Vitiation from accidental sources consists of dust particles of organic and inorganic detritus which are added to the air from walls, floors, furniture and hangings. Other processes such as laundering will add a certain amount of poisons and suspended matter to the air. All this reveals the necessity for exchanging the inside air for pure outside air. Requirements of a Ventilating System A ventilating system must not only bring about an exchange of air, but it must also keep the inside air in proper circulation during the time it is contained inside. Proper circulation of air is one of the most important functions of ventilating systems. The value of air circulation will be appreciated when it is known that an aerial envelope is formed around the body when the air is not kept moving and the temperature and humidity of this air will resemble that of a very hot, humid summer day. The effects will also be similar to those of heat exhaustion. This shows the importance of keeping the air in circulation in order to carry away the poisons that are being constantly excreted from the skin and through the respiratory tract. The effects of foul air are usually manifested as headache, fatigue, lassitude, vertigo, nausea, vomiting, collapse and even death. The chronic effects are anemia, debility, lowered vitality and disturbances in digestion. Prolonged exposure to vitiated air will necessitate increased adaptation on the part of Innate Intelligence. If this exposure is carried to an excess it will draw upon the adaptative forces of the body and make it more susceptible to the invasion of toxins and subject to changes which it would not be when under more natural environmental conditions. It is now affirmed by the best hygienists that there is no great objection to rebreathing air if provision is made for diffusion of the carbon dioxide and if the bodily odors are eliminated. In this way air may be properly warmed and kept recirculating; thus there is a saving on fuel. If air is to be rebreathed it must first be washed. There is, of course, a limit to the length of time air can be recirculated. At no time is the recirculated air equal to outside air and because this method is not properly regulated and carefully controlled it is not considered safe for use. Before considering methods for obtaining pure air it is interesting to look at Nature’s purifying system. The effect of vegetation upon the air is to consume carbon dioxide, especially in the sunlight. The purifying effects of the sun’s rays on the organic particles, the washing of the air by the rain which carries down the dissolved gases and suspended impurities, the natural constant diffusion of the air due to the wind, are all natural processes tending to keep the outside air in a state of purity. Man so far has been unable to find any artificial means of purifying the air and therefore must provide means of admitting air from the outside in a state conducive to sustaining indoor life.