THE AIR AND VENTILATION 221 



air, or with a few other substances. Its discovery has not 

 modified our ideas of the physiological effects of the air. 



379. Dust in the lungs. If the dust in the air is small in 

 amount, it adheres to the moist surface of the nose and pharynx, and 

 does not enter the trachea. If some enters the trachea, it becomes 

 entangled in the cilia of the epithelial cells and is forced back towards 

 the mouth and then coughed out. If the air is very dusty, some dust 

 will enter the air sacs. Then the dust particles will be carried by the 

 lymphatics to the nearest lymph nodes, where they will be deposited 

 and remain harmless. But the greatest danger from dust is that many 

 of the particles may consist of disease germs (p. 418). 



380. Occupation diseases. Even though the lymph nodes 

 take care of inhaled dust, after a while the continuous irritation of the 

 hard particles injures the delicate lining of the bronchi and air sacs, 

 and causes bronchitis or asthma or pneumonia. Tool grinders are 

 especially liable to the trouble, for the fine particles of stone and steel 

 which fly off in their work and are inhaled, cannot be taken up by the 

 lymphatics. Potters, miners, flax workers, and pearl button makers 

 are all subject to lung troubles to a greater degree than workers in a 

 dustless atmosphere. Those who work with quicksilver or phosphorus 

 are liable to inhale the fumes and be severely poisoned. 



381. Amount of oxygen needed to support life. When 

 inspired air contains less than 20 per cent of oxygen, a 

 shortness of breath comes on, which is in proportion 

 to the lack of oxygen. A candle will not burn in air 

 containing less than 17 per cent of oxygen, while air 

 containing only 15 per cent of oxygen will support life, 

 but there will be great shortness of breath. In old wells 

 and cellars oxygen is often replaced by carbonic acid gas, 

 and men have been suffocated in them. A simple test of 

 the safety of entering them is to lower a lighted candle 

 into the suspected place. If it burns, there is surely 

 enough oxygen to support respiration. When the amount 

 of oxygen is diminished to ten per cent, animals die in a 

 few moments with all the symptoms of suffocation. 



