast, asthmatic subjects had less damage in their leukocytes than did nonasthmatic individuals. These findings suggest that the hyperreactivity of the nasal epithelium prevents systemic effects from air pollutants, as reflected by less DNA injury to leukocytes of the asthmatic group. Asthmatic's nasal epithelial cells were more sensitive to DNA damage than were those of nonasthmatics-perhaps as a consequence of increased fragility induced either by air pollution or by a chronic inflammatory response.