Highly soluble gases exchange primarily with the bronchial circulation through pulmonary airway tissue. Because of this airway exchange, the assumption that end-exhaled alcohol concentration (EEAC) is equal to alveolar alcohol concentration (AAC) cannot be true. During exhalation, breath alcohol concentration (BrAC) decreases due to uptake of ethanol by the airway tissue. It is therefore impossible to deliver alveolar gas to the mouth during a single exhalation without losing alcohol to the airway mucosa. A consequence of airway alcohol exchange is that EEAC is always less than AAC. In this study, we use a mathematical model of the human lung to determine the influence of subject lung size on the relative reduction of BrAC from AAC. We find that failure to inspire a full inspiration reduces the BrAC at full exhalation, but increases the BrAC at minimum exhalation. In addition, a reduced inhaled volume and can lead to an inability to provide an adequate breath volume. We conclude that alcohol exchange with the airways during the single-exhalation breath test is dependent on lung size of the subject with a bias against subjects with smaller lung size.