“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]
The absorbents used in both the British and American gas mask canister, which afforded a degree of protection far superior to that of any other allied or enemy nation except Germany, consisted of a mixture of charcoal and soda-lime, as described in the preceding chapter. In general, a gas mask absorbent must have certain requirements. These are: absorptive activity, absorptive capacity, versatility, mechanical strength, chemical stability, low breathing resistance, ease of manufacture and availability of raw materials.
Absorptive activity, or a very high rate of absorption, is one of the more important properties of a satisfactory absorbent. A normal man when exercising violently breathes about 60 liters of air per minute, and since inhalation occupies but slightly more than half of the breathing cycle, the actual rate at which gas passes through the canister during inhalation is about 100 liters per minute. Calculated on the basis of the regular army canister, this corresponds to an average linear air velocity of about 80 cm. per second. On the average, therefore, a given small portion of the air remains in contact with the gas absorbent for only about 0.1 second. Besides this, the removal of the toxic material must be surprisingly complete. Though the concentration entering the canister may occasionally be as high as one half per cent, even the momentary leakage of 0.001 per cent (ten parts per million) would cause serious discomfort and the prolonged leakage of smaller amounts would have serious results in the case of some gases. The activity of the present gas mask charcoal is shown by the fact that it will reduce a concentration of 7000 parts per million of chloropicrin to less than 0.5 part per million in less than 0.03 second.
Of equal importance is the absorptive capacity. That is, the absorbent must be able to absorb and hold large amounts of gas per unit weight of absorbent. Its life must be measured in days against ordinary concentrations of gas. It is further necessary that the gas be held firmly and not in any loose combination which might give up minute traces of gas when air is, for long periods of time, breathed in through a canister which has previously been exposed to gas.
The absorbents used must be of a type which can be relied upon to give adequate protection against practically any kind of toxic gas (versatility). The need of this is apparent when the difficulty of having separate canisters for various gases is considered, as well as the difficulty in rapidly and accurately identifying the gases and the possible introduction of new and unknown gases. Fortunately, practically all of the toxic gases are very reactive chemically or have relatively high boiling points and can therefore be absorbed in large amounts by charcoal.
Absorbents must be mechanically strong in order to retain their structure and porosity under conditions of transport and field use. Further, they must not be subject to abrasion for the production of a relatively small amount of fines would tend to plug the canister or to cause channels through which the gas would pass without being absorbed.
Since the canister is filled several months before it is first used in the trenches, and since the canister may be used over a period of months before it is discarded, it is obviously the ultimate activity and capacity (not the initial efficiency) which determines the value of an absorbent. It must therefore have a very considerable degree of chemical stability. By this is meant that the absorbent itself is not subject to chemical deterioration, that it does not react with carbon dioxide, that it does not disintegrate or become deliquescent even after being used and that it has no corrosive action on the metal container.
In a good general absorbent there must be a proper balance between its various essential qualities, and hence the most suitable mixture will probably always be a compromise.