VENTILATING THE HOME 19 



the oxygen is taken by the blood to all parts of the body. 

 Here it unites with the food that has been digested and 

 absorbed, and burning takes place slowly, in much the same 

 way that coal and wood burn in the stove, only much less 

 rapidly. As a result of this burning, the body is kept warm 

 and power is given to use the muscles. In this process of 

 burning, carbon dioxid is formed and carried back by the 

 blood to the lungs, from which it is exhaled. As a result of 

 this process of breathing, the oxygen in the air has been re- 

 duced from 21 per cent to 16 per cent, and the carbon dioxid 

 increased from one twenty-fifth of one per cent to about 4 per 

 cent ; and there has been also an increase in the amount of 

 water. 



The ill effects of lack of ventilation. The ill effects of 

 lack of ventilation, as frequently shown in crowded rooms 

 and lecture halls, are well known. Drowsiness, fatigue, 

 lack of attention, headache, and a general feeling of dis- 

 comfort are the results. Frequent colds and similar ail- 

 ments are the common results in winter of lack of proper 

 ventilation. Likewise, the ill effects on those who are 

 obliged to work for long periods of time in poorly ventilated 

 rooms are shown on every hand, especially by contrast with 

 those who work outdoors. There is a tendency toward a 

 general undermining of the health, which may develop into 

 tuberculosis and other lung troubles, while those who are 

 much in the open are healthier and much less likely to con- 

 tract these diseases. 



Chemical causes of ill effects. In order to know how to 

 ventilate properly, we need to know what are the causes of 

 these ill effects, so that we may know what to seek and what 

 to avoid. Two classes of causes have been proposed at 

 various times, the chemical and the physical. As the air 

 undergoes chemical changes in the process of breathing, the 

 early theory was that the injurious effects of bad air were 

 due to a lack of oxygen or to an excess of carbon dioxid. 



