672 PHYSIOLOGY OF RESPIRATION. 



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

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

 safely an indefinite number of times provided the excess of CO 2 is 

 removed by absorption and new oxygen is supplied 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.* As opposed to these negative 

 results, Weichardt reports a series of experiments upon mice in 

 which the expired air of a number of animals was passed through 

 acidulated water, and the latter was then condensed in a vacuum to 

 a small volume and neutralized. When injected into a fresh ani- 

 mal this material brought on a soporific condition, fall of body tem- 

 perature, and diminution in output of carbon dioxid. The author 

 explains these results on the assumption that some of the so-called 

 fatigue-toxin (kenotoxin) is excreted by way of the lungs, and he be- 

 lieves that the known depressing effects of poor ventilation are an 

 expression of the action of this substance. His results have not 

 been confirmed and, at present, the definitely known evil results 

 of breathing the air of crowded, poorly ventilated rooms must be 

 referred to other causes, such as the increase in temperature and 

 moisture. These two conditions cause depression and malaise 

 even when an adequate supply of air is provided. If the ventila- 

 tion 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 percentage 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, 

 whether from products of expiration, or, according to recent views, 

 from changes in temperature and moisture. The statement was 



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

 23, 1913. 



