ID Number		615
Study Type		In Vivo
Model		600, 2450 MHz exposure to small animals and analysis of thresholds for core body and skin temperature increase
Details		New Zealand white rabbits and Fischer 344 rats were exposed to 600 MHz RF for 90 minutes using a waveguide system. For rabbits, the threshold for measurable colonic and ear skin temperature increases (as measured using thermistor probes and type-T thermocouples) at an ambient temperature of 20°C was 0.64 and 0.26 W/kg, respectively. The threshold for measureable temperature increases at an ambient temperature of 30°C was 0.26 and 0.19 W/kg, respectively. For rats, the threshold for a 1 degree colonic temperature increase was 4.5 W/kg at an ambient temperature of 20°C, 1.8 W/kg at an ambient temperature of 28°C, and a linear relationship between SAR and colonic temperature was observed at an ambient temperature of 35°C. In a similar set of studies, BALB/c mice and golden hamsters were exposed to 2450 MHz microwaves for 90 minutes. The threshold SARs for elevating colonic and ear skin temperature at 32.2°C for the mouse were 4.3 and 6.5 W/kg, respectively. The threshold SARs for elevating colonic and ear skin temperature at 35°C for the mouse were 0.12 and 0.63 W/kg, respectively, and for the hamster were 0.68 and 1.1 W/kg, respectively. In another study, Sprague-Dawley rats were maintained at an ambient temperature of 10°C and exposed to 600 MHz (near resonant frequency) while measuring metabolic rate via O2 consumption. The average metabolic rate was reduced significantly after 30 to 60 minutes of exposure at SARs of 2-5 W/kg. This decrease amounted to 37% of the total RF heat load, with the remaining 63% either stored in the body or dissipated by thermoregulatory mechanisms. The increase in colonic temperature was equal to 0.17°C/W/kg, and heat gain exceeded heat loss. In contrast, exposure of squirrel monkeys to 2450 MHz results in a rapid thermoregulatory response with skin warming but limited core temperature increase. In a review of the ANSI guidelines for whole body RF exposure (0.4 W/kg / 6 minutes ) (Bioelectromagnetics (1987) 8:111-118), the authors suggest the guideline may be too high because surface area provides a better factor to normalize biological effects between species than body weight. Using this approach, comparable thresholds from mouse, hamster, rat, and rabbit data for raising colonic temperatures in humans would be closer to 0.006 W/kg. In a theoretical study using information from the literature on thermal thresholds due to RF exposure, mice (having a surface area:mass ratio of 14X greater than humans and a metabolic rate of 9.0 W/kg vs 1.39 W/kg in humans) can dissipate a whole body average 1 W/kg SAR absorption better than humans. The authors calculate a theoretical threshold SAR (heat gain = 25% basic metabolic heat production) of about 0.25 W/kg at resonant frequency in humans with an SAR of ~22 W/kg to cause a similar temperature elevation in mice. In CBA/J mice exposed to 2450 MHz (CW) MW for 90 minutes at an SAR of 29 W/kg. The ambient temperature threshold necessary to increase evaporative water loss at this level was 30-33 degrees. Above 29 W/kg, a linear elevation in evaporative heat loss was observed with increasing SAR. The authors conclude that an SAR of 29 W/kg in a 3.4 gram mouse would be equivalent to an SAR of 0.25 W/kg in a 70 kg man. A technical note to this study by Adair (J Microwave Power (1983) 18:209-211) suggested that the study may have omitted metabolic heat generation when they calculated threshold conditions for an elevation in evaporative heat loss. Gordon counters (J Microwave Power (1983) 18:377-382) that the purpose of the study was to relate only SAR to evaporative heat loss and not the combined effects of evaporative heat loss and metabolic rate. The authors concluded that body mass was the critical parameter in determining an animals sensitivity to the thermal effects of RF.
Findings		Effects (only at thermal levels)
Status		Completed With Publication
Principal Investigator		EPA, NC, USA
Funding Agency		EPA, USA
Country		UNITED STATES
References		Gordon, CJ et al. Rev Environ Health, (2005) 20:235-263 Gordon, CJ BIOLOGICAL EFFECTS AND SAFETY ASPECTS OF NUCLEAR MAGNETIC RESONANCE IMAGING AND SPECTROSCOPY, (1992) 649:273-284 Gordon, CJ et al. Comp. Biochem Physiol. [A], (1987) 88:107-112 Gordon, CJ J. Appl. Physiol., (1987) 62:1814-1818 Gordon, CJ et al. Int. J. Radiat. Biol., (1986) 49:987-997 Gordon, CJ et al. Health Phys., (1986) 50:781-787 Gordon, CJ Bioelectromagnetics, (1987) 8:111-118 Gordon, CJ et al. Int. J. Radiat. Biol., (1984) 46:387-397 Gordon, CJ J. Microwave Power, (1983) 18:377-383 Gordon, CJ J. Appl. Physiol.: Respirat. Environ. Exercise Physiol., (1983) 55:1242-1248 Gordon, CJ J. Microwave Power, (1982) 17:145-150 Gordon, CJ ISI Atlas of Science, Animal and Plant Sciences. , (1988) 1:245-250 Gordon, CJ J Appl Physiol Respir Environ Exerc Physiol. 1982 Aug;53(2):316-23., (1982) 53:316-323 Gordon, CJ et al. J. Therm. Biol. , (1986) 11:59-65
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