Exposure and energy absorption

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Exposure and energy absorption
Unfortunately, no simple relationship exists between exposure fields outside the
human body and induced fields within the body which may cause a biological response.
Time-varying electric and magnetic fields induce electric fields and corresponding
electric currents in people exposed to these fields. The intensities and spatial distribution
of induced currents and fields are dependent on various characteristics of the exposure
field, the exposure geometry and the exposed person. The exposure field characteristics
that play a role include the type of field (electric or magnetic), frequency, polarization,
direction and strength. Important characteristics of the exposed person include shape or
orientation to the field, and electrical properties of body tissues. Current views are that
the biological responses and effects due to exposure to electromagnetic fields depend on
the strength of induced currents and fields. Dosimetric techniques have been developed
to correlate the induced currents with the external exposure field.
In the 10-100 MHz frequency range, the commonly used dosimetric quantity is the
specific absorption rate (SAR). The SAR is defined as "the time derivative of the
incremental energy, dW, absorbed by, or dissipated in an incremental mass, dm,
contained in a volume element, dV, of a mass density, " (NCRP, 1986). The SAR is
most often expressed in units of watts per kilogram (W/kg) and can be averaged over
localized regions of tissue or over the whole body mass.
The SAR is related to the induced electric field strength in biological tissue by the
electrical properties of the tissues through which the current flows. It is possible to
obtain a measure of induced current under certain exposure conditions which enables the
SAR to be calculated from knowledge of anatomical cross-sections and the conductivity
of the relevant tissues. This can be expressed as:
SAR = E²/ = J²/ (W/kg)
where: J = current density (A/m²)
= conductivity (S/m)
= density (kg/m³)
SAR is related also to temperature rise; a direct calculation of the expected
temperature rise (T Kelvin) in tissue exposed to RF radiation for a time (t seconds) can
be made from the equation:
T = (SAR) t/C
where C is the heat capacity expressed in J kg-1 K-1. This equation, however, does not
include terms to account for heat losses via processes such as thermal conduction and
convection. The SAR concept has proved to be a useful tool in quantifying the
interactions of RF radiation with biological material. It enables comparison of
experimentally observed biological effects for various species under various exposure
conditions and it provides a means of extrapolating animal data to potential hazards to
human beings exposed to RF radiation.
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