64 PURE AIR 



oxygen and has much carbon dioxide and other waste 

 products as well. It is therefore highly important that 

 we force out as much of this air as possible rather frequently, 

 and we can do this only by deep breathing. One of the 

 best deep-breathing exercises, which should be practiced 

 several times daily out of doors, is to raise the arms well 

 from the sides and breathe deeply while we do this, hold 

 the breath for three seconds, exhale, and repeat fifteen 

 times. 



Water, carbon dioxide, and other wastes given off 

 through the lungs. A study of chemistry shows that as 

 wood, which contains carbon and hydrogen, combines with 

 oxygen it produces two substances, carbon dioxide and 

 water. A chemist writes these two substances as CO 2 

 (carbon dioxide) and H 2 O (water) . Chemical elements are 

 very simple substances of which oxygen, hydrogen, and 

 carbon are examples. They always unite in definite propor- 

 tions. For example, water is always composed of f hydro- 

 gen and ^ oxygen by volume, no matter how much of it 

 there may be. Foods contain carbon, hydrogen, and 

 oxygen, and when they are oxidized in the body, produce other 

 substances ; among these are carbon dioxide and water. You 

 are, perhaps, familiar with the odor that is noticed when 

 one passes from out of doors into a closed room where sev- 

 eral people have been for some time. The " stuffy " odor 

 is caused, in part at least, by certain waste materials which 

 have been passed off through the lungs and skin into the air. 



Need of ventilation. It therefore becomes necessary 

 in rooms where people are gathered, such as schoolrooms, 

 churches, " movie " theaters, and the rooms of our own 

 homes, to have the air constantly changed in order to 

 get rid of the odor, moisture and carbon dioxide, and 



