PRINCIPLES OF VENTILATION 317 



Now exhaled air contains much moisture it is nearly 

 saturated at blood temperature, or 98F. Our bodies also 

 give off considerable quantities of moisture in the form of 

 perspiration through the skin. In poorly ventilated rooms 

 where crowds gather, the humidity of the air rises rapidly. 

 The result is that the evaporation from the skin is checked 

 and we soon become uncomfortable. This effect of increased 

 humidity is often greatly aggravated by increase in tempera- 

 ture due to the heat given off from our bodies. Taken 

 together, the three conditions: (1) high humidity, (2) high 

 temperature, and (3) foul odors soon produce headache and 

 a feeling of weariness and exhaustion. Many recent experi- 

 ments indicate that these three conditions of the air, high 

 temperature and high humidity, and disagreeable odors in 

 crowded, ill-ventilated rooms are large factors in producing 

 what we know as vitiated air. 



347. Control of the Temperature of the Body. The tem- 

 perature of the human body in health is maintained with great 

 constancy at 98%F. No matter what the fluctuations of 

 external temperature, the mechanism of the human body is so 

 delicately adjusted that, in health, it perfectly corrects the 

 effects of all temperature changes. If the external tempera- 

 ture is low, the temperature of the body is kept up by an in- 

 crease of heat production within the body. If the external 

 temperature is high, the temperature of the body is kept down 

 by the cooling device of increased perspiration, and the 

 consequent increased surface evaporation and cooling. But 

 any effort which the body must thus put forth to counteract 

 external temperature is necessarily a drain upon the vital 

 forces of the body. 



348. Metabolism. By METABOLISM we mean a sort of 

 double process: On the one hand, the living cells of the body 

 are built up and nourished by the food materials assimilated. 

 On the other hand, it includes the breakdown of some of the 

 living material of the cells into waste products. This latter 

 phase of the process is always accompanied by the liberation of 



