(_c_) As will be noted below, the effect of resistance is a tendency to decrease the minute-volume of air breathed. Dead air space increases the minute-volume. Accordingly, if breathing is accomplished against resistance and through a large volume of dead air space, the volume of air breathed is reduced more in proportion to the actual needs of the body than when breathing against resistance without the additional factor of dead space; this, again, causes the level of carbon dioxide in the blood and tissues to be raised to a higher level than normal, and thus again there is some reserve power wasted. 2. Interpretation from the standpoint of the canister. The life of the canister depends on the volume of the gas-laden air passed through it. The dead space increases the minute-volume of air passed through the canister and, therefore, shortens its life. Physiologically, the reason for the harmful effects of breathing resistance is more involved: “The importance of resistance to breathing lies in: (1) the effect on the circulation of the blood, and (2) the changes in the lung tissue, which seriously interfere with the gas exchange between the outside air and the blood. Data have been presented to draw attention to the seriousness of resistance to inspiration. In these reports, it was suggested that the deleterious effects on the body consist in changes in the blood pressure, increased work of the right side of the heart, and an increase in the blood and lymph content of the lungs. Resistance also decreases the minute-volume of air breathed and thereby increases the percentage of carbon dioxide in the expired air. The foregoing changes are all deleterious. “Although the chief problem of resistance in gas mask design concerns inspiration, nevertheless _resistance to expiration_ is an important factor. The expired air of the lungs contains carbon dioxide for which means of escape must be provided. The expiratory act is more passive than the inspiratory act, and resistance to expiration is, therefore, more keenly felt than resistance to inspiration. It is then imperative that the exhale valve be so arranged as to allow for the escape of the entire amount of air during the time of expiration with the least possible resistance. The data of the laboratory indicate that seldom, if ever, do expiratory rates rise above a velocity of 150 to 175 per minute. The effect of resistance to exhalation upon the vital organs of the body is not dissimilar to that of inspiration.” CHAPTER XIII ABSORBENTS[27]