Transmission Coefficient of Power Density into Skin Tissue Between 6 And 300 GHz

Andreas Christ, Theodoros Samaras, Esra Neufeld, Niels Kuster, Radiation Protection Dosimetry 2020, Volume 192, Issue 1, pages 113-118, online 04 December 2020; doi: 10.1093/rpd/ncaa179

The latest electromagnetic safety guidelines define transmitted or epithelial power density as the basic restriction above 6 GHz. In this note, we derive an approximation for a conservative transmission coefficient for quasi plane wave incidence as a function of the frequency for the normal component of the Poynting vector with respect to the evaluation plane or tissue surface |Sz inc| and for its modulus ||Sinc||. The maximum transmission coefficient for the normal component of the Poynting vector Tzmax is 1 independent of tissue composition and frequency. Approximations of Ttotalmax normalized to ||Sinc|| for thin and thick stratum corneum (SC) are provided allowing higher exposures. These approximations allow to conservatively demonstrate compliance with basic restrictions when quasi plane-wave conditions are locally satisfied and enhancement effects of standing waves between source and body can be neglected. The reported results are important to regulators and standardization bodies regarding revisions of compliance requirements and safety guidelines.

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

  • The study identifies the conditions that maximize the power density transmitted into the skin covering the upper frequency limit of the most recent exposure guidelines issued by the ICNIRP and the IEEE
  • For compliance testing against exposure limits, the evaluation of the modulus of the Poynting vector is favorable, as it allows higher incident power densities in the frequency range utilized by the latest 5G standards; in contrast, the evaluation of the normal component of the Poynting vector requires the assumption of a power transmission coefficient equal to 1 due to enhanced absorption at the Brewster angle and for particular tissue layer sequences
  • Future research should be focused on validation and improvement of the skin models particularly regarding frequencies above 100 GHz, as well as human validation studies to determine the statistical distribution of the SC thickness and the confirmation of the dielectric properties of the tissue layers of the skin