NON-IONIZING RADIATIONS 785
sources may operate continuously as in the case of some
communications systems or intermittently, e.g. in microwave
ovens, induction heating equipment and diathermy equip-
ment or in the pulsed mode in radar systems. Natural sources
of RF and microwave energy also exist. For example, peak
fi eld intensities of over 100 V/m are produced at ground
level by the movement of cold fronts. Solar radiation intensi-
ties range from 10^ ^18 to 10^ ^17 watts per square meter per Hz
(Wm^ ^2 Hz^ ^1 ) however, the integrated intensity at the earth’s
surface for the frequency range of 0.2–10 GHz is approxi-
mately 10^ ^8 mW/cm^2. This value is to be compared with an
average of 10^2 mW/cm^2 on the earth’s surface attributable to
the entire (UV, visible IR and microwave) solar spectrum.Biological Effects of Microwave RadiationThe photon energy in RF and microwave radiation is con-
sidered to be too low to produce photochemical reactions in
biological matter. However, microwave radiation is absorbed
in biological systems and ultimately dissipated in tissue as
heat. Irradiation of the human body with a power density of
10 mW/cm^2 will result in the absorption of approximately
58 W with a resultant body temperature elevation of 1C, a
value which is considered acceptable from a personal hazard
standpoint. By way of comparison, the human basal meta-
bolic rate is approximately 80 W for a person at rest; 290 for
a person engaged in moderate work.
Microwave wavelengths less than 3 cm are absorbed in
the outer skin surface, 3–10 cm wavelengths penetrate more
deeply (1 mm–1 cm) into the skin and at wavelengths from
25–200 cm penetration is greatest with the potential of causing
damage to internal body organs. The human body is thought
to be essentially transparent to wavelengths greater than about
200 cm. Above 300 MHz the depth of penetration changes rap-
idly with frequency, declining to millimeter depths at frequen-
cies above 3000 MHz. Above 10 GHz the surface absorption
of energy begins to approach that of the IR radiation.
The observed effects of radiofrequency radiation on bio-
logical systems seem to depend more on a differential rate
of energy deposition than in the case with ionizing radiation
where biological effects seem to be related more to energy and
integral (time independent) quantities, such as absorbed dose.
The National Council on Radiation Protection and
Measurements (NCRP) has attempted to consolidate the many
quantities and units used to describe absorption of radio fre-
quency electromagnetic energy by introducing the term “spe-
cifi c absorption rate” (SAR). The specifi c absorption rate is the
rate at which electromagnetic energy is absorbed at a point in
a medium per unit mass of the medium, and is expressed in
W/kg. Energy absorption is a continuous and differentiable
function of space and time and one may speak of its gradient
and its rate, hence the time derivative of the incremental energy
(d W ) absorbed in an incremental mass (d m ) contained in a
volume element (d V ) of a given density ( r ) may be expressed:SARd
dd
dd
dd
d
tW
mtw
V⎛
⎝⎜⎞
⎠⎟⎛
⎝⎜⎞
r ⎠⎟.Carpenter and Van Ummersen (1968) investigated the effects
of microwave radiation on the production of cataracts in
rabbit eyes. Exposures to 2.45 GHz radiation were made at
power densities ranging from 80–400 mW/cm^2 for different
exposure times. They found that repeated doses of 67 J/cm^2
spaced a day, a week, or 2 weeks apart produced lens opaci-
ties even though the single threshold exposure dose at that
power density (280 mW/cm^2 ) was 84 J/cm^2. When the single
exposure dose was reduced to 50 J/cm^2 opacities were pro-
duced when the doses were administered 1 or 4 days apart,
but when the interval between exposures was increased to
7 days no opacifi cation was noted even after 5 such weekly
exposures. At the low power density of 80 mW/cm^2 (dose of
29 J/cm^2 ) no effect developed but when administered daily
for 10 or 15 days cataracts did develop. The conclusion is
that microwaves may exert a cumulative effect on the lens of
the eye if the exposures are repeated suffi ciently often. The
interval between exposures is an important factor in that a
repair mechanism seems to act to limit lens damage if ade-
quate time has elapsed between exposures.
Certain other biological effects of microwave radiation
have been noted in literature. One of these is the so-called
“pearl chain effect” where particles align themselves in chains
when subjected to an electric fi eld. There is considerable dis-
agreement as to the signifi cance of the pearl chain effect.
Investigators at the Johns Hopkins University have sug-
gested a possible relationship between mongolism (Down’s
Syndrome) in offspring and previous exposure of the male
parent to radar. This suggested relationship was based on the
fi nding that of 216 cases of mongolism, 8.7% of the fathers
having mongol offspring vs. 3.3% of the control fathers (no
mongol offspring) had contact with radar while in military
service. This possible association must be regarded with
extreme caution because of many unknown factors includ-
ing the probability of a variety of exposures to environmental
agents (including ionizing radiation) while in military service.
Soviet investigators claim that microwave radiation pro-
duces a variety of effects on the central nervous system and
without a temperature rise in the organism. Claims are also
made for biochemical changes, specifi cally a decrease in
cholinesterase and changes in RNA at power density levels
of approximately 10 mW/cm^2. The reported microwave
effects on the central nervous system usually describe ini-
tial excitatory action, e.g. high blood pressure followed by
inhibitory action and low blood pressure over the long term.
Electroencephalographic data have been interpreted as indi-
cating the presence of epileptiform patterns in exposed sub-
jects. Other reported effects ranged from disturbances of the
menstrual cycle to changes in isolated nerve preparations.
Field interactions with brain tissue in cats have been
assessed by effects on calcium ion fl uxes. Increases in cal-
cium effl ux of the order of 20% have been reported under
conditions of direct stimulation of synaptic terminals.
Moreover, exposure of intact animals (cats) to a 450 MHz
0.375 mW/cm^2 fi eld, amplitude modulated at 16 Hz pro-
duced a sharp rise in calcium effl ux, with a response curve
identical to that obtained by direct electrical stimulation of
brain tissue at the same intensity.C014_004_r03.indd 785C014_004_r03.indd 785 11/18/2005 3:09:24 PM11/18/2005 3:09:24 PM