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Asthma prevalence in the Cataño Air Basin of Puerto Rico is 27% for children aged 13–14 years and 45% for children aged 5–6 years. There is concern that these rates are related to air pollution. The authors conducted a nested case-control study to evaluate whether proximity to air pollution point sources was associated with increased risk of asthma attacks. For 1997–2001, 1,382 asthma-related medical visits (International Classification of Diseases, Ninth Revision, codes 493 and 493.9) in children under 17 were identified through health insurance claims. Controls were children with no asthma attacks who were randomly selected from enrollees in two health insurance companies by incidence density sampling (1:5) and matched to cases on gender, age, insurance company, and event date. The distance from a point source to the subject's residence area represented a surrogate exposure measurement. Odds ratios for a 1-km decrease in distance were obtained by conditional logistic regression. Risk of asthma attack was associated with residing near a grain mill (odds ratio (OR) = 1.35), petroleum refinery (OR = 1.44), asphalt plant (OR = 1.23), or power plant (OR = 1.28) (all p's < 0.05). Residence near major air emissions sources (>100 tons/year) increased asthma attack risk by 108% (p < 0.05). These results showed that proximity to some air pollution sources is associated with increased risks of asthma attacks. [ABSTRACT FROM AUTHOR]