## 3 Results
### Brillouin Precursors  
Many countries have not yet introduced the mmW band into what is being called 5G by some and 4.5G by others. In these cases, beamforming is being transmitted at a mid- band around 3.5 GHz. Penetration of the focused energy at 6 GHz is around 8 mm [2] (p. 504). Since the penetration depth increases as the frequency drops, it will be deeper than this at 3.5 GHz. Skin is approximately 2 mm thick, so, under this scenario, all skin layers and near-surface blood vessels will be being heated by all 5G frequencies. The addition of using mmW frequencies for beamforming will reduce the depth of primary penetration, and structures nearest the skin's surface will be preferentially heated in this frequency band. For instance, at 30 GHz, penetration is 0.92 mm (ICNIRP 2020 Table 10, p. 504) [2]. However, the upper layer of the dermis--the papillary dermis-- and the nerve endings immediately in front of the ear are shallower (Table 1). At a depth of approximately 1.15-1.45 mm [10], the skin is rich in papillae, containing an extensive capillary network and neural structures (Table 1). Since penetration is approximately 0.23 mm at 300 GHz [2] (p. 504), even fully operational 5G will impact on upper cutaneous blood vessels and free nerve endings in some parts of the body (Table 1). Most of the transmitted mmW energy is deposited in the tissues within a mm of the skin or eye surface. Using these and higher frequencies for 5G may create pulses that carry some of the energy more deeply into the body. Sommerfeld and Brillouin precursor pulses were eponymously predicted in 1914, and first demonstrated in 1969 [13]. The Sommerfeld pulse occurs immediately after the relevant light pulse's arrival at a dispersive dielectric surface, such as skin. It displays a signal that is a considerably higher frequency than that originally generated, but it is weak and declines rapidly. It is not thought to be of concern in the current scenario. However, directly after the Sommerfeld pulse, another precursor (also known as forerunner) pulse appears, this time starting well below the transmitted frequency but rising very rapidly; if the incident wave has a sudden start, this second precursor covers the full spectrum of frequencies from \(-\infty\) to \(\infty\)[14]. This second pulse is the Brillouin \begin{table}
\begin{tabular}{|c|c|c|} \hline
**Depth of Upper Cutaneous Blood** & **Depth of Upper Cutaneous Blood** & **Approx. Depth of Preamicular Free Nerve** \\
**Vessel from Skin Surface** & **Vessel from Skin Surface** & **Endings from Healthy Skin Surface (mm)** \\
**(mm)** & **Sensitive Skin (mm)** & 0.025  \\ \hline
1.15–1.45 & 0.19–0.23 & 0.025  \\ ([PERSON] et al., 2016, pp. 269–280) [10] & ([PERSON] et al., 2020, pp. 431, 433) [11] & ([PERSON] et al., 2022, p. 1194 table 1) [12] \\ \hline \end{tabular}
\end{table} 
Table 1: Average depth microvasculature and free nerve endings. pulse, which functions thus: the generation of electrical charge through living tissue carries mechanical force. Many membranes have charged surfaces; there are dissociated ionic sites in proteins and DNA, and there are a host of chemical ions in tissue. All these are subject to these forces, so they, in turn, \"radiate a portion of that energy as a propagating electromagnetic field\" [15]. Therefore not only does the energy propagate more deeply than expected, but there is then an increased rate of collisions as they pass on their mechanical energy. This progression raises the total kinetic energy and, thus, the temperature of the medium as a whole. [PERSON] et al. point out that \"deposition of thermal energy in a tissue can offset homeostatic mechanisms in the organism and cause physiological stress; at high temperature, deleterious phenomena such as protein denaturation can occur\" [15]. In 1994, [PERSON] et al. raised the possibility of these electromagnetic transients being generated by modern transmitting devices, with regard to potential use for medical applications but also the creation of unintended adverse biological effects [16].