Threshold Values of Ultra-Violet Radiations. ∗

The question of glazing the windows of modern homes and hospitals so that winter sunshine with its low content of ultra-violet may enter, and the newer problem of so-called “dual purpose” lighting of houses has raised the question of threshold values.

When one selects ultra-violet sources of greatly reduced intensity and thus reduces sufficiently the size of the unit of ultraviolet irradiation it will be found that the amount necessary for the first detectable effects is much below the amount necessary for complete prevention of rickets. This threshold amount is between one-fourth and one-sixth that needed for complete prevention. Rays from very low power sources, applied in the most advantageous way, can be made quite efficient and practical. The difficulty is that the distance factor and the covering of the animal, when varied, make such enormous differences that results in the hands of different investigators may seem entirely contradictory.

Two series of tests have been directed specifically toward the problem of threshold values. In one, completed in June, 1930, there were 8 groups of chickens containing 17 birds each and 6 pens of rats with 4 rats in each. The lamps were 60 watt Mazda CX bulbs of varying intensities of ultra-violet, except in one control group where a mercury arc was used. The intensity of ultra-violet from the tungsten filament of these lamps can be varied by differences in size and length of the filament. This alters the temperature, and the higher the temperature the greater the amount of ultra-violet. The irradiation in some of the groups was also increased by increasing the time of exposure. A second set of experiments recently completed made use of the quartz mercury arc for a control group, a Model E-1 GE Sunlamp and three 115 watt CX bulbs, one 60 watt, one 300 watt and a third of 500 watt size.