Several types of filtering (air-purifying) devices, air-line (supplied-air) apparatus, and self-contained breathing apparatus are used at work. The most important parameters of the respiratory protective devices that affect physical work capacity are additional inspiratory and expiratory breathing resistance, dead space, and weight. The main physiological effects of the respirators are alterations in breathing pattern, hypoventilation, retention of carbon dioxide, and increase in the work of breathing. The undesirable effects are accentuated during heavy physical work, maximal physical work capacity thereby being reduced.