VENTILATION. 361 



rooms, etc., which are occupied by many persons, the air soon 

 becomes appreciably changed by their breathing. 



The most important changes are (1) removal of oxygen, (2) 

 increase in carbonic acid, and (3) the appearance of some poi- 

 sonous materials which, though highly injurious, cannot be deter- 

 mined. The deficiency in oxygen never causes any inconvenience, 

 as it is never reduced below what is sufficient for the saturation 

 of the haemoglobin. The excess of CO 2 seldom gives any incon- 

 venience, since the air becomes disagreeably fusty or stuffy from 

 breathing long before the amount of CO 2 has reached 0.1 per 

 cent., which amount of pure CO 2 can be inspired without any 

 unpleasantness. It is, then, the exhalations coming from the 

 lungs, and probably skin, some of which must have a poisonous 

 character, that render the proper supply of fresh air imperative. 



The difficulty of determining the presence of the poisonous 

 organic materials makes it convenient to use the amount of CO 2 

 present in the air as the means of measuring its general purity. 

 We suppose, then, that the relation between the poisonous organic 

 ingredients and the CO 2 is constant. 



Air which is rendered impure by breathing, becomes disagree- 

 able to the sense of smell when the CO 2 has reached the low 

 standard of .06 or .08 per cent. ; that is to say, scarcely twice as 

 much CO 2 as is contained in the pure atmosphere. Supposing 

 that air is unwholesome when its impurities are appreciable by 

 the senses, then, if the animal body be the source of the CO 2 , .06 

 per cent, of this gas makes the air unfit for use. 



An adult man disengages more than half a cubic foot of CO 2 

 in one hour (.6, Parkes), and consequently in that time he renders 

 quite unfit for use more than 1000 cubic feet of air, by raising 

 the percentage of CO 2 to .1 (.04 being initial, and .06 respiratory). 



It is obvious that the smaller the space and the more confined, 

 the more rapidly will the air become vitiated by respiration. It 

 becomes necessary for health, therefore, to have not only a certain 

 cubic space and a certain change of air for each individual, but 

 the cubic space and the change of air should bear to each other 

 a certain proportion in order that the air may remain sufficiently 

 pure. 



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