Taking into consideration the amount of air breathed into the lungs per hour and the amount of impurities carried into the breathing zone by the expired air, it can readily be seen that the problem of supplying a sufficient volume of pure air in the house is no small problem. The amount of fresh air needed for the average adult is estimated to be 3,000 cubic feet per hour. The ventilating system that does not provide this amount of air per hour without objectionable draughts does not meet the requirements of modern hygiene. In a system of ventilation it is not only a question of providing a certain volume of air from the outside, but the great problem is to provide a sufficient volume of pure air of proper temperature and of proper humidity. Air that is laden with smoke, dust and suspended matter, or that which contains gas or foul odors, is objectionable and does not meet the requirements. Therefore, the source of the air becomes important. The ventilating system must also keep the air in proper circulation at a proper velocity. The ventilating system of an ordinary building might seem a very simple proposition, but when considered carefully it is a very great problem and one that requires the careful attention of engineers trained in that particular line. It is not the object of ventilation to provide an indoor condition identical with that outdoors, but it is to maintain a condition indoors conducive to a normal expression of indoor life. It is quite obvious that if indoor conditions were maintained identical with outdoor conditions the desired results could not possibly be obtained, for indoor life demands a much different environment. It is necessary to give any system of ventilation as much attention as is given a heating system in order to obtain the best results. No system will work itself. Sources of Impurities in Air There are many processes carried on within the house that add to the impurities of the air. Such sources may be classified as follows: respiration of persons, impurities from heating and illumination, and accidental sources from processes carried on within the house, such as house cleaning and laundering. It will be observed from a study of the amount of air inspired and expired and the impurities carried from the body in the process that one of the most common sources of vitiation of the air in houses is respiration. That these expired poisons may be diluted or carried out of the breathing zone it becomes necessary to have an adequate ventilating system. If the room is heated by a stove impurities will be added from coal dust and the dust of ashes. When the iron is overheated it gives off carbon dioxide and other gases. Open fire or an open blaze for lighting purposes consumes oxygen, gives off carbon dioxide, raises the temperature and increases the amount of aqueous vapor.