## 2 Materials and Methods
### Numerical Simulations  
Numerical analysis was performed using the FDTD method [33] implemented in XFdtd (Remcom, State College, PA, USA). An automatic non-uniform mesh was chosen with a minimum size of 0.08 mm and a maximum size of 1.071 mm. The maximum cell size was automatically chosen to be equal to one-tenth of the free space wavelength in compliance with the well-known rule to suppress the numerical dispersion error in FDTD simulations [34; 35]. Using the intelligent meshing option, we set the minimum number of grid cells per wavelength to be equal to 15. The material properties of dielectric materials, such as human tissues, have been used to determine the wavelength. As a result of this automatic meshing, the human model's discretization had a resolution of 0.15 mm in the region of interest. The adopted resolution enabled obtaining good computational accuracy while also considering the penetration depth of radiation in the skin, which is about 1 mm at 28 GHz. The convergence of the results was reached with a threshold of 30 dB. Due to the computational limit, to reduce the total mesh size, the non-exposed part of the human models was not included in the mesh. Specifically, for all the human models, in the phone call scenario, the mesh extended from the crown of the head to the jugular notch, in the  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Phone Call**} & \multicolumn{2}{c}{**Message Writing**} & \multicolumn{2}{c}{**Browsing**} \\ \hline  & d1 (mm) & d2 (mm) & d1 (mm) & d2 (mm) & d1 (mm) & d2 (mm) \\ \hline [PERSON] & 2 & 124 & 157 & 346 & 207 & 642 \\ \hline [PERSON] & 2 & 121 & 157 & 317 & 207 & 596 \\ \hline Billie & 2 & 110 & 157 & 281 & 207 & 541 \\ \hline [PERSON] & 2 & 108 & 157 & 241 & 207 & 457 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Human models-antenna distances for each exposure scenario.  
Figure 2: Exposure scenarios: (**a**) phone call, (**b**) message writing, (**c**) browsing.  
message writing scenario from the crown of the head to the line of last ribs, and in the browsing scenario from the jugular notch to the genitals (see Figure 2).  
We used perfect matched layers (seven layers) [33] with 20 cells of free space surrounding the voxel domain as boundary conditions.  
All of the simulations were performed using a workstation Intel Core i5 @2.67 GHz, RAM 4 GB (Intel Corporation Italia spa, Milan, Italy), equipped with a graphical card NVidia Geforce GTX 750 GPU (NVIDIA Corporation, Santa Clara, CA, USA) and Windows 10 (Microsoft Italia, Milan, Italy) operating system.  
Four available realistic human models belonging to the Virtual Population [34] were used in our simulations: [PERSON] (adult male, age 34, height 1.77 m, mass 70.2 kg, BMI 22.4 kg/m\({}^{2}\), 146 tissues), [PERSON] (adult female, age 26, height 1.63 m, mass, 57.3 kg, BMI 21.5 kg/m\({}^{2}\), 76 tissues), [PERSON] (pre-teenager female, age 11, height 1.49 m, mass 34.0 kg, BMI 15.4 kg/m\({}^{2}\), 112 tissues), and [PERSON] (male child, age 6, height 1.15 m, mass 18.6 kg, BMI 14.1 kg/m\({}^{2}\), 76 tissues).  
All these models were based on high-resolution magnetic resonance (MR) images of healthy volunteers.  
Electric properties (dielectric constant and conductivity) of the tissues included in each model were calculated at 28 GHz according to the Cole-Cole method [35]. All the models were in a standing position with arms by their side.  
For each human model, all the three exposure scenarios described previously were simulated, with a total number of simulations performed of 12.