This investigation determined the cardiorespiratory responses of subjects with normal lung function and exercise tolerance and compared them with subjects with moderate and severe impairment of lung function and exercise tolerance. Comparisons were made during work while wearing an industrial respirator. The respirator was a full-face mask (MSA-ultravue) "demand" breathing type equipped with an inspiratory resistance of 85 mm H2O at 85 L/min air flow and an expiratory resistance of 25 mm H2O at 85 L/min air flow. Physiologically and subjectively the response of the normal and moderately impaired subjects to respirator wear during rest, 35%, 50% and 63% of their maximal workloads were not different. However, when the moderately impaired worked at 63% of their maximum the workload was equivalent to 50% of maximum of the normal subject. Significant differences in the peak flow/pressure ratio of the severely impaired compared to the normals and moderately impaired were found. By relating work performance to the dyspnea index (VI/MBC) it was suggested that the relationship between maximal lung function and maximal work performance needs to be identified both with and without respirators. This relationship may prove suitable in predicting performance during respirator wear.