SUNLIGHT TREATMENT. (See also HELIOTHERAPY; VITAMINS.) In countries where, relatively, there is much per sistent cloud, in extreme northern or southern lands where night is long in winter, on the sunless slopes of mountains or in sunless valleys, in factory towns where smoke defiles the atmosphere with its soot particles, in slums where the houses are crowded together and men, women and children are herded in dark damp rooms with vitiated air it is found that resistance to infective disease is low, rheumatism and tuberculosis are rife, expectation of life is below normal, infant mortality is high. To these conditions, no doubt, many causes contribute but it is generally believed a deficiency of sunlight is a powerful factor. The tendency at the present day regarding the attributes of the sun's rays is to lay much stress upon the invisible ultra-violet rays of the solar spectrum as well as upon the invisible infra-red and the visible rays.
Treatment by Direct Sunlight.—This is difficult or impos sible under such conditions as have been mentioned above; some methods of supplying it are by the provision of open spaces and playing fields in the neighbourhood of crowded areas, establish ment of open-air schools, cult of outdoor sports, etc. But even if sunlight be available the atmospheric conditions may permit of little penetration by the ultra-violet rays. Hence, methods have been adopted to supply these from abundant supplies such as the electric arc and the mercury-vapour quartz lamp, and to minimize screening effects by the use in buildings of such material as glass made of quartz which admits a maximum of ultra-violet and other beneficial rays, by the use of lighter and more porous clothing, or even by exposure of much of the body. Thus in one way or an other, whether as a curative or preventive measure against dis ease there is a great movement towards supplying, naturally or artificially, that sunlight which is deficient. As a preventive meas ure the use of sunlight is modern, but the curative side has been recognized for centuries, having been used by the Chinese, Egyp tians and the South American Indians and is represented by the numerous health spas and resorts throughout the world.
Mode of Action of Direct Sunlight.—The rays of sunlight, when analysed by the prism, range between the infra-red (heat) rays of relatively long wave-length and slow frequency to the ultra-violet (chemical) rays of shorter wave-length and greater frequency. Within this range the penetrating power of the infra red and of ultra-violet rays into human tissues is definitely less than that of the visible rays. Hence heat from the visible rays of
sunlight passes through the skin and, largely, is taken up by the blood and distributed by way of the circulation throughout the body; infra-red heat rays warm the superficial layers of the skin and from them heat is carried to other parts. Ultra-violet light produces its effects in the skin itself and its action is manifested by the inflammation, peeling and tanning that exposure to the sun occasions in most persons. The pigment of coloured races pre vents the penetration of ultra-violet rays deeper than the pigment layer and thus is a protection in tropical countries.
Warmth.—The increased warmth, when moderate, stimulates metabolism, induces a sense of well-being, increased appetite and mental activity; when greater, as in the height of summer or in the tropics, leads to sweating, desire for cool drinks and cool breezes and disinclination for food, particularly sugars and fats the oxidation of which supplies a large proportion of animal heat; and when excessive it leads to the pathological condition of sun stroke or heat-stroke (q.v.).
Ultra-violet Rays.—But the ultra-violet rays are now known to have special qualities. For long it has been known that the skin has the property of regulating heat loss and so contributing towards maintenance of a stable body temperature in warm blooded animals. Recently it has been shown experimentally that even a short exposure to ultra-violet radiation modifies the bacteri cidal power of the blood and its leucocytic content. Moreover, the discovery that vitamin D (see VITAMINS) can be produced in ergo sterol—an impurity apt to be present in all cholesterols but found originally in ergot of rye (q.v.)—has indicated a variety of action of which the full importance is, probably, still unknown. In investigations on rickets (q.v.) it was found that cod liver oil is beneficial ; later, that ultra-violet irradiation is beneficial, even in the absence of cod liver oil, and ultimately that ultra-violet rays act on the non-saponifiable part of a natural fat, i.e., the sterol, and produces vitamin D. From this it appears that the ultra violet rays of summer sunlight act upon the ergosterol impurity in the cholesterols present in all animal cells and build up a store of vitamin D for our use during the winter when the ultra-violet radiation of sunlight is deficient.