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Abstract
This report examines the bases
for developing radiofrequency exposure standards which can be related to the thermogenic properties of electromagnetic fields. A review
of selected biological effects, including dosimetric
data and simulation of human thermodynamic characteristics that are pertinent
to standards development, is presented. Based on the analogy of
thermal-stress standards that have been developed for hot industrial
environments, limits on increases of body temperature are proposed as
criteria for limiting exposure to radiofrequency fields, i.e., occupational
exposures involving deep heating of the whole body should not increase core
temperature in excess of 1 °C.
Since energy deposition from exposure to some RF fields is likely to be
non-uniform and may be high in tissues that are not adapted to high rates of
absorption or dissipation of thermalizing energy,
means are needed to adjust focal thermal loading against the whole-body
averages. A limit on core temperature is inadequate when focal elevations of
temperature are close to the limits for protein denaturation,
as may well occur even though the core temperature may rise less than 1°C. Safety limits for the
general population are also discussed and here the permissible thermal load
should be low enough to cause no more than an insignificant increase in core
temperature. Areas needing further research to reduce the uncertainties in
developing safe exposure limits for man are delineated. Even in highly
adverse environmental conditions the gross thermal load and consequential
heat stress from exposure to radiofrequency fields at the 10 m/cm² level will
be small compared with that generated by any physical effort. On the basis of
available data, it is concluded that the safe value for continuous exposure
to 10 mid/cm², widely used in Western countries, appears to provide an
adequate margin of safety for both occupational and environmental exposure
for frequencies above about 1 GHz. This limit may well be too high (perhaps
by an order of magnitude) for some frequencies below 1 GHz where body
resonances cause a significant increase in energy deposition and where local
temperature rises occur. At the same time the present averaging period of 0.1
h seems unjustifiably short.
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