THE AIR AND VENTILATION 223 



the liberated gases to expand slowly. The ear drums 

 could be easily ruptured by a quick change in pressure. 

 Sometimes the pressure causes a severe injury of the 

 spinal cord. 



384. Effects of carbonic acid gas. Carbonic acid gas 

 itself has very little harmful effect upon the body. When 

 air containing one fourth its bulk of carbonic acid gas is 

 inhaled, the air sacs soon contain more of the gas than 

 is found in the blood. Then carbonic acid gas is no longer 

 given off, but remains in the blood and air sacs, and pre- 

 vents the entrance of oxygen. Shortness of breath, uncon- 

 sciousness, and death soon occur, caused mainly by the 

 displacement of the oxygen. Carbonic acid has been used 

 to produce insensibility during surgical operations but its 

 effects cannot be controlled, and its use is unsafe. 



When many persons are confined in a small room, the 

 oxygen is speedily used up, and carbonic acid gas takes its 

 place. When the amount of oxygen is diminished to ten 

 per cent, death will occur, caused rather by the lack of 

 oxygen than by the presence of the carbonic acid gas or 

 other substances in the expired air. But discomfort will 

 be felt long before the oxygen is diminished to an appre- 

 ciable degree. 



385. Foul air. Besides the carbonic acid gas, the ex- 

 pired air contains a greater or less quantity of water and 

 of foul-smelling vapors. Odors are constantly given off 

 also by the skin of the most cleanly persons. In the air 

 of a closed room in which several people have been for 

 some time, there is a characteristic odor which belongs to 

 man, just as certain odors are peculiar to different lower 

 animals. These odors are very oppressive. They cause 

 sickness in sensitive persons mainly because of their 

 unpleasantness. This effect passes off when pure air is 



