INTRODUCTION Although sulfur dioxide produces subjective complaints of irritation at concentrations as low as 6 ppm@ the usual quantitative measures of pulmonary function have not shown significant degradation even after exposures to 8 ppm. We found no effects of consequence in airway conductance (Gaw)@ forced vital capacity (FVC)@ forced expiratory volume in 1 second (FEV 1 sec)@ peak expiratory flow (V max)@ and maximum expiratory flow at 50% of the vital capacity (V max @ 50% VC) after 120 hours of continuous exposure to 3@6@ or 8 ppm in 12 healthy male subjects. (ref 1). Static compliance and lung volumes remained unchanged by exposures to SO2 which suggested no damage to lung parenchyma. Deleterious changes in the small conducting airways are difficult to detect with non-invasive techniques. Since the small airways contribute only 10 to 30% of total airway resistance in normal subjects@ it would take relatively large changes in the resistance of the small airways to significantly alter total airway resistance. Therefore@ measurement of total resistance would not detect small airways disease until it was well advanced.