Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> C. Contribution of This Paper <s> This work presents a comparison of simulated and measured Specific Absorption Rate (SAR) levels of wearable antennas intended for voice communication for security applications. The compared antennas operate at UHF frequencies (360 – 420 MHz) and can be used for TETRA or TETRAPOL communication systems. Four different antenna prototypes have been considered, analyzed and SAR values simulated and measured. Two of the compared antennas are Planar Inverted F Antennas (PIFAs) and they are proposed by the authors, while the other two are dipole based antennas. One of the dipole antennas is commercially available, while the second one is reported in the literature. The antennas are modeled and simulated with a commercial software, while the measurements are performed with a special equipment intended for SAR measurements. <s> BIB001 </s> Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> C. Contribution of This Paper <s> Abstract At ultra-high fields, the assessment of radiofrequency (RF) safety presents several new challenges compared to low-field systems. Multi-channel RF transmit coils in combination with parallel transmit techniques produce time-dependent and spatially varying power loss densities in the tissue. Further, in ultra-high-field systems, localized field effects can be more pronounced due to a transition from the quasi stationary to the electromagnetic field regime. Consequently, local information on the RF field is required for reliable RF safety assessment as well as for monitoring of RF exposure during MR examinations. Numerical RF and thermal simulations for realistic exposure scenarios with anatomical body models are currently the only practical way to obtain the requisite local information on magnetic and electric field distributions as well as tissue temperature. In this article, safety regulations and the fundamental characteristics of RF field distributions in ultra-high-field systems are reviewed. Numerical methods for computation of RF fields as well as typical requirements for the analysis of realistic multi-channel RF exposure scenarios including anatomical body models are highlighted. In recent years, computation of the local tissue temperature has become of increasing interest, since a more accurate safety assessment is expected because temperature is directly related to tissue damage. Regarding thermal simulation, bio-heat transfer models and approaches for taking into account the physiological response of the human body to RF exposure are discussed. In addition, suitable methods are presented to validate calculated RF and thermal results with measurements. Finally, the concept of generalized simulation-based specific absorption rate (SAR) matrix models is discussed. These models can be incorporated into local SAR monitoring in multi-channel MR systems and allow the design of RF pulses under constraints for local SAR. <s> BIB002 </s> Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> C. Contribution of This Paper <s> Numerous antenna design approaches for wearable applications have been investigated in the literature. As on-body wearable communications become more ingrained in our daily activities, the necessity to investigate the impacts of these networks burgeons as a major requirement. In this study, we investigate the human electromagnetic field (EMF) exposure effect from on-body wearable devices at 2.4 GHz and 60 GHz, and compare the results to illustrate how the technology evolution to higher frequencies from wearable communications can impact our health. Our results suggest the average specific absorption rate (SAR) at 60 GHz can exceed the regulatory guidelines within a certain separation distance between a wearable device and the human skin surface. To the best of authors' knowledge, this is the first work that explicitly compares the human EMF exposure at different operating frequencies for on-body wearable communications, which provides a direct roadmap in design of wearable devices to be deployed in the Internet of Battlefield Things (IoBT). <s> BIB003
To this end, this paper provides an extensive overview on the human EMF exposure in wearable communications, addressing the aforementioned limitations of the prior surveys  BIB001 BIB002 . The key contributions of this paper are highlighted as follows: • Investigation on SAR Levels of Off-the-shelf Wearable Products: The exact SAR levels generated by the popular commercial wearable devices-i.e., Apple AirPod, Fitbit, Snap wearable video camera, and Apple Watch-are investigated. -The values have been found via (i) investigation of a product's data sheet and external articles, and (ii) calculation referring to the standard analysis and measurement frameworks in case a SAR level is not revealed. • State-of-the-art Standards and Measurement Methodologies: This paper features in-depth information on how the SAR values shown in the products are computed. • SAR Analysis According to Spectrum Band: An explicit comparison of SAR at different operating frequencies is provided-i.e., Wi-Fi, Universal Mobile Telecommunications System (UMTS), Long-Term Evolution (LTE), etc. -Some products support multiple spectrum bandse.g., Apple Watch series, as shall be discussed in Section V. The contribution of this paper is highlighted in such a case: we lead the customers to precise understanding on differentiated impacts according to carrier frequencies. • SAR Evaluation of Wearables Operating in mmW Spectrum: Section VI compares the SAR caused by a wearable device in 2.4 GHz and 60 GHz. -Wearable communications at 60 GHz adopt a more directional radiation pattern to (i) overcome the excessive path loss and (ii) achieve higher data rates at the user end compared to the previous-generation technologies BIB003 . Such directivity elevates the SAR on human skin. II. STATE-OF-THE-ART This paper starts the survey with review on the current understanding in the literature. This section can suggest a taxonomy of the current study on human EMF exposure.