Discussion on Spatial and Time Averaging Restrictions Within the Electromagnetic Exposure Safety Framework in the Frequency Range Above 6 GHz for Pulsed and Localized Exposures

Esra Neufeld, Theodoros Samaras, Niels Kuster, Bioelectromagnetics, Volume 41, Issue 2, February 2020, pp. 164–168, online 30 December 2019, doi:10.1002/bem.22244

Both the current and newly proposed safety guidelines for local human exposure to millimeter‐wave frequencies aim at restricting the maximum local temperature increase in the skin to prevent tissue damage. In this study, we show that the application of the current and proposed limits for pulsed fields can lead to a temperature increase of 10°C for short pulses at frequencies between 6 and 30 GHz. We also show that the proposed averaging area of 4 cm2, which is greatly reduced compared to the current limits, does not prevent high‐temperature increases in the case of narrow beams. A realistic Gaussian beam profile with a 1 mm radius can result in a temperature increase about 10 times greater than the 0.4°C increase that the same averaged power density (PD) would produce for a plane wave. In the case of pulsed narrow beams, the values for the time and spatially averaged PD allowed by the proposed new guidelines could result in extreme temperature increases.

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

  • For very short pulses, pulse‐duration‐independent limits imposed on transmitted energy density (fluence) alone cannot preclude the induction of high‐temperature increases in the skin; pulse‐duration‐dependent limits should be applied for pulses of <1 s, likely as well for frequencies <30 GHz
  • To ensure the safety of new devices based on emerging technologies, these assumptions should be explicitly stated in the guidelines, or the limits should be adapted to be intrinsically safe
  • For spatial averaging, an averaging area of <4 cm2 should be used; further from the antenna, the increased beam radius allows the averaging area to be increased
  • Duration‐independent limits on the fluence of pulses should be replaced by duration‐dependent fluence limits for pulses or by limits on the peak exposure times, and limits should be set after taking narrow‐beam exposures into consideration; these limits will depend on the chosen spatial and temporal averaging schemes and the maximum temperature increase deemed acceptable