662 PHYSIOLOGY OF RESPIRATION. 



had been attributed mainly to this supposed substance. The 

 investigations that have been made upon this substance are, un- 

 fortunately, far from being conclusive.* The expired air, according 

 to work of this character, contains no organic poison which can be 

 regarded as a product of respiration, and this conclusion is sup- 

 ported in a general way by the experience that air can be re- 

 breathed safely an indefinite number of times provided the excess 

 of CO2 is removed by absorption and new oxygen is supphed to 

 replace that used up in the body. 



Some observers (Hermann, Haldane and Smith) have made 

 direct experiments upon men which also throw much doubt upon 

 the existence of a toxic substance in expired air. Individuals 

 kept in a confined space for a number of hours show no evil effects, 

 provided they are not doing muscular work, except when the ac- 

 cumulation of the carbon dioxid has reached a concentration of 

 over 4 per cent. At this concentration rapid breathing is apparent, 

 and if it rises to 10 per cent, great distress is felt and the face be- 

 comes congested and blue. These authors conclude that expired 

 air is injurious in itself only from the carbon dioxid it contains, and 

 only when this gas accumulates to a percentage such as is not 

 found in the worst ventilated rooms, f At present the definitely 

 known evil results of breathing the air of crowded, poorly venti- 

 lated rooms must be referred to other causes, such as the increase 

 in temperature and moisture. These two conditions cause de- 

 pression and malaise even when an adequate supply of air is 

 provided. If the ventilation is so poor that the carbon dioxid 

 accumulates to the extent of 3 to 4 per cent., then this factor 

 begins to exercise a direct effect upon the respiratory movements 

 and the general condition — an effect which increases as the per- 

 centage of carbon dioxid rises (see p. 703). 



Ventilation. — It is obvious from the foregoing statements that 

 our. knowledge is not yet sufficiently complete to enable us to say 

 positively at what point air in a room becomes injurious to breathe. 

 The statement was formerly made in the books that, when the 

 air contains as much as 1 per cent, of carbon dioxid (Smith) that 

 has been produced by breathing, evil results, as judged by one's 

 feehngs, are sure to occur, but this statement was founded on the 

 belief that an organic poison is present in expired air. As we have 



* See Haldane and Smith, "Journal of Pathology and Bacteriology," 

 1, 168, and 318, 1893; Merkel, "Archiv f. Hygiene," 15, 1, 1892; Formdnek, 

 ^'Archiv f. Hygiene," 38, 1, 1900; Weichardt, ibid., 65, 252, 1908. Amoss, 

 "Journal of Exp. Medicine," 17, 1.32, 1913. 



t See L. Hill, et al., "Smithsonian Miscellaneous Collections," vol. 60, No. 

 23, 1913; also L. Hill, "The Science of Ventilation and Open-air Treatment," 

 London, 1920 



