Biological Effects - RF/Microwave Interaction with Biological Tissues - page 129

middle of the twentieth century [95]. Hence, the picture about protection is
complicated, especially when remembering that guidelines for limiting EM
field exposure provide protection against known adverse health effects while
biological effects may or may not result in a health effect.
In the present recommendations, two kinds of limitations are considered:
• Basic restrictions that should be always respected.
• Reference levels that could be exceeded when the basic limitations are
not exceeded.
The reason is simple. The basic restrictions are expressed in quantities that are
internal to the body and are not measured, such as SAR. On the other hand,
the reference levels are expressed in quantities that are measured in the
absence of human beings , such as an electric field. There are theories and esti-
mations relating these two sets of quantities.
Only one biological effect of microwaves is well known: heating . Hence, the
present recommendations, being based only on scientific evidence, are limited
to heating processes. As an example, the Scientific Steering Committee of the
European Commission stated in June 1998 that as regards nonthermal expo-
sure to EMF, the available literature does not provide sufficient evidence to
conclude that long-term effects occur as a consequence of EMF exposure.
The conclusion was therefore that any recommendation for exposure limits
regarding nonthermal long-term effects cannot be made at this stage on a sci-
entific basis. Arguments other than scientific might however be considered,
for instance, observations made by medical doctors on public health, published
in the 2002 Freibuerger Appeal [113].
The recommendations are based on one single source: They originally come
from the World Health Organization (WHO), 1993 [2]. Today, they are essen-
tially based on documents produced by the International Commission on Non-
Ionizing Radiation Protection (ICNIRP), with a main document establishing
in 1998 guidelines for limiting exposure to EM fields up to 300 GHz [114].
There are, however, ambiguities in the basic texts. The 1993 WHO document
states that a biological effect is produced from 1 to 4 W kg -1 while calculating
the safety factor from 4 and not from 1 W kg -1 . A further factor of 5 is recom-
mended for the public at large, yielding safety factors of 50 and 12.5, when
starting from 4 and 1 W kg -1 , respectively. Most documents, however, refer to
a safety factor of 50. The same discrepancy is found in the 1998 ICNIRP
document.
The text of the WHO 1993 document [2, p. 21] is based on the known effect
of “increasing the body central temperature by less than 1°C when exposing
healthy adults for 30 minutes to a microwave exposure of 1-4 W/kg.” The
safety factor has to take into account several elements: The temperature
increase should be much less than 1°C; the exposure may be 24 h a day and
not 30 min; adults are not healthy; there are nonadults (children); all children
are not healthy; and there are “unfavorable, thermal, environmental, and pos-