9.3.4 Averaging, Brief Exposure, And Multiple Frequencies

An ICNIRP restriction is defined together with its spatial and temporal averaging or integrating conditions. These conditions are part of the restriction rather than optional measurement details, and every applicable condition must be satisfied.

Whole-body average SAR and the corresponding whole-body reference-level quantities are averaged over 30 minutes. This represents the time required for core body temperature to approach a steady state under thermal loading.

Localized SAR, absorbed power density, and their corresponding localized reference-level quantities are averaged over 6 minutes. This shorter interval approximates the thermal time constant for localized exposure and limits temperature rise within a restricted tissue region.

Above 400 MHz, additional restrictions apply to brief localized exposure. Specific energy absorption is used above 400 MHz and up to 6 GHz, while absorbed energy density is used above 6 GHz. The permitted total energy is a function of the integration time, and every interval greater than zero and shorter than 6 minutes must comply, including individual pulses, groups of pulses, portions of pulse trains, and combined pulsed and non-pulsed exposure. No additional brief-exposure restriction is specified at or below 400 MHz.

Where several frequencies are present simultaneously, their contributions cannot be assessed independently and then ignored. ICNIRP provides summation rules for combining the relevant fractions of the limits. Contributions associated with nerve stimulation and heating are assessed separately, using the spatial and temporal conditions that apply to each quantity.