REGIONAL BRAIN HEATING DURING MICROWAVE EXPOSURE (2.06 GHZ), WARM-WATER IMMERSION, ENVIRONMENTAL HEATING AND EXERCISE

Nonuniform heating may result from microwave (MW) irradiation of tissu es and is therefore important to investigate in terms of health and sa fety issues. Hypothalamic (T-hyp), cortical (T-ctx), tympanic (T-ty), and rectal (T-re) temperatures were measured in rats exposed in the fa r field, k-polarization (i.e., head pointed toward the transmitter hor n and E-field in vertical direction) to two power densities of 2.06 GH z irradiation. The high-power density (HPM) was 1700 mW/cm(2) [specifi c absorption rate (SAR): hypothalamus 1224 W/kg; cortex 493 W/kg]; the low-power density (LPM) was 170 mW/cm(2) (SAR: hypothalamus 122.4 W/k g; cortex 49.3 W/kg). The increase (rate-of-rise, in degrees C/s) in T -hyp was significantly greater than those in T-ctx or T-re when rats w ere exposed to HPM. LPM produced more homogeneous heating. Quantitativ ely similar results were observed whether rats were implanted with pro bes in two brain sites or a single probe in one or the other of the tw o sites. The qualitative difference between regional brain heating was maintained during unrestrained exposure to HPM in the h-polarization (i.e., body parallel to magnetic field). To compare the temperature ch anges during MW irradiation with those produced by other modalities of heating, rats were immersed in warm water (44 degrees C, WWI); expose d to a warm ambient environment (50 degrees C, WSED); or exercised on a treadmill (17 m/min 8% grade) in a warm ambient environment (35 degr ees C, WEX). WWI produced uniform heating in the regions measured. Sim ilar rates-of-rise occurred among regions following WSED or WEX, thus maintaining the pre-existing gradient between T-hyp and T-ctx. These d ata indicate that HPM produced a 2-2.5-fold difference in the rate-of- heating within brain regions that were separated by only a few millime ters. In contrast more homogeneous heating was recorded during LPM or nonmicrowave modalities of heating. (C) 1998 Wiley-Liss, Inc.