Biological Effects - RF/Microwave Interaction with Biological Tissues - page 116

permeability, degenerative cell changes in both the iris and retina, and altered
electroretinograms, indicating a significant decrease in visual functions at
10 mW cm -2 ; (3) corneal damage at a SAR of 2.6 W kg -1 at 2.45 GHz; and (4)
retinal damage at a SAR of 4 W kg -1 in the frequency range 1.25-2.45 GHz.
Pulsed microwaves with an average power density of 10 mW cm -2 (SAR
2.6Wkg -1 ) produced effects for which levels of 20-30 mW cm -2 (SAR 5.3-
7.8Wkg -1 ) CW exposure were required to produce similar changes [91].
Later, another series of experiments was conducted where the nonhuman
primate eye was submitted to an ophthalmic pretreatment. Immediately
before microwave exposure, one or both eyes of an anesthetized primate were
treated topically with one drop of 0.5% timolol maleate or 2% pilocarpine.
The power density threshold was observed to decrease by a factor of about 10
(from 10 to 1 mW cm -2 ) for induction of corneal endothelial lesions and for
increased vascular permeability of the iris at a SAR of 0.26 W kg -1 at pulsed
2.45 GHz [92]. In another experimental investigation, however, the same
authors reported the absence of any detectable ocular damage after either
single or repeated exposure to 10 mW cm -2 from a 60-GHz CW source [93].
Possible effects of RFs and microwaves on the heart have been investigated
also. The location the heart, however, placed quite inside the body, together
with the small penetration of microwaves due to the skin effect are such
that the heart is not submitted to high microwave fields. Hearts of chicken
embryos have been isolated. Heartbeat stimulation and the control effect of
microwaves on the electrical activity of the heart have been analyzed. The
hearts were exposed to low-power, pulse-modulated microwaves at 2.45 GHz,
10 mW peak power, and 10% duty cycle. The estimated incident peak power
density was 3 mW cm -2 . The repetition frequency was within normal physio-
logical limits (1-3 Hz). Before being exposed, the heart rhythm was rather
irregular. When microwaves with a pulse repetition rate of 2.4 Hz were tuned
on, the heartbeat became regular at about the same frequency. By increasing
the repetition frequency, the heartbeat increased likewise until, above 2.65 Hz,
the heart came back to beat irregularly. Hence, the heartbeat was synchro-
nized with the signal from the source within normal physiological limits [94].
This phenomenon is explained by an effect of the pulsed modulation of
the source on currents due to the calcium ions, mentioned in Sections 3.2
and 3.3.
Further experimental results by the same authors showed that the dragging
and regularization effects observed when the samples are exposed for short
durations also appear during longer exposure durations, lasting through all the
exposure time. However, CW exposure at the same peak power as the exper-
iments with pulsed modulation does not show any significant modification of
the heartbeat. As the authors have pointed out, this suggests a nonthermal
effect induced by the pulse-modulated microwaves. The temperature of the
sample is lower than at CW excitation; therefore, no heartbeat increase can
be related to temperature variations [95].