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7 Light in Biology and Medicine
intensity, namely. the energy arriving at or leaving from a surface perpendicular
to the direction of propagation of the wave per unit time per unit area.
The theoretical description of radio and microwave radiation absorption in
materials begins with Maxwell’s equations. Absorption can be treated by taking
the electric permittivity and magnetic permeability μ as complex numbers. As
magnetic effects are much weaker than electric effects in biological tissue, the
magnetic permeability can be taken real and the same value as in a vacuum.
However, because electric polarizabilities of bound charges and induced electric
currents in tissue can be significant, the real part of the electric permittivity can be
large (causing a slowing down of the wave), and the imaginary part can also be large,
causing heat production. In fact, by applying Maxwell’s equations, the imaginary
part, b , comes from the motion of bound charges on molecules and membranes.
Their coupling to adjacent molecules dissipates their energy, generating heat. We
write
= ε r 0 − ii b ,
(7.24)
where ε r is called the relative permittivity 53 and 0 is the vacuum electric permittivity. 54
The electric field in the tissue induces a current density J that will have
contributions from real currents and bound currents, expressed as
J = (σ + 2πf f b )E.
(7.25)
In this relation, a form of Ohm’s Law, σ is the conductivity of free charges (electrons
and ions) within the tissue, f the frequency of the passing electromagnetic wave,
and E the electric field of the wave.
As yet, there is no indisputable evidence that low frequency radiation to the
public has caused cancer. Static magnetic fields can be detected by some animals,
such as pigeons, who use the Earth’s field to help their navigation. The Earth’s
magnetic field is around 20–70 μT, and is biologically detectable by the effect on
microferrous materials embedded in nervous tissue. Very strong static magnetic
fields, tens of thousands times stronger than the Earth’s field, still do no damage to
human tissue. Strong (several tesla) low frequency magnetic fields can cause effects
to nerve tissue from the induced electric currents in the tissue. These effects can
also be caused by movement of the body tissue through a static magnetic field.
The current near the sinoatrial node of the heart induced by a 5 T field is about
100 mA/m 2 , which is below the cardiac excitation threshold. 55 Oscillating magnetic
53 Relative permittivity is the same as the older term, ‘dielectric constant’.
54 It follows, again from Maxwell’s equations, that the tissue’s index of refraction squared is given
by n 2 ε r .
55 Kinouchi et al., Theoretical analysis of magnetic field interactions with aortic blood flow,
Bioelectromagnetics 17:1, 21–32 (1996).
7 Light in Biology and Medicine
intensity, namely. the energy arriving at or leaving from a surface perpendicular
to the direction of propagation of the wave per unit time per unit area.
The theoretical description of radio and microwave radiation absorption in
materials begins with Maxwell’s equations. Absorption can be treated by taking
the electric permittivity and magnetic permeability μ as complex numbers. As
magnetic effects are much weaker than electric effects in biological tissue, the
magnetic permeability can be taken real and the same value as in a vacuum.
However, because electric polarizabilities of bound charges and induced electric
currents in tissue can be significant, the real part of the electric permittivity can be
large (causing a slowing down of the wave), and the imaginary part can also be large,
causing heat production. In fact, by applying Maxwell’s equations, the imaginary
part, b , comes from the motion of bound charges on molecules and membranes.
Their coupling to adjacent molecules dissipates their energy, generating heat. We
write
= ε r 0 − ii b ,
(7.24)
where ε r is called the relative permittivity 53 and 0 is the vacuum electric permittivity. 54
The electric field in the tissue induces a current density J that will have
contributions from real currents and bound currents, expressed as
J = (σ + 2πf f b )E.
(7.25)
In this relation, a form of Ohm’s Law, σ is the conductivity of free charges (electrons
and ions) within the tissue, f the frequency of the passing electromagnetic wave,
and E the electric field of the wave.
As yet, there is no indisputable evidence that low frequency radiation to the
public has caused cancer. Static magnetic fields can be detected by some animals,
such as pigeons, who use the Earth’s field to help their navigation. The Earth’s
magnetic field is around 20–70 μT, and is biologically detectable by the effect on
microferrous materials embedded in nervous tissue. Very strong static magnetic
fields, tens of thousands times stronger than the Earth’s field, still do no damage to
human tissue. Strong (several tesla) low frequency magnetic fields can cause effects
to nerve tissue from the induced electric currents in the tissue. These effects can
also be caused by movement of the body tissue through a static magnetic field.
The current near the sinoatrial node of the heart induced by a 5 T field is about
100 mA/m 2 , which is below the cardiac excitation threshold. 55 Oscillating magnetic
53 Relative permittivity is the same as the older term, ‘dielectric constant’.
54 It follows, again from Maxwell’s equations, that the tissue’s index of refraction squared is given
by n 2 ε r .
55 Kinouchi et al., Theoretical analysis of magnetic field interactions with aortic blood flow,
Bioelectromagnetics 17:1, 21–32 (1996).