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Background Mexican-born children living in the US have a lower prevalence of asthma than other US children. While children of Mexican descent near the Arizona-Sonora border are genetically similar, differences in environmental exposures might result in differences in asthma prevalence across this region. Objective To determine if the prevalence of asthma and wheeze in these children varies across the AZ-Sonora border. Methods The International Study of Asthma and Allergy in Children written and video questionnaires were administered to 1753 adolescents from five middle schools: Tucson (school A), Nogales, AZ (schools B, C), and Nogales, Sonora, Mexico (schools D, E). Prevalence of asthma and symptoms was compared, with analyses in the AZ schools limited to self-identified Mexican-American students. Results Compared with the Sonoran reference school E, the adjusted odds ratio for asthma was significantly higher in US schools A (OR 4.89, 95%CI 2.72-8.80), B (3.47, 1.88-6.42), and C (4.12, 1.78-9.60). The adjusted odds ratio for wheeze in the past year was significantly higher in schools A (2.19, 1.20-4.01) and B (2.67, 1.42-5.01) on the written questionnaire and significantly higher in A (2.13, 1.22-3.75), B (1.95, 1.07-3.53) and Sonoran school D (2.34, 1.28-4.30) on the video questionnaire compared with school E. Conclusion Asthma and wheeze prevalence differed significantly between schools and was higher in the US. Environmental factors that may account for these differences could provide insight into mechanisms of protection from asthma.

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