222
7 Light in Biology and Medicine
7.11 Radio Wave Absorption in Tissue
Generally, biological tissue requires extraordinarily intense radio waves before
absorption effects are deleterious. This follows from the fact that such tissue has
no appreciable resonances at radio frequencies, the wavelengths of radio waves is
large compared to the tissue extent, and no harmful currents are ordinarily induced.
An exception occurs when metal is present, such as fillings in teeth. Individuals
with such fillings who live near a commercial radio transmission antenna might be
able to hear transmissions by the vibrations from their filling in their teeth caused
by the passing radio waves stimulating currents. The interaction of these currents
produce forces which vibrate the fillings not at the carrier-wave frequency, but rather
at the much lower AM signal frequency due to a non-linear response to the induced
voltage. The resulting sound vibration spreads through the upper jaw into the inner
ear.
Radio waves are absorbed in tissue largely by stimulating ions to vibrate and by
causing polar molecules to rotate. The dominant effect of this absorption in tissue
is heating, and not chemical transformations, and thus induced cancer in cells is
not likely. The tissue absorption is calibrated using the ‘Specific Absorption Rate’
(SAR) (α R ), i.e. the rate at which radio-wave energy is absorbed per unit mass.
Below f = 10 MHz,
SAR ≡ α R ≈ 2 (f/10 MHz)
2 mW/kg
for human tissue. 22
Radio Wave Dosimetry
To minimize harm, there is general agreement that the resulting SAR should be less
than 1.6 W/kg for radio waves and less than 0.4 W/kg for microwaves. The U.S.
recommended safe limit for the incoming radio wave intensity is 10 mW/cm 2 . More
specific recommendations are imposed on Magnetic Resonance Imaging (MRI)
scanners.
Damaging effects of radio waves produced by power transmission lines are
much less than microwaves of the same intensity, since the absorption in tissue
is significantly less. Moreover, as the frequency of the alternating current in a
transmission line is 60/s, the radiation from transmission wires is far less than wires
carrying current with a higher frequency of oscillation, since the radiative power
from a given segment of wire is proportional to the fourth power of the frequency.
The (2016) standard limit for cell phone radiation is 1.6 W/kg of tissue. A typical
22 A Practical Guide to the Determination of Human Exposure to Radio-frequency Fields, NCRP
Report 119 (2015).
7 Light in Biology and Medicine
7.11 Radio Wave Absorption in Tissue
Generally, biological tissue requires extraordinarily intense radio waves before
absorption effects are deleterious. This follows from the fact that such tissue has
no appreciable resonances at radio frequencies, the wavelengths of radio waves is
large compared to the tissue extent, and no harmful currents are ordinarily induced.
An exception occurs when metal is present, such as fillings in teeth. Individuals
with such fillings who live near a commercial radio transmission antenna might be
able to hear transmissions by the vibrations from their filling in their teeth caused
by the passing radio waves stimulating currents. The interaction of these currents
produce forces which vibrate the fillings not at the carrier-wave frequency, but rather
at the much lower AM signal frequency due to a non-linear response to the induced
voltage. The resulting sound vibration spreads through the upper jaw into the inner
ear.
Radio waves are absorbed in tissue largely by stimulating ions to vibrate and by
causing polar molecules to rotate. The dominant effect of this absorption in tissue
is heating, and not chemical transformations, and thus induced cancer in cells is
not likely. The tissue absorption is calibrated using the ‘Specific Absorption Rate’
(SAR) (α R ), i.e. the rate at which radio-wave energy is absorbed per unit mass.
Below f = 10 MHz,
SAR ≡ α R ≈ 2 (f/10 MHz)
2 mW/kg
for human tissue. 22
Radio Wave Dosimetry
To minimize harm, there is general agreement that the resulting SAR should be less
than 1.6 W/kg for radio waves and less than 0.4 W/kg for microwaves. The U.S.
recommended safe limit for the incoming radio wave intensity is 10 mW/cm 2 . More
specific recommendations are imposed on Magnetic Resonance Imaging (MRI)
scanners.
Damaging effects of radio waves produced by power transmission lines are
much less than microwaves of the same intensity, since the absorption in tissue
is significantly less. Moreover, as the frequency of the alternating current in a
transmission line is 60/s, the radiation from transmission wires is far less than wires
carrying current with a higher frequency of oscillation, since the radiative power
from a given segment of wire is proportional to the fourth power of the frequency.
The (2016) standard limit for cell phone radiation is 1.6 W/kg of tissue. A typical
22 A Practical Guide to the Determination of Human Exposure to Radio-frequency Fields, NCRP
Report 119 (2015).