Respirators, in general, are worn by only about 20% of workers in need of respiratory protection against hazardous substances in the workplace. This has been principally due to a generalized "discomfort" experienced by workers. Breathing resistance and equipment dead space are two of the most important design factors contributing to this discomfort. The principal physiological responses to added breathing resistance appear to be hypoventilation, reduced oxygen consumption, a "flattened" and prolonged pattern in the breathing phase to which resistance has been added, increased respiratory work, and a tendency for increased Functional Residual Lung Capacity and increased carbon dioxide retention (when compensation is incomplete). Added dead space forces rebreathing of exhaled carbon dioxide which, in turn, stimulates compensatory hyperventilation to maintain a normal alveolar carbon dioxide tension. Tidal volume increases and, thereafter, breath frequency rises. The mechanism for maintaining a normal carbon dioxide tension seems effective up to 2% carbon dioxide for short-term and to 1% carbon dioxide for long-term exposures. Additional studies of physiological responses under stress and reconsideration of current standards are recommended.