758 METABOLISM 



conducts heat away. At about 70 F. the cooling effect of air disappears 

 and sweating occurs. The evaporation of the sweat now causes cooling, 

 the degree of which varies inversely with the relative humidity. Be- 

 tween these two temperatures, i.e., 65 and 70 there is a range in which 

 humidity has little influence a neutral region. The influence of high 

 relative humidity on bodily comfort at temperatures above the neutral 

 temperature becomes very marked indeed at 85 F. and at a relative hu- 

 midity of 90 per cent, for example, unfavorable symptoms appear in a 

 few minutes, when there is no movement of the air. 



Relative humidity and temperature alone are not, however, the only 

 physical conditions to be considered. Another is the movement of the 

 air, for even under the unfavorable conditions just cited, immediate 

 relief is afforded if an electric fan be started, as it will be recalled was 

 the result in Hill's experiment. The movement of the air enables it, 

 though nearly loaded to its full capacity with moisture, to carry away 

 considerable quantities of heat in small loads. 



The wearing of clothes greatly affects the rate with which these changes occur. The 

 clothes act as barriers, preventing the movement and exchange of air around the body. 

 The garment next the skin entraps a layer of air which is more or less at the same 

 temperature as the skin, and which soon becomes saturated with moisture at that tem- 

 perature. Between the inner garments and those over them other layers of air are en- 

 trapped, each one being at a somewhat lower temperature and containing less moisture 

 than the one inside. These layers of air, therefore, form stepping stones, as it were, be- 

 tween the extreme conditions next the surface of the skin, and the environment of the 

 clothed body. Obviously if the layers of air next the skin are to be renewed at such 

 a rate that they remain cooler than the skin and unsaturated with moisture the clothing 

 must be adjusted to suit the outside conditions. 



There is every reason for believing that it is because of interference 

 with the processes of heat loss that improperly ventilated and over- 

 crowded places are uncomfortable. The moisture exhaled and evapo- 

 rated from the bodies soon raises the relative humidity so that heat loss 

 is retarded from the skin, and the heat that is actually given off raises 

 the temperature so that loss from the body by radiation and convection 

 becomes suppressed. As the temperature steadily rises, the air takes up 

 more and more moisture, with the result that less and less heat comes to 

 be lost from the lungs in saturating the expired air with vapor. The 

 physical conditions of the environment become unsuitable for the physi- 

 ological mechanism of heat loss, although meanwhile heat production 

 goes steadily on. The body furnaces are not damped down in propor- 

 tion as the loss of heat diminishes, and the consequence is a rise in the 

 temperature of the blood a mild fever. Now it is well-known that the 

 cellular activities, which, taken together, make up the life process of 

 the body are extraordinarily sensitive to change of temperature; their 

 chemical activities become interfered with, they demand more oxygen. 



