McIntosh RL, Anderson V. SAR versus S(inc): What is the appropriate RF exposure metric in the range 1-10 GHz? Part II: Using complex human body models. Bioelectromagnetics. March 30, 2010 Ahead of print.
The two fundamental metrics (or basic restrictions) commonly specified for limiting human exposure to radiofrequency (RF) electromagnetic fields are the specific energy absorption rate (SAR) and incident power flux density (Sinc). The specific absorption rate (SAR) is expressed as W/kg of tissue mass and can be calculated at any point in the exposed tissue from knowledge of the magnitude of the internal electric field strength. While the power flux density (Sinc) is expressed as W/m2 and is a measure of the RF power transmitted through a unit area. As the RF exposure frequency increases, the depth of penetration (i.e., where point (unaveraged) SAR is attenuated by 95%) reduces from approximately 55mm at 1 GHz to 4mm at 10 GHz. This article is the second part of a study that present modeling data for specifying the appropriate crossover frequency as the designated basic restriction for protecting against radiofrequency electromagnetic heating effects in the 1–10 GHz range. This crossover frequency is the point at which incident power flux density (Sinc) replaces the peak 10 g averaged value of the specific energy absorption rate (SAR). The hypothesis in this study is to examine whether the mass-averaged SAR value is appropriate at the lower range, and whether the incident power flux density, Sinc, is suitable at the upper end of the range.
The work presented in this article was performed using the results of another study in which, multiplanar layered models of human tissue were used instead of complex human body models. After considering several sites on the body and the range of tissue thicknesses at these sites across the population, over 3,200 models were developed for each of 10 frequencies (1–10 GHz), enabling RF exposure effects to be examined for a large sample of the human population. The two realistic human body model types were a full body model representing an adult, ‘‘NORMAN,’’ and a head model representing a 12-year-old child. SAR and Sinc were calculated using the commercially available finite-difference time-domain (FDTD) software, XFDTD. The use of anatomically complex models of the human body in this study will provide insight to be obtained on different sources of variation between localized SAR or Sinc and peak tissue temperature rise in different part of the body.
The results suggest neither peak 10 g SAR nor Sinc are consistently accurate at predicting the peak temperature rise. Certainly, the factors that play a major role in modifying the thermal profile of the body (such as significantly differing tissue properties of blood perfusion and thermal conductivity) add to the complexity of relating temperature change with incident and internal power levels. Temperature change for the ICNIRP public 10 g SAR limit is approximately 12 times higher than for the ICNIRP Sinc limit in the 6–10 GHz range.
The authors recommend that the breakpoint between 10 g SAR and Sinc exposure limits is set at 6 GHz. They also recommend that the methodology for determining 10 g SAR values near surface