Thermal and endocrinological effects of protracted irradiation of rats by 2450‐MHz microwaves

Eighty‐six male Long‐Evans rats, 63–64 days of age, were subjected to 2450‐MHz CW microwave irradiation after a two‐week “gentling” procedure and three daily sessions of sham exposure. Exposures occurred without anesthesia at power densities of 0, 1, 5, 10, or 20 mW/cm 2 for 1, 2, 4, or 8 hours. Rectal temperatures of exposed rats were found to be higher than those of sham‐irradiated rats at power densities of 1 mW/cm 2 for four hours, 5 mW/cm 2 for one and two hours, 10 mW/cm 2 for two and four hours, and 20 mW/cm 2 for all of the durations of exposure. Circadian rhythmicity of rectal temperature was noted in sham‐irradiated rats. Except for rats exposed at 20 mW/cm 2 for four and eight hours, none of the averaged rectal temperatures of exposed groups reliably exceeded those of sham‐irradiated rats after eight hours of exposure. We concluded that the effect of exposure to microwaves at power densities below 10 mW/cm 2 accelerated the appearance of the peak rectal temperature to an earlier time of day. Serum corticosteroid (CS) levels were signficantly lower in rats exposed at 20 mW/cm 2 for eight hours than in animals sham exposed for the same duration. A significant correlation between rectal temperature and CS level was observed in the sham‐irradiated rats. Certain combinations of duration of exposure and power density of incident energy could dissociate this relationship, i.e., temperature increased sometimes without corresponding elevation of CS level. Serum thyroxine levels were depressed in rats exposed to radiation at 20 mW/cm 2 for four or for eight hours. A stimulatory effect of microwaves on thyroid function was noted in rats exposed at 1 mW/cm 2 for four hours, but the effect was transitory. Levels of growth hormone did not change.