To compare the effects of exposure to a near-resonant frequency of microwaves at two orientations with a higher frequency exposure, five rhesus monkeys were exposed for 4 hr to 225 MHz, electric field oriented parallel to the long axis of the body (225 MHz-E), and to 225 MHz, magnetic field orientation (225 MHz-H), or to 1290 MHz, electric field orientation. On a separate occasion, the monkeys were exposed at night to 225 MHz-E. Exposures were conducted with the animal chair restrained in an anechoic chamber with rectal temperature continuously monitored. Blood samples were taken hourly during the 225-MHz-E exposures for cortisol analysis. The power densities used were 0, 1.2, 2.5, 5.0, 7.5, 10.0, and <tex-math>$15.0\ {\rm mW}/{\rm cm}^{2}$</tex-math> for 225 MHz-E (day), 0 and <tex-math>$5\ {\rm mW}/{\rm cm}^{2}$</tex-math> (225 MHz-E night and 225 MHz-H), and 0, 20, 28, and <tex-math>$38\ {\rm mW}/{\rm cm}^{2}$</tex-math> (1290 MHz). The monkeys were unable to tolerate exposure at power densities equal to or greater than <tex-math>$7.5\ {\rm mW}/{\rm cm}^{2}$</tex-math> (5.1 W/kg) at 225 MHz-E for longer than 90 min. The criterion for tolerance was that the rectal temperature would not exceed 41.5°C. Average rectal temperature increases for day exposure to 225 MHz-E were 0.4 and 1.7°C for 4-hr exposures to 2.5 and <tex-math>$5.0\ {\rm mW}/{\rm cm}^{2}$</tex-math> (1.7 and 3.4 W/kg). No changes in circulating cortisol levels occurred during any exposures to <tex-math>$5\ {\rm mW}/{\rm cm}^{2}$</tex-math> or less. Night exposures to <tex-math>$5\ {\rm mW}/{\rm cm}^{2}$</tex-math> (3.4 W/kg) at 225 MHz-E raised mean rectal temperature 2.1°C. Exposure to <tex-math>$5\ {\rm mW}/{\rm cm}^{2}$</tex-math> (1.2 W/kg) at 225 MHz-H for 4 hr resulted in a 0.2°C rise in mean rectal temperature. For 4 hr of 1290-MHz exposure to 20, 28, or <tex-math>$38\ {\rm mW}/{\rm cm}^{2}$</tex-math> (2.9, 4.0, and 5.4 W/kg), the mean body temperature increases were 0.4, 0.7, and 1.3°C, respectively. The degree of hyperthermia caused by radiofrequency (rf) exposure was shown to be frequency and orientation dependent for equivalent power densities of exposure.

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