6.5.2 Dosimetric Quantities
RF dosimetry uses several internal quantities because the physical interaction between an RF field and the body changes with frequency, exposure duration, and the spatial extent of the exposure. Some quantities describe fields induced within tissues, while others describe the rate or total amount of energy absorbed. They must be distinguished from external exposure quantities such as incident electric-field strength, magnetic-field strength, and incident power density.
Induced electric-field strength describes the electric field established within biological tissues. It is expressed in volts per meter (V m⁻¹) and is particularly important at the lower end of the RF range, where sufficiently strong internal fields can stimulate excitable tissue. ICNIRP specifies induced electric-field restrictions from 100 kHz to 10 MHz. Because heating-based restrictions also apply within this frequency range, both induced-field and energy-absorption quantities may need to be considered.
Specific absorption rate (SAR) is the rate at which RF energy is absorbed per unit mass of tissue and is expressed in watts per kilogram (W kg⁻¹). Whole-body average SAR describes the total power absorbed by the body divided by body mass and is related to the overall thermal load. Localized SAR describes absorption within a limited region and is commonly averaged over a specified mass of tissue, such as 10 g. ICNIRP uses localized SAR for frequencies from 100 kHz to 6 GHz, while whole-body average SAR remains relevant across the RF range. SAR describes absorbed power rather than temperature rise itself; the resulting temperature also depends on exposure duration, heat transfer, blood perfusion, and environmental conditions. For application of the ICNIRP and RPS S-1 restrictions, whole-body average SAR is temporally averaged over 30 minutes, while localized SAR is averaged over 6 minutes.
Specific energy absorption (SA) is the energy absorbed per unit mass of tissue and is expressed in joules per kilogram (J kg⁻¹). It is the time integral of SAR. SA is useful for brief localized exposures, for which the total energy deposited during a short interval may be more informative than power averaged over a longer period. In the ICNIRP and RPS S-1 frameworks, additional SA restrictions apply to brief localized exposures above 400 MHz and up to 6 GHz. The absorbed energy must comply for every applicable integration interval greater than zero and shorter than 6 minutes.
Absorbed power density describes the RF power absorbed per unit area at the body surface and is expressed in watts per square meter (W m⁻²). It is used for localized exposure above 6 GHz, where RF energy is absorbed increasingly close to the surface and an area-based quantity is more appropriate than a mass-based localized SAR. Absorbed power density is not the same as incident power density, because some incident energy is reflected and the remainder is distributed within the tissues. For sustained localized exposure above 6 GHz, absorbed power density is averaged over 6 minutes.
Absorbed energy density is the energy absorbed per unit area and is expressed in joules per square meter (J m⁻²). It is used for brief localized exposure above 6 GHz, for which the absorbed energy must comply with time-dependent restrictions over every applicable interval greater than zero and shorter than 6 minutes.
These quantities describe different aspects of the internal interaction and are not interchangeable. The appropriate quantity depends on frequency, exposure duration, and whether the concern is whole-body or localized exposure. IEEE uses a broadly similar approach, although some terminology and averaging conventions differ. Compliance must therefore be assessed using the definitions and procedures of the applicable guideline or standard. The following section describes how dosimetric quantities are determined in practice.