SECTION IV. 



AIR AND VENTILATION. 



A PLENTIFUL supply of pure air is the first essential for health, 

 and the fact that it is made impure by animals, and that the processes 

 of contamination if allowed to go on unchecked would ultimately 

 render the air incapable of supporting life, makes it necessary that 

 the hygienist should have a clear conception of (1) the impurities 

 that exist in the air; (2) how they accumulate; (3) to what degree 

 they can be tolerated ; and (4) how they may be rectified. So long 

 as animals are scattered and live a free and open-air existence the 

 pollution they cause is of no practical moment. When animal 

 life is crowded in a restricted area, with restriction of natural 

 purifying means, the composition and character of the air becomes 

 changed from the normal state to one which, if not necessarily 

 fatal to life, may at least appreciably reduce vitality. The more 

 confined the space in proportion to the animal life contained within 

 it, the more serious is the pollution and the more rapidly does it 

 take place. 



Pure air is a mechanical mixture of various gases holding in 

 suspension a quantity of water vapour. Such uncontaminated air 

 has the following composition by volume : 



Oxygen 20-94 per cent. 



Carbon dioxide 0-03 to 0-035 

 Nitrogen 78-09 



Argon 0-94 



Helium, krypton, neon, &c., traces. 



There are also traces of ammonia, ozone, nitric acid, free 

 hydrogen and methane. On an average air also contains about 

 1 4 per cent, of moisture. 



Pure air is never found in the vicinity of animals or people, 

 because by them the proportions of its normal constituents are 

 altered and foreign matters, some of which may be harmful, are 

 added to it. The normal physiological processes cause a reduction 

 in the amount of oxygen, an increase in the carbon dioxide and 

 methane and an alteration in the physical character of the air by 

 increasing its moisture and temperature. The combustion of coal 

 and other fuel, the decomposition of animal and vegetable matter, 



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