Monday, February 15, 2016

Growth Rate of Irradiated Mice

In a 1954 study by E. Lorenz et al. on the "effects of long-continued total body gamma irradiation on mice, guinea pigs, and rabbits," it was found that the growth rate of mice increased in proportion to the exposure, up to the level of 1.0 cGy (1,000 mrad) per day. [Lorenz, Egon, et al. Effects of long-continued total body gamma irradiation on mice, Guinea pigs and rabbits, in Biological Effects of External X and Gamma Radiation, Vol. 1, Zirkle, R. E., ed., McGraw-Hill, New York, 1954.]

Above this point, as shown in Figure 6, the growth rate decreased, until it was the same as the control group at approximately 7.5 cGy per day. Irradiation, which lasted eight hours per day, commenced one month after birth and continued until death. This study did not, however, examine the question of the relative longevity of the mice involved - a subject taken up later in this chapter.



[Caption for Figure 6: Growth Rate of Irradiated Mice - Note: "Control" or "Controls" refers to a group of unexposed animals often as large as the entire exposed cohort. The average behavior of this group is the norm to which all other groups are compared. Source: Lorenz, E., Jacobson, L.O., Heston, W.E., Shimkkin, M., Eschenbrenner, A.B., Deringer, M.K., Doniger, J., and Schweistal, R., Effects of long-continued total body gamma irradiation on mice, Guinea pigs, and rabbits. III. Effects on life span, weight, blood pressure and carcinogenesis and role of intensity of radiation. In Biological Effects of External X and Gamma Radiation, Vol. 1, Zirkle, R. E., ed., McGraw-Hill, New York, 1954.]

The size advantage (considered "healthy" in mice, though probably not popular in Minnie's ballerina class) continued into late maturity - about 100 weeks.

If we consider the background level to be the U.S. average of 0.3 cSv or 300 mrem per year (about 1 mrem per day) [the dose, expressed in units of energy per mass, is independent of the size of the recipient], then the fastest-growing rodents received about 1,000 times their normal background radiation - hardly in keeping with the LNT - which would predict unhealthiness at any level above background. It strongly suggests beneficial radiation effects are at work.