Biological Effects - RF/Microwave Interaction with Biological Tissues - page 128

pacemaker with respect to hand-held phones. Recent pacemaker models are
better protected against interference risk. The conclusion is that it is desirable
for implanting physicians to use only pacemakers with immunity against
mobile phones as guaranteed by the manufacturers.
Electromagnetic interference has also been evaluated from a global system
for mobile communication telephones with a model of a hearing aid used in
the ear canal [110]. The electric and magnetic fields were calculated in the ear
canal using a finite-difference method for two models of the ear and three
positions of the telephone. One result was that, for a given model of telephone,
the level of EMI-induced sound pressure levels vary by more than 40 dB,
depending on the design of the hearing aid.

3.11 RADIATION HAZARDS AND EXPOSURE STANDARDS

Although equipment that utilizes or emits EM energy provides benefits to
mankind, it also constitutes hazards to the individual through uncontrolled and
excessive emissions. There is a need to set limits on the amount of exposure
to radiant energies individuals can accept with safety. These limits are still
subject to change. It can be expected that adequate limits should be frequency
dependent. At higher frequencies, indeed, the depth of penetration in bio-
logical tissue is limited to the superficial layers. Hence, any concern about
potential hazards should focus on the tissues that are both superficial and
biologically sensitive. Ocular tissue meets both of these criteria because of its
unique structure, location, biochemistry, physiology, and sensitivity to various
physical agents. Limits can be based on body-averaged SAR, specifying mostly
SAR in 1 or 10 g tissue. Of course, local SAR cannot be measured, in the
human brain for instance, and there is a need for reliable evaluation of expo-
sure risks. Investigation of human exposure is difficult because the human
body possesses a complex geometry and heterogeneous tissues.

3.11.1 Standards and Recommendations

The limits have been quite different from one country to another in the
past. In 1975 already, there was a ratio of 1000 between the U.S. standard
(10 mW cm -2 ) and the USSR standard (10mWcm -2 for an exposure time
beyond 2 h a day). This has been the subject of a number of investigations [111,
112]. Such a huge difference in representing unhealthy effects comes from the
fact that the USSR standards took into account not only thermal but also
isothermal effects. These have been described in Section 3.8. The controversy
has not ceased because, as said above, accepting the possibility of nonthermal
effects introduces an extra safety factor of at least 100. It is sometimes said
that the Soviet/Russian standard for microwaves is the single standard that
does not require a modification, because it already accounts for the existence
of isothermal effects, as a result of a large experimental program made in the