Relationship between power density and surface temperature elevation for human skin exposure to electromagnetic waves with oblique incidence angle from 6 GHz to 1 THz

Abstract

This study presents an investigation of human skin exposure to obliquely incident electromagnetic waves at frequencies from 6 GHz to 1 THz. We aim to clarify the relationship between the power density and the skin surface temperature elevation under various exposure conditions. A Monte Carlo simulation was conducted to assess the transmittance and surface temperature elevation considering the variation of skin tissue thickness. For the case of TM wave injection, transmittance increases with increasing incidence angle from the normal incidence because of the Brewster effect. The normal incidence is confirmed as the worst-case exposure condition when the incident power density is defined in an area normal to the propagation direction. In addition, we investigated the power density required to obtain the equivalent temperature elevation over the skin surface. The analysis shows that the incident power density defined in the direction normal to the skin surface may underestimate the temperature elevation when TM waves are incident over the normal incidence up to the maximum transmittance angle. Our results also show that the power density inside the skin surface strongly correlates with the surface temperature elevation but less dependent on the frequency and independent of the oblique incidence angle and polarization. The findings of this study are expected to be valuable for discussing how to use the different definitions of power density based on dosimetric characteristics as measures in safety guidelines to protect humans from excessive temperature elevation by millimeter and submillimeter-wave exposure.

Skin Temperature Elevation for Incident Power Densities From Dipole Arrays at 28 Ghz

“…For conservative compliance, the IPD in the propagation beam should be decreased by a factor of up to two (an inverse of cosine at 60 • ) to obtain a conservative heating factor. At beam directions lower than 60 • , the power transmittance decreased and increased by 40% for TE-and TM-like polarizations, respectively [26]. In contrast, the heating factor S n decreased and increased by 30% for TE-and TM-like polarizations, respectively.…”

“…Li, et al [26] and Samaras and Kuster [22] considered a two-dimensional infinite slab and subsequently reported a larger heating factor at a larger incidence angle for TM-like exposure. This tendency was confirmed in a threedimensional analysis, as shown in Figs.…”

“…A two-dimensional analytical solution suggests that almost all energy is deposited for transverse-magnetic (TM) mode exposures at an angle of incidence equal to Brewster's angle [25]; for biological tissues, this angle ranges from 65 to 80 • [22], [26]. Therefore, the power deposited in a human would be enhanced compared with the power at normal incidence [22], [26].…”

“…A two-dimensional analytical solution suggests that almost all energy is deposited for transverse-magnetic (TM) mode exposures at an angle of incidence equal to Brewster's angle [25]; for biological tissues, this angle ranges from 65 to 80 • [22], [26]. Therefore, the power deposited in a human would be enhanced compared with the power at normal incidence [22], [26]. However, only a few studies [19], [27], [28] have computed the relationship between IPD and surface temperature elevation in 3-D local exposure scenarios but not for different incident angles, although this relates better to the rationale for relating the exposure guidelines and product safety standards.…”

Impact of Textile on Electromagnetic Power and Heating in Near-Surface Tissues at 26 GHz and 60 GHz

“…A normally impinging plane wave was considered as a source. Note that an oblique incidence would result in a lower PD and temperature elevation [29]. Aiming at the worst case condition, the temperature elevation was investigated at steady-state.…”