In recent years marked improvements in theater planning and equipment have been effected, and corresponding steps in advance have been made in matters relating to theater hygiene. It should therefore be understood that my remarks are intended to apply to the average theater, and in particular to the older buildings of this class. There are in large cities a few well-ventilated and hygienically improved theaters and opera houses, in which the requirements of sanitation are observed. Later on, when speaking more in detail of theater ventilation, instances of well-ventilated theaters will be mentioned. Nevertheless, the need of urgent and radical measures for comfort and health in the majority of theaters is obvious. Much is being done in our enlightened age to improve the sanitary condition of school buildings, jails and prisons, hospitals and dwelling houses. Why, I ask, should not our theaters receive some consideration? The efficient ventilation of a theater building is conceded to be an unusually difficult problem. In order to ventilate a theater properly, the causes of noxious odors arising from bad plumbing or defective drainage should be removed; outside fumes or vapors must not be permitted to enter the building either through doors or windows, or through the fresh-air duct of the heating apparatus. The substitution of electric lights in place of gas is a great help toward securing pure air. This being accomplished, a standard of purity of the air should be maintained by proper ventilation. This includes both the removal of the vitiated air and the introduction of pure air from outdoors and the consequent entire change of the air of a hall three or four times per hour. The fresh air brought into the building must be ample in volume; it should be free from contamination, dust and germs (particularly pathogenic microbes), and with this in view must in cities be first purified by filtering, spraying, or washing. It should be warmed in cold weather by passing over hot-water or steam-pipe stacks, and cooled in warm weather by means of ice or the brine of mechanical refrigerating machines. The air should be of a proper degree of humidity, and, what is most important of all, it should be admitted into the various parts of the theater imperceptibly, so as not to cause the sensation of draught; in other words, its velocity at the inlets must be very slight. The fresh air should enter the audience hall at numerous points so well and evenly distributed that the air will be equally diffused throughout the entire horizontal cross-section of the hall. The air indoors should have as nearly as possible the composition of air outdoors, an increase of the CO_{2} from 0.3 to 0.6 being the permissible limit. The vitiated air should be continuously removed by mechanical means, taking care, however, not to remove a larger volume of air than is introduced from outdoors.