Three regions of thermal response are distinguished: 1) a non-thermal regulatory region, where no measurable temperature response occurs, 2) a thermal regulatory region, where the regulatory system is able to compensate the absorbed energy (to achieve a stable body temperature), and 3) a thermal breakdown region, where the organism is not able to compensate the temperature increase.
For B6C3F1 mice, the thermal response was investigated for three different weight groups (20 g, 24 g, 29 g), both genders and for pregnant mice. In addition, NMRI mice with a weight of 36 g were examined for an interstrain comparison.
Exposure duration: continuous for 2 h
|Distance between exposed object and exposure source||111.5 mm|
|Chamber||The mice were exposed inside a 'Mini Wheel Setup', the concept of which was derived from the 'Ferris Wheel Setup' [Balzano et al., 2000]. The mini wheel setup was designed as a mono-mode cavity resonant waveguide structure of two parallel, circularly shaped metallic plates (diameter 332 mm, separation 120 mm) shorted with metallic bars along the outer edge and with an isotropic antenna at the centre. The system was matched for resonance (return loss ≤ -18 dB) using a triple stub tuner.|
|Setup||The mice were circularly positioned in restrainer tubes at a radial distance of 111.5 mm from the antenna with their body axis oriented parallel to the E-field.|
|Sham exposure||A sham exposure was conducted.|
|Additional info||Groups of eight mice were exposed to a series of increasing SAR levels on consecutive days (one exposure per mouse group per day). Four additional animals of the same breed, gender and weight group were always sham exposed in a second setup.|
|SAR||20 W/kg||average over mass||measured and calculated||whole body||2, 5, 7.2, 10, 12.6, 20 W/kg|
The thermal regulatory threshold of in-tube restrained mice was revealed at SAR levels between 2 W/kg and 5 W/kg, whereas the breakdown of regulation was determined at 10.1 +/- 4.0 W/kg for B6C3F1 mice and 7.7 +/- 1.6 W/kg for NMRI mice. The thresholds show a clear dependence upon the metabolic rate and weight. NMRI mice were more sensitive to thermal stress and responded at lower SAR values with regulation and breakdown.
The data suggest that the thermal breakdown for restrained mice, exposed to radiofrequency fields, may occur at SAR levels of 6 W/kg at laboratory conditions.