RESPIRATION 303 



arterial blood remains practically unaffected as regards either 

 CO 2 or oxygen. 



Although apart from CO 2 no appreciable amount of any 

 poisonous substance is given off to the air by the body, various 

 substances which affect the olfactory nerves are given off in minute 

 amounts from persons or furniture in a room. As a rule these sub- 

 stances are only perceived on entering a room, and are not noticed 

 after a short time by those who remain in it. In sensitive persons, 

 however, they may produce an unpleasant reflex effect; and for 

 this reason apart from any other a good ventilation is desirable. 

 When, however, there is no musty furniture, and the bodies and 

 clothing of those present are fairly clean, there is little or no in- 

 convenience from this cause. 



A far more important factor in connection with the physio- 

 logical effects of the air in rooms is temperature, and along with 

 it moisture. The maintenance of a constant internal body tempera- 

 ture depends on constant physiological adjustment between ac- 

 tual heat loss from the body and variations in environmental con- 

 ditions which tend to make the heat loss greater or less than the 

 heat production. The variations in environmental conditions con- 

 sist in variations in temperature, moisture content, and movement 

 of the air, and also variations in the radiant heat gained or lost 

 by the body, apart from the actual temperature of the air. The 

 actual heat loss is regulated physiologically, apart from conscious 

 regulation by variation of clothing, etc., partly by varying the 

 rate of blood circulation through the skin, and partly by varying 

 the amount of water evaporated by the skin. The latter means of 

 regulation does not come into play unless the air is warm, or heat 

 production in the body is greatly increased by muscular exertion. 



When the air of a room is so cold, or the movement of the air 

 is so great, that the skin, or parts of it, become uncomfortably 

 cold, we are always clearly aware of the cause of discomfort. But 

 when the air is so warm as to lead to the skin being uncomfortably 

 warm we are apt to attribute the discomfort to some other cause 

 than the heat. The matter is also complicated by the fact that in 

 different persons the air temperature at which discomfort is 

 felt varies considerably. Thus persons who have been undergoing 

 "open-air" treatment and are accustomed to rooms with open 

 windows feel much discomfort in rooms with closed windows 

 where other persons are just comfortable. Similarly Americans 

 accustomed to the warm air associated with central heating find 

 British houses with fires very uncomfortably cold in winter, while 



