Biological Effects - RF/Microwave Interaction with Biological Tissues - page 101

further investigation. Such investigations are attracting the attention of many
in the field [44].

3.2.4 Influence of Parameters of Microwave Exposure

The parameters of microwave exposure are very important to consider when
investigating biological effects. As an example, different durations of acute
exposure lead to different biological effects; consequently, different long-term
effects may occur after repeated exposure. The waveform of the radiation is
also important. This has been observed when comparing pulsed- versus con-
tinuous-wave exposure and plane- versus circularly polarized wave exposure.
Furthermore, the pattern of energy absorption in the body also contributes to
the microwave effect. These findings raise the question of whether the whole-
body average SAR can be used as the only determining factor in evaluating
biological effects of low-level microwaves. Other features of the radiation also
need to be considered [45, 46].
The effects of CW, sinusoidal-amplitude-modulated, and pulsed square-
wave-modulated 591-MHz microwave exposure on brain energy metabolism
in male Sprague-Dawley rats (175-225 g) have been compared [47]. Brain
nicotinamide adenine dinucleotide reduced form (NADH) fluorescence, ATP
concentration, and CP concentration were determined as a function of mod-
ulation frequency. Brain temperatures of animals were maintained within -0.1
and -0.4°C from the preexposure temperature when subjected to as much as
20 mW cm -2 (average power) CW, pulsed, or sinusoidal-amplitude-modulated
591-MHz radiation for 5 min. Sinusoidal-amplitude-modulated exposures at
16-24 Hz showed a trend toward preferential modulation frequency response
in inducing increased brain NADH fluorescence. The pulse-modulated and
sinusoidal-amplitude-modulated at 16-Hz microwaves were not significantly
different from CW exposures in inducing increased brain NADH fluorescence
and decreased ATP and CP concentrations. When the pulse modulation fre-
quency was decreased from 500 to 250 pulses per second, the average incident
power density threshold for inducing an increase in brain NADH fluorescence
increased by a factor of 4, that is, from about 0.45 to about 1.85 mW cm -2 . Since
brain temperature did not increase, the microwave-induced increase in brain
NADH and decrease in ATP and CP concentrations were not due to hyper-
thermia. These data suggested to the authors a direct interaction mechanism
that is consistent with the hypothesis of microwave inhibition of mitochondr-
ial electron transport chain function of ATP production.
Microwave evoked body movements are one of the biological effects asso-
ciated with high-peak-power although low- to average-power microwaves.
These evoked body movements were studied in mice [48]. A resonant cavity
was used to provide head and neck exposure of the mouse to pulsed and gated
1.25-GHz CW. No difference in response to pulsed and gated CW stimuli of
equal average power was found. The incidence of the microwave evoked body
movement increased, however, proportionally with specific absorption (dose)