Amount of Ultra-violet Radiation Necessary to Cure Rickets.

It has been amply proved that ultra-violet radiations have value in the cure and prevention of disease. Wave lengths of therapeutic value may vary for different diseases, but for rickets the band having a high antirachitic potency is a comparatively narrow one centering around line 2968 A. U.

The depth of penetration of the “vital rays” into the skin is perhaps not greater than 1 ½ mm., 95% of the rays being absorbed by the more superficial layers. Some of them do not penetrate the corneum. However, the most superficial blood vessels and nerves of the skin are within 0.2 of a mm. of the surface and are no doubt reached by many of the rays, such as the shortest received from the sun (2900 A. U.).

Rickets, a disease characterized by low blood calcium, lack of bone formation, low metabolism, retarded development, and weakness, responds so promptly and with such uniformity to irradiations from a source of known strength, that results can be predicted with almost the exactness of a chemical reaction.

In experimental and clinical work it has been the usual practice, in the past, to give much longer irradiations than are necessary. After four years experimenting with the problems of the minimum dose of ultra-violet rays necessary for satisfactory results in cure of rickets in chickens, we felt justified in making the following conclusions:

There was a total of 459 animals distributed among 7 series and 44 groups.

1. At a lamp floor distance of 22 1/2; inches, one minute daily irradiation will bring about 95% cure in 28 days. It takes approximately 28 days to cure a severe form of the disease in chickens, no matter how much the irradiation is increased. At 30 inches about double this length of exposure is needed to bring about the same result. This corresponds to the law which states that the intensity is inversely proportional to the square of the distance.