202
6 Electric and Magnetic Fields in Life
The dosimetry for non-ionizing (low frequency) electromagnetic radiation will
be described in Sect. 7.31.
6.7.1 Strong Static Electric Fields
Electric fields, such as those under a thundercloud, can reach 1000 kV/m, sufficient
to cause lethal current discharge within tissue, with damage largely due to heating.
Electric fields above around 100 mV/m across tissue has measurable consequences.
The field particularly affects the behavior of cell membranes since cells develop
charge separation across their cell wall as an energy source to power membrane
motors, pumps, and nerve pulses.
For humans, however, because strong electric fields will cause charges on the
skin to accumulate, little static electric field penetrates the skin, being attenuated by
a factor of a trillion by the epidermis. 16 In effect, the skin acts as a Faraday cage.
Strong static electric fields are detectable by humans by the effect those fields
have in aligning hairs on the skin. The ‘pin-prick’ sensation caused by tissue heating
from charges accumulated on the body discharging when touching a grounded piece
of metal is secondary evidence that a strong external electric field was present,
sometimes as large as 3000 kV/m. However, there are so few charges producing
this field that their discharge generates little total energy. Large fields exist near
operating cathode ray tubes (‘CRT’), up to 20 kV/m at a distance of 30 cm from the
tube. 17
6.7.2 Strong Static Magnetic Fields
The magnetic fields used in MRI medical imaging, measured in 1–8 T range,
are relatively large compared to typical environmental magnetic field strengths.
(The field of the Earth is more than 20,000 times smaller.) The Food and Drug
Administration sets limits on not only the strength of the magnetic field to which
patients are subjected, but also the rate of change of these fields in both time and
across space. Gradients over space are necessary for imaging. They are produced
by radio waves at about 64 MHz with powers at the source below 25 kW. Time
variations occur when the magnetic field is switch on and off. These variations in
the magnetic field produce local electric fields, also varying with time.
Important issues include:
16 Polk C, Postow E., Handbook of biological effects of electromagnetic fields, 2nd ed. Boca
Raton: [CRC Press] (1996).
17 The greater danger here is not the strength of the field, but the X-rays produced by electrons of
kinetic energy of 25 keV hitting the phosphor and glass in a color CRT.
6 Electric and Magnetic Fields in Life
The dosimetry for non-ionizing (low frequency) electromagnetic radiation will
be described in Sect. 7.31.
6.7.1 Strong Static Electric Fields
Electric fields, such as those under a thundercloud, can reach 1000 kV/m, sufficient
to cause lethal current discharge within tissue, with damage largely due to heating.
Electric fields above around 100 mV/m across tissue has measurable consequences.
The field particularly affects the behavior of cell membranes since cells develop
charge separation across their cell wall as an energy source to power membrane
motors, pumps, and nerve pulses.
For humans, however, because strong electric fields will cause charges on the
skin to accumulate, little static electric field penetrates the skin, being attenuated by
a factor of a trillion by the epidermis. 16 In effect, the skin acts as a Faraday cage.
Strong static electric fields are detectable by humans by the effect those fields
have in aligning hairs on the skin. The ‘pin-prick’ sensation caused by tissue heating
from charges accumulated on the body discharging when touching a grounded piece
of metal is secondary evidence that a strong external electric field was present,
sometimes as large as 3000 kV/m. However, there are so few charges producing
this field that their discharge generates little total energy. Large fields exist near
operating cathode ray tubes (‘CRT’), up to 20 kV/m at a distance of 30 cm from the
tube. 17
6.7.2 Strong Static Magnetic Fields
The magnetic fields used in MRI medical imaging, measured in 1–8 T range,
are relatively large compared to typical environmental magnetic field strengths.
(The field of the Earth is more than 20,000 times smaller.) The Food and Drug
Administration sets limits on not only the strength of the magnetic field to which
patients are subjected, but also the rate of change of these fields in both time and
across space. Gradients over space are necessary for imaging. They are produced
by radio waves at about 64 MHz with powers at the source below 25 kW. Time
variations occur when the magnetic field is switch on and off. These variations in
the magnetic field produce local electric fields, also varying with time.
Important issues include:
16 Polk C, Postow E., Handbook of biological effects of electromagnetic fields, 2nd ed. Boca
Raton: [CRC Press] (1996).
17 The greater danger here is not the strength of the field, but the X-rays produced by electrons of
kinetic energy of 25 keV hitting the phosphor and glass in a color CRT.