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Spirometric measures of airway obstruction are strongly influenced by the neurally mediated reduction in total lung capacity that accompanies ozone exposure. This study was conducted to evaluate a method for quantifying the effects of performing spirometry from a reduced inspiratory position on measures of airway obstruction, and to apply the method to published ozone-response data. Nineteen healthy, nonsmoking adults, 18-30 yr of age, each performed 17 forced expiratory maneuvers from inspiratory volumes that ranged from functional residual capacity to total lung capacity. In general, measures of airway obstruction were related strongly to total expired volume, and approximately 80% of the ozone-induced decrease in forced expiratory volume in 1 s (FE[V.sub.1.0]) resulted from the reduction in forced vital capacity. Results indicated that, for young, healthy, nonsmoking adults, spirometric values from maneuvers conducted from reduced inspiratory positions can be predicted accurately and precisely in the unexposed state, allowing interpretation of measures of ozone-induced airway obstruction., AIRWAY CALIBER, lung elastic recoil, and forced expiratory flow (FEF) are related to the lung volume at which they are measured and to volume history. This dependence of FEF on [...]