Biological Studies in the Tolerance Range

Abstract

The experiments to be discussed here were started early in 1941 and were greatly expanded during the war. Although they are not yet completed, it is not expected that the results will be altered significantly by future findings. Gamma rays were used throughout the experiments. The daily doses were 8.8 r, 4.4 r, 2.2 r, 1.1 r, and 0.11 r, given either in eight hours or twenty-four hours per day. Three radiation sources were used: 1 gm., 250 mg., and 100 mg. of Ra element filtered with 0.5 mm. of platinum. The shelves carrying the cages of the experimental animals were arranged so that each cage in a given set was approximately at an equal distance from the source for each exposure level. Figure 1 shows the experimental set-up used with the 1 gm. source. After the daily eight-hour exposure, the source was lowered underground into a shaft to a depth of 21 feet. In the two other experimental set-ups the shelves for animal cages were arranged similarly. The gamma ray sources were kept in lead boxes provided with hinged lids. Doses were measured at several places within each cage with calibrated 0.25 r and 25 r Victoreen ionization chambers. The error of the dose varied with the size of the cages. It was estimated to be ± 10 per cent in the mouse and guinea-pig cages; the average total dose for each animal can be expressed by the dose measured in air. The average total dose received by the rabbits was probably smaller by 25 per cent than the dose as measured in air in the middle of the rabbit cages. Three inbred strains of mice were used in the experiments, strain A, C3H, dba and LAF1 hybrids, these latter animals also being genetically homogeneous. The guinea-pigs used were genetically heterogeneous hybrids and two inbred strains. The rabbits were crosses of Dutch and American Blues. The exposures were started at the age of two to three months for the mice, three to six months for the guinea-pigs, and five months for the rabbits; the animals were exposed daily until in extremis or dead, with the exception of those used in term exposures. Blood counts were taken every four weeks at the beginning and more frequently in the later stages of the experiments. The highest accumulated doses were 5,900 r for mice, and are at present 6,000 r for guinea-pigs and 12,000 r (air) for rabbits. Figure 2 shows the percentage survival of LAF1 mice per thirty-day intervals. The animals of the 8.8 r group from the beginning show an increased mortality rate in comparison with untreated controls. In the lower exposure groups, the increase in mortality rate begins later, the smaller the daily dose. The decreased initial slope of the curve for the animals of the 0.11 r group may be attributable to biological variations or may indicate a general stimulating effect of an obscure mechanism of the radiation.