7.6.3 Exposure Limits And Dependencies
7.6.3.1 Occupational And General-Public Exposure
As explained in Chapter 6, occupational restrictions apply only to informed adults working under controlled conditions with suitable training, supervision, and the ability to apply RF risk controls. A person is not classified as occupationally exposed merely because an exposure occurs at work.
General-public restrictions apply where people may be unaware of the source, unable to control their exposure, or not covered by an appropriate occupational safety program. They also accommodate the diversity of ages and health conditions in the community and therefore incorporate more conservative reduction factors.
Neither exposure category should be assumed to protect the operation of implanted medical devices. Device-specific electromagnetic compatibility requirements and clinical or manufacturer advice apply separately.
7.6.3.2 Frequency-Dependent Exposure Limits
Exposure restrictions vary with frequency because the coupling of RF fields into the body, the spatial distribution of absorption, and the relevant interaction mechanism all vary with frequency. The transitions are gradual rather than abrupt biological boundaries.
Induced electric-field restrictions apply from 100 kHz to 10 MHz to control electrostimulation. Heating-based restrictions overlap this range and continue throughout the RF spectrum. Localized SAR is used up to 6 GHz where mass-based absorption remains appropriate; above 6 GHz, absorbed power density is used for localized surface absorption. Whole-body thermal loading and brief localized exposures require their own quantities and averaging provisions.
7.6.3.3 Time-Dependent Exposure Limits
The thermal exposure restrictions operate on three related timescales, as explained in Section 7.3 and illustrated in Figure 7.3. Whole-body quantities are averaged over 30 minutes, localized power-absorption quantities over 6 minutes, and brief localized exposure above 400 MHz is additionally controlled through energy integrated over every applicable interval shorter than 6 minutes. These requirements apply simultaneously where relevant.
The 30-minute and 6-minute intervals principally reflect thermal response. Restrictions protecting against nerve stimulation are specified through induced electric-field quantities and their applicable frequency, spatial, and waveform conditions; they should not be described as using the same thermal averaging times.
The prescribed averaging or integrating interval defines the exposure quantity used for compliance. It is not necessarily the amount of time for which an instrument must remain at one measurement point. A shorter measurement may provide a valid estimate where an applicable assessment method, source-operating information, and suitable extrapolation demonstrate the required time-averaged quantity. Practical measurement considerations are discussed in Chapter 8.