VENTILATION, the process of supplying or removing air by natural or mechanical means into or out of any enclosed space. Ventilation generally is associated with control of temperature and odour, and in removal of dust and industrial by-products. The problem of securing adequate ventilation involves far more than merely the provision of enough proper air for respiration, since changing of the air around the body also has been proved necessary to comfort and health. A classical illustration of this is the test of placing a man within a small air tight cabinet while he breathes air from outside the cabinet through a mask and tubes, compared with rebreathing the air inside the cabinet through the mask and tubes while the body of the man is outside the cabinet. Discom fort develops quickly in the first experiment and is postponed indefinitely in the second one. After a small electric fan is oper ated within the cabinet during the first experiment the objection able effects are delayed appreciably. Tests of this sort were con ducted repeatedly in both England and America and confirmed the hypothesis that ventilation must embrace far more than consider ation of the comparatively small quantity of respired air; less than 0.3 cu.ft. per minute in the case of an ordinary adult.
Atmospheric air is a complex mixture of many gases, principally, however, one-fifth oxygen and about four-fifths nitrogen. Oxygen is the active and important component without which life or com bustion cannot exist. A small proportion of carbon dioxide always is present in atmospheric air, along with inconsequential amounts of other gases. An increased proportion of carbon dioxide in in spired air accelerates the rate of respiration by reaction of certain involuntary nerves and advantage is taken of this fact in resusci tation after partial asphyxiation, as in drowning. Since the propor tion of carbon dioxide in air easily can be measured by laboratory analysis of a bottled sample, it is a useful index of the distribution of air within occupied enclosures. Outside atmosphere in cities
contains about four parts of carbon dioxide in io,000 parts of air.
If the air in a room contains more than ten parts of carbon dioxide in io.000 it may be reasoned that the air has had sufficient contact with animals to require dilution. The significant reasons for such dilution however, have been established not as carbon dioxide, but as the combination of temperature, water vapour con tent, movement, dusts, and odours.
A human, for comfort and well being, must have a bodily temperature of around 98.6°F. The normal person balances the heat output from his body, largely by transferring the surplus heat originally derived from food, into the surrounding air. It is necessary, then, in order to achieve this temperature regulation, that the air surrounding the body shall be somewhat cooler than 98.6°F. and also shall be sufficiently free from moisture to permit it to accept and carry away from the body the perspiration which is so vital a part of the temperature regulating function. Movement of air is an important consideration in ventilation because with out movement there is a definite tendency for a comparatively thin layer of too-warm, too-moist air to lie against the skin ; thus retarding the desirable transfer of the surplus heat from the body to the general atmosphere. Dust in air can be annoying as well as dangerous, depending on its source, and while dust is always pres ent, the larger particles can be removed. Organic dust particles such as pollens usually can be controlled by filtering the air, and dangerous inorganic dusts such as that from silica can be removed by local high velocity exhaust ducts. Odours become manifest in air which has had intimate contact with people. Such odours have not been to be harmful in the concentrations encoun tered in unventilated relatively leaky buildings, but they do cause loss of appetite and a general feeling of ill being.