We do not want to pass over this subject without calling the student’s attention to the effect which may be produced by the physical, mental and nervous strain under which the work is performed. If the worker is under great tension a great amount of energy will be used up and fatigue will appear early. This will have a marked effect upon the individual and tend to lessen his efficiency. If work is performed under great physical strain, the muscular activity is greater than the muscles are able to express. Then there will be not only fatigue but actual injury done the muscles and other structures. Under such conditions the vital organs will also be overtaxed. This may result in dilatation of the heart, hernia, aneurysms, or in a general weakened condition of the body. The results of mental strain, responsibility and worry are not without their adverse effects. Lighting, Heating and Ventilating Work Places Industrial hygiene should be concerned about the temperature, humidity, air pressure, light, ventilation, and heating of the work place. The building should be well lighted. If possible natural light should be provided even though the first cost is considerably greater than that of artificial lighting. There will not only be a better grade of work done in natural light and with less eye strain, but the general health of the workmen will be better. If artificial lighting is used it should be electric light, since it produces less impurities and provides a more even light. The light should be evenly distributed and of a proper distance from workmen so that it will not be reflected directly into their eyes. If the occupation takes the worker out of doors then these features can not be controlled, but in this event attention must be given to the clothing of the workman. Sudden changes in the temperature should be avoided as much as possible. Innate is capable of adapting the body to extremes in temperature, but must be given a sufficient amount of time in which to bring about these changes. Men may work in the frigid temperature or in the torrid temperature and if properly clothed and acclimated will suffer no ill effects. In many industries the laborers are forced to work in artificially high temperatures. Blast-furnace workers, glass blowers, miners, bakers, and stokers must remain in extremely high temperatures. The body may be adapted to such temperature, but the effect of the continued expenditure of energy necessary to bring about this adaptation will show adverse effects upon the body. There is a tendency to respiratory disorders and a thickening of the blood plasma which disrupts the circulatory system. Heat also has a detrimental effect upon the eyes and there is a tendency to rheumatism.