HELIOTHERAPY (see PUBLIC HEALTH ; THERAPEUTICS). Heliotherapy, or treatment by sunlight, has been intermittently practised since the dawn of history, but its scientific use dates from the time of Finsen, who developed light treatment by em ploying artificial light, notably the carbon arc. It owes much to the work of Bernhard and Rollier, who commenced treatment with natural sunlight early in the present century. The latter has done much to popularize its use, especially in non-pulmonary tuberculosis.
There are great variations in the optimum exposure of dif ferent individuals. Generally speaking, brunettes tolerate expo sure better than blondes ; individuals who freckle but whose skins do not brown must be exposed with great caution, while albinos can barely tolerate exposure at all. On the normal skin an erythema develops within six hours of exposure ; care should be taken not to exceed this erythema, and blistering is to be avoided. Gradually the skin becomes bronzed, and as this bronz ing deepens so may exposure be prolonged, the pigment protect ing the patient from the harmful effects of the actinic rays.
Insolation necessarily involves exposure of the body to the cold air, and the latter has been shown to increase the metabolic activity of the subject treated. The rays of greatest therapeutic value are the ultra-violet or actinic rays, the light waves of shortest wave-length in the solar spectrum. These are most intense at the seaside or on the mountains. Excess of heat rays is to be avoided, and insolation for its therapeutic value is thus better carried out in temperate than in tropic regions. Properly timed exposure on the individual responding to treatment is associated with a tonic and exhilarating effect ; exposure unduly prolonged results in fatigue and exhaustion.
The remote beneficial effects are more marked but less easy to explain. Eidenow has shown that the haemobactericidal power is often raised after an erythema dose. This enhanced power of the blood to destroy pathogenic organisms is obtainable equally when a portion or the whole of the skin is exposed, and, if increased haemobactericidal power is alone sought, fractional exposures of different portions of the skin at consecutive sessions is the method of treatment of choice. Sonne has demonstrated that the visible rays of the solar spectrum have great penetrative power, pass through the skin, and are absorbed in the subjacent blood, their physical being transformed into thermal energy. He has suggested that this local heating effect is of value in destroying deleterious circulating toxins. Investigations have shown that the calcium, phosphorus and iron content of the blood may be raised by insolation.
Sunlight treatment has its greatest therapeutic value in increas ing and maintaining bodily tone and energy, in the treatment of the various manifestations of surgical tuberculosis, in the treat ment of rickets and the relief of nervous asthma. It is of proved value in certain affections of the skin, notably psoriasis. It is of great assistance in a variety of conditions, accelerating and con solidating the cure, particularly in convalescence from debili tating diseases such as infectious fevers. Its stimulating effect is seen in increasing fecundity and in the treatment of certain defects of the endocrine glands. The range of its usefulness is being rapidly extended, in the main, as an aid to cure rather than as a specific treatment.
See A. Rollier and others, Heliotherapy (1923). (H. J. G.)