If the human skin is not exposed to solar radiation (direct or scattered) for long periods of time, disturbances will occur in the physiological equilibrium of the human system. The result will be functional disorders of the nervous system, a vitamin D deficiency, a weakening of the body’s defenses, and an aggravation of chronic diseases. Sunlight deficiency is observed more particularly in persons living in the polar regions and in those working underground or in windowless industrial buildings. The simplest and at the same time the most effective measure for the prevention of this deficiency is the irradiation of human beings by means of uv lamps. Such irradiation is conducted either in special rooms called photaria or directly in locations where persons are regularly present—in workshops, schools, hospitals, etc. As a rule, the daily dosage of uv does not exceed half of the average dose which produces a just perceptible reddening of an untanned human skin. It is preferable to use fluorescent lamps which use phosphorous and have maximum emission of 315 nm. The beneficial effect of uv irradiation has been confirmed by many years of experience. Ultraviolet irradiation is also beneficial for agricultural animals. In addition to the lamps used for irradiation of human beings, high pressure mercury lamps in a quartz envelope are used to irradiate animals. Irradiation doses vary according to the species and age of the animals.
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