20
Electromagnetic Fields in Biological Systems
• Electric and magnetic fields are decoupled inside a biological body.
• Magnetic fields inside a body are equal to the field applied externally.
• Electric fields applied through air are weakened by a factor of 10 −8 −10 −6 upon penetration into biological tissues.
• Electric fields in the immediate vicinity of the biological body are perpendicular
to the surface of the body.
• Electric fields are enhanced near the surface at points of sharp curvatures.
• Magnetically induced electric fields encircle the magnetic field axis and produce
an eddy current whose magnitude increases with distance from the center of the
body.
• Eddy current calculation must be applied to each region inside the body with
a different conductivity, which behaves as a unit with its own body center and
radius or an equivalent radius.
• The highest induced electric field and eddy current density occur with the large
dimensions associated with the outer layers of a body with finite conductivity.
• Relative significance of electrically or magnetically induced coupling in humans
and animals is a function of body size and biological properties.
• Conduction current is much greater than displacement current in biological
materials at low frequencies.
• Biological materials may be considered as conducting media at low frequencies.
• Biological bodies or models are small compared with a wavelength at low
frequencies.
1.10 Radiofrequency Fields and Energy Deposition
The coupling of incident RF fields into biological tissues is influenced by the geometry
and composition of the exposed object and the frequency and configuration of the source,
as noted in Section 1.8. In addition, the width and strength of the incident field differ
according to distance from a source and the specific source type, that is, whether the
source is handheld, land mobile, base, or broadcast station. As an example, the incident
field strength ranges from 3 to 10 V/m (24−265 mW/m 2 ) at typical far-zone distances from
a wireless cellular mobile telephone base station. An important distinction in assessing
the coupling of RF energy into biological systems is the determination of whether the
exposure is taking place in the near zone or the far zone of a given source. The 2D 2 /λ distance is approximately 6 cm for a 10-cm RF antenna operating at 900 MHz in free space.
Clearly, both near-zone inductive and far-zone field interactions are encountered in the
vicinity of RF mobile telecommunication services. In the near field, quasistatic interactions prevail and the aforementioned discussions can be applied to understand and estimate induced fields. (See also the subsequent section (Section 1.10.4) on “RF Coupling in
the Near Zone” for further information and discussions.)
In the far zone of an antenna, the radiated RF energy propagates as plane waves. The
interaction of RF radiation with biological systems is independent of the source configuration as can be seen from Equations 1.36 through 1.38. The electric and magnetic
fields are uniquely defined and are related through a constant factor η, the intrinsic
Electromagnetic Fields in Biological Systems
• Electric and magnetic fields are decoupled inside a biological body.
• Magnetic fields inside a body are equal to the field applied externally.
• Electric fields applied through air are weakened by a factor of 10 −8 −10 −6 upon penetration into biological tissues.
• Electric fields in the immediate vicinity of the biological body are perpendicular
to the surface of the body.
• Electric fields are enhanced near the surface at points of sharp curvatures.
• Magnetically induced electric fields encircle the magnetic field axis and produce
an eddy current whose magnitude increases with distance from the center of the
body.
• Eddy current calculation must be applied to each region inside the body with
a different conductivity, which behaves as a unit with its own body center and
radius or an equivalent radius.
• The highest induced electric field and eddy current density occur with the large
dimensions associated with the outer layers of a body with finite conductivity.
• Relative significance of electrically or magnetically induced coupling in humans
and animals is a function of body size and biological properties.
• Conduction current is much greater than displacement current in biological
materials at low frequencies.
• Biological materials may be considered as conducting media at low frequencies.
• Biological bodies or models are small compared with a wavelength at low
frequencies.
1.10 Radiofrequency Fields and Energy Deposition
The coupling of incident RF fields into biological tissues is influenced by the geometry
and composition of the exposed object and the frequency and configuration of the source,
as noted in Section 1.8. In addition, the width and strength of the incident field differ
according to distance from a source and the specific source type, that is, whether the
source is handheld, land mobile, base, or broadcast station. As an example, the incident
field strength ranges from 3 to 10 V/m (24−265 mW/m 2 ) at typical far-zone distances from
a wireless cellular mobile telephone base station. An important distinction in assessing
the coupling of RF energy into biological systems is the determination of whether the
exposure is taking place in the near zone or the far zone of a given source. The 2D 2 /λ distance is approximately 6 cm for a 10-cm RF antenna operating at 900 MHz in free space.
Clearly, both near-zone inductive and far-zone field interactions are encountered in the
vicinity of RF mobile telecommunication services. In the near field, quasistatic interactions prevail and the aforementioned discussions can be applied to understand and estimate induced fields. (See also the subsequent section (Section 1.10.4) on “RF Coupling in
the Near Zone” for further information and discussions.)
In the far zone of an antenna, the radiated RF energy propagates as plane waves. The
interaction of RF radiation with biological systems is independent of the source configuration as can be seen from Equations 1.36 through 1.38. The electric and magnetic
fields are uniquely defined and are related through a constant factor η, the intrinsic