With the coming of modern air conditioning, there is more and more tendency toward 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, motion-picture 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 reasonably stable level.

In maintaining the cleanliness of air all sorts of useful devices have been developed. Air-conditioning and airfiltering apparatus help to remove pollen and dust.