On the averaging area for incident power density for human exposure limits at frequencies over 6 GHz

Abstract

Incident power density is used as the dosimetric quantity to specify the restrictions on human exposure to electromagnetic fields at frequencies above 3 or 10 GHz in order to prevent excessive temperature elevation at the body surface. However, international standards and guidelines have different definitions for the size of the area over which the power density should be averaged. This study reports computational evaluation of the relationship between the size of the area over which incident power density is averaged and the local peak temperature elevation in a multi-layer model simulating a human body. Three wave sources are considered in the frequency range from 3 to 300 GHz: an ideal beam, a half-wave dipole antenna, and an antenna array. 1D analysis shows that averaging area of 20 mm × 20 mm is a good measure to correlate with the local peak temperature elevation when the field distribution is nearly uniform in that area. The averaging area is different from recommendations in the current international standards/guidelines, and not dependent on the frequency. For a non-uniform field distribution, such as a beam with small diameter, the incident power density should be compensated by multiplying a factor that can be derived from the ratio of the effective beam area to the averaging area. The findings in the present study suggest that the relationship obtained using the 1D approximation is applicable for deriving the relationship between the incident power density and the local temperature elevation.

Theoretical and numerical assessment of maximally allowable power‐density averaging area for conservative electromagnetic exposure assessment above 6 GHz

“…These conditions could be relaxed, should a temperature threshold >1 K be selected or for larger distances from the source. The suggested areas are smaller but comparable to the 4 cm 2 previously suggested by Hashimoto et al [] based on computational modeling of localized exposure‐induced heating.…”

“…For this study, the frequency dependence was approximated as . It should be noted that the transfer factor values obtained for the conservative skin configurations from Christ et al [] are significantly higher than those in Hashimoto et al [] and Sasaki et al [].…”

“…The frequency dependence of the relationship between averaging area and temperature increase has been studied by Hashimoto et al [] by means of simulations of human exposure to beams, dipoles, and dipole arrays. They conclude, based on the quality of the correlation between averaged power and temperature increase, that an averaging area of 4 cm 2 is best suited and that the areas recommended in the current standards are too large.…”

Limitations of Incident Power Density as a Proxy for Induced Electromagnetic Fields

“…These aspects are discussed, e.g. in Hashimoto et al [2017], Laakso et al [2017], Neufeld et al [2018], and Neufeld and Kuster [2018].…”

Comparison of Thermal Response for RF Exposure in Human and Rat Models

“…Foster et al derived an analytic formula to estimate the thermal time constant in a one-dimensional homogeneous model for plane wave exposure [49]. Hashimoto et al proposed the effective area of the SAR pattern as an evaluation index of temperature elevation, which is defined by the area where SAR is larger than 1/ e of the peak value in the averaging area using a multilayer cubic model [51]. The effective SAR volume V eff [cm 3 ] is defined as a metric where SAR is larger than 1/ e of the SAR max to consider both the depth and the spread on the model surface.…”