IS AIR CONDITIONING IMPORTANT? The human being is sensitive to the atmosphere in which he lives. Air is the most crucial of all substances needed for life. A human being can live about forty days without food, about four days without water, but only about four minutes without air. Air is a mixture containing approximately 21 per cent oxygen, 78 per cent nitrogen, and a fraction of 1 per cent of a number of other gases. The air of our large cities is contaminated by the gases that come from the bodies of man and from industry, by dust and germs of various kinds, and even by droplets of fluid. The air that surrounds the human body is used, first, for breathing, second, to help control the temperature of the body. There is a heatregulating center in the brain. When the body gets cool, the amount of blood circulating through the skin is increased and radiation takes place from the surface of the body. All air contains a certain amount of moisture. The moisture increases with the temperature. The moisture in the air is reflected by the measurement of the humidity. The effects of bad air are due to high temperature, the high relative humidity, and the lack of motion, When the air is in motion, it tends to evaporate water from the surface of the body more rapidly than when it is not in motion. This produces a cooling effect. The best temperature for a room is from 65 to 68 degrees, with sufficient water vapor in the air to produce a relative humidity of from 30 to 60 per cent. Once it was thought that only the chemical constituents of the air were important in distinguishing good air from bad air. More recently it has been recognized that the physical qualities of the air are also important. Dusts and pollens may be primarily responsible for symptoms like those of hay fever and asthma from which many people suffer. When a room contains a great many people without arrangements for free ventilation they begin to feel drowsy and fatigued. This sensation results from a combination of the warmth and the accumulation of a certain amount of carbon dioxide which is exhaled into the air from the lungs. Once all sorts of diseases were believed to be due to bad ventilation and to gases coming from marshes and sewers. Now it is known that these diseases are actually due to germs which are transferred from one person to another. Moreover, the marshes were breeding places of mosquitoes and other insects which carried diseases. Once it was thought night air was unhealthful. Night air is just as healthful as day air, provided it is not contaminated by germs or infested with insects which carry germs. Everyone knows now that the wind comes up and the mosquitoes fly toward nightfall. In cold weather there is a tendency to overcrowding and lack of ventilation. This means a greater likelihood of infections being transmitted from one person to another. Unsatisfactory ventilation may be a factor in the increase of coughs, colds, and other respiratory diseases during the winter. With the coming of modern air conditioning, there is more and more opportunity for proper ventilation. The average home, however, can easily be ventilated by opening the windows occasionally and allowing a brisk circulation of air through the rooms. Indeed the ordinary home as now built contains enough chinks, cracks, and space under doors and windows to permit a good deal of fresh air to circulate constantly. Mechanical systems of air conditioning have value in crowded places, like factories, workrooms, motionpicture theaters, and public halls. Frequently such systems constitute the only means by which a sufficient amount of good pure air can be secured. Unfortunately most air-conditioning apparatus is operated without any recognition of the functioning of the human body. The human body adapts itself to a good many extraordinary conditions. Sudden changes in temperature from cold to warm or from warm to cold place considerable stress on the physiology of the human body. Most experts believe that the difference in temperature between the inside and outside air should not be more than from ten to fifteen degrees in warm weather. When the temperature outside is exceedingly cool, people should wear plenty of warm clothing in order to maintain the body temperature at a reasonably stable level. In maintaining the cleanliness of air all sorts of useful devices have been developed. Air-conditioning and air-filtering apparatus help to remove pollen and dust. Few housewives understand how to clean a room without filling it with dust. The modern vacuum cleaner takes dust out of the air and out of furnishings in a room without stirring it up. The old-fashioned carpet sweeper is better than a broom. The obsolete feather duster scatters dust in the air, whereas a moist or oiled cloth removes it without disturbing it. Good ventilation is particularly important during sleep. Sleep occupies or should occupy approximately one third of the twenty-four hours. Good ventilation during the hours of sleep helps take the place of bad ventilation which may be likely during hours of work indoors. During sleep the temperature of the body may be controlled by suitable covering. Thus the temperature of the room may be somewhat lower during sleep than is permissible during the day. Old people and invalids usually require a higher temperature for comfort because their bodies produce less heat than do the bodies of younger people. Because air is so universal, characteristics of the air such as odor, moisture, or movement are likely to be much more emphasized than their importance in relation to health merits. Apparently much of our belief in the importance of fresh air is based on the will to believe.