Biological Effects - RF/Microwave Interaction with Biological Tissues - page 135

Biological effects due to microwave exposure depend upon the electric field
inside the tissues. As has been seen earlier, thermal effects depend on the SAR
distribution (Section 3.1). Are there nonthermal effects? Some say yes, many
say no. The so-called nonthermal effects might be microthermal effects, while
there might also be isothermal effects (Section 3.8). It should be well remem-
bered that microwaves as a whole form a “family” of frequencies, tradition-
ally from 100 MHz to 1 THz, inducing essentially the same effects throughout
the whole frequency range. From a biological point of view, it is even wise to
consider microwaves from 50 MHz up, because of the TV emissions at those
frequencies.
Absorbed power density is the main cause of effects: 1 W kg -1 may yield an
increase of (less than) 1°C in human body, taking thermal regulation into
account [2]. Measurements on the face have shown that the temperature
increase is obtained after 10-12 min [7]. It should be well noted that the SAR
is the ratio of absorbed power to absorbing mass: The skin effect has to be
taken into account. At 900 MHz, the lowest GSM cellular telephone frequency,
the skin depth is approximately 1.5 cm. As an indication, using a simple cylin-
drical approximation, the absorbing mass of a human body weighting 65 kg
and 1.70 m high is approximately 8.5 kg at 900 MHz. Pulsed waves produce
a detectable effect at power levels smaller than at CW (Section 3.5.2). For
mobile telephony, the interaction between a handset antenna and the human
body is obviously of special interest [18].
The question of the “microwave syndrome” (Section 3.5.1) was raised
several decades ago in Eastern Europe, related to a number of manifestations,
such as headache, perspiration, emotional instability, irritability, tiredness, som-
nolence, sexual problems, loss of memory, concentration and decision difficul-
ties, insomnia, and depressive hypochondriac tendencies. The evaluation is
difficult because of the absence of a control group and well-established dosi-
metric data. A recent paper supports the RF sickness syndrome as a possible
medical entity [83].
Experiments concerning thermal effects due to GSM exposure from hand-
held mobile telephones have been made [7]. The time evolution of different
parts of the head has been measured when exposed to microwaves from GSM
at 900 GHz. Three different phones have been used from three different man-
ufacturers located in Europe, the United States, and Japan, respectively. An
IR camera measured the temperature evolution by taking a thermographic
image every 20 s using a pattern of 10,000 thermocouples located in the lens.
Measurements were made during the different phases of the phone operation:
stand-by, ringing, talking, and switching off. Mainly two different locations
were used: favorable reception-transmission conditions on the fourth floor of
a building and unfavorable conditions in the basement, 4 m below ground
level. Total temperature was measured, taking into account microwave
heating, possible heating due to the electronic part of the device, and possible
reduction in ventilation because of the presence of the device near the face.
The most significant temperature increase has been observed on the ear lobe,
at the end of the talking phase, for the highest duration (20 min.). This tem-