Biological Effects - RF/Microwave Interaction with Biological Tissues - page 119

boundary conditions. It must be noticed, however, that temperature is not
an EM parameter: It is a consequence of energy absorption at RFs and
microwave frequencies. This is illustrated for instance by Eqns. (2.19) and
(2.21): The SAR is proportional to absorption losses, and there is a tempera-
ture elevation when the SAR is positive; if there is absorption, there is a tem-
perature elevation. Hence, from a phenomenological point of view, it should
be emphasized that EM theory does not possess the mathematical tool for
imposing a constant temperature. As a consequence, it cannot investigate the
possibility of nonthermal effects: When using EM theory, only thermal effects
can be evaluated. In other words, using exclusively EM tools offers no chance
to display nonthermal effects.
If EM energy is not an adequate tool for investigating nonthermal RF and
microwave effects, then what is the good tool?
Obviously, other considerations have to be taken into account in which tem-
perature is a parameter. This of course leads to thermodynamics. Contrary
to EM theory, thermodynamics has no connection with the structure of the
system. It considers the system as a “black box,” with four parameters: volume,
pressure, temperature, and entropy. Hence, thermodynamics is able to inves-
tigate effects at constant volume or constant pressure or also constant tem-
perature. In other words, to investigate the possibility of isothermal effects,
electromagnetics and thermodynamics have to be used jointly, combining
Poynting's theorem with basic thermodynamic equations. Hence, one has to
investigate to what extent energy and entropy can be used in combination to
evaluate isothermal effects. This of course seriously complicates the study.
On the other hand, investigating the possibility of isothermal effects does
not preclude the attention to be paid to “nonthermal” effects, which should
probably better be termed microthermal effects [11]. The question is:
Can extremely weak EM exposure have large biological effects, and how is
this possible? One then has to consider the possibility of trigger action by
microwaves.
The study of nonthermal/microthermal effects, on the one hand, and
isothermal effects, on the other, will be separated and handled in Sections 3.8.1
and 3.8.2, respectively. It should be reminded that biomolecules, like proteins
for instance, have remarkable dielectric properties, different in many ways
from more ordinary, small molecules.

3.8.1 Microwaves as a Trigger

It must be emphasized that an appropriate discussion on experimental results
with regard to microthermal effects must frequently consider thermal effects
as well. Energy transfer from the radiation to the material produces thermal
effects. These vary slowly with frequency and are largely governed by dielec-
tric loss. Within reasonable limits this loss is proportional to the intensity of
the radiation. Microthermal effects, on the other hand, occur in certain fre-
quency regions only and usually exhibit saturation at rather low intensity. As