9.1.4 Biological Research And The Development Of RF Dosimetry
Biological research expanded substantially during the 1960s and 1970s. Laboratory studies investigated how biological responses varied with frequency, exposure duration, modulation, body geometry, environmental conditions, and the spatial distribution of absorbed energy.
These studies demonstrated that exposure could not be characterized adequately by transmitter power or external field strength alone. Researchers needed to determine the internal electric fields and absorbed-energy distributions produced within biological tissue. This requirement encouraged the development of RF dosimetry: the measurement or calculation of internal fields, energy absorption, and related thermal quantities.
Early dosimetric models represented the body using relatively simple shapes and homogeneous materials. Advances in computing and measurement techniques later allowed anatomically realistic models, tissue-specific electrical properties, numerical electromagnetic methods, physical phantoms, and specialized probes to be used.
RF dosimetry made it possible to distinguish between whole-body and localized absorption and to examine how absorption varies with frequency, posture, body dimensions, and exposure configuration. It therefore provided the connection between an external RF field and the biological quantities upon which modern exposure restrictions are based.
9.1.5 From External Fields To Internal Dosimetric Quantities
Specific absorption rate (SAR) became a central dosimetric quantity for describing the rate at which RF energy is absorbed per unit mass of tissue. It is expressed in watts per kilogram and may be averaged over the whole body or over a specified mass of tissue.
The adoption of SAR represented an important development in RF protection. It allowed exposure restrictions to be related more directly to internal energy absorption and temperature rise while accounting for the frequency-dependent interaction between RF fields and the body.
Modern standards nevertheless use several internal quantities rather than relying on SAR alone. At the lower end of the RF range, induced electric fields are relevant to protection against electrostimulation. SAR is used where RF energy penetrates sufficiently deeply for absorption to be meaningfully averaged within a tissue mass. At frequencies above 6 GHz, absorption becomes increasingly confined to superficial tissues, and absorbed power density is used for localized exposure.
Corresponding external quantities remain essential for practical assessment. Incident electric-field strength, incident magnetic-field strength, and incident power density may be used as reference levels where their relationship to the applicable basic restriction is sufficiently reliable. For brief exposures, energy-based quantities may also be required. These distinctions are examined in greater detail later in this chapter.