Systematic Derivation of Safety Limits for Time Varying 5G Radiofrequency Exposure Based on Analytical Models and Thermal Dose

Esra Neufeld and Niels Kuster, Health Physics, December 2018, Volume 115, Issue 6, pp. 705–711; Early View online 21 September 2018, doi: 10.1097/HP.0000000000000930

Very broadband wireless devices (bandwidth >1 GHz) operating at frequencies >10 GHz may transmit data in bursts of a few milliseconds to seconds. Even though the time- and area-averaged power-density values remain within the acceptable safety limits for continuous exposure, these bursts may lead to short temperature spikes in the skin of exposed people. In this paper, a novel analytical approach to pulsed heating is developed and applied to assess the temperature-enhancement and -oscillation magnitude relative to continuous exposure as a function of the pulse fraction, which is related to the peak-to-average (PAR) exposure ratio. This has been analyzed for the published perfusion- and diffusion-related thermal time constants observed for plane wave and localized exposures. To manage peak temperatures that considerably exceed an increase of 1 K, the CEM43 tissue damage model, which has a threshold of 600 min (CEM43 dose) for human skin damage based on experimental data, was used to allow large temperature oscillations that remain below the level at which tissue damage occurs. To stay consistent with current safety guidelines, safety factors of 10 for occupational exposure and of 50 for the general public were integrated into the model. The assumptions and limitations of the new approach, such as the thermal- and tissue-damage-models employed, homogeneous skin, and consideration of localized exposure by means of a modified time-constant, are discussed in detail.

The scientific and technical impact of the study can be summarized as:

• Transient exposure with high PAR as permitted by current safety guidelines can lead to large temperature oscillations due to the very localized energy deposition, with peak temperature increases in the skin reaching tens of degrees, thus exceeding exposure thresholds for tissue damage
• A novel analytical approach for assessment of the magnitude of the temperature oscillations during transient exposures in the quasi-steady state regime has been developed and applied to plane-wave and localized exposures
• Application of thermal dose considerations and limits determined from experimental data on thermal skin damage allow derivation of maximal averaging times that are compliant with specific safety criteria
• The proposed methodology can also be applied to evaluate the consistency of safety guidelines or safety factors of specific communication systems operating at frequencies >10 GHz

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