Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> V. ANALYSIS OF SAR DATA FROM COMMERCIAL WEARABLE COMMUNICATIONS DEVICES <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> BIB001 </s> Human Electromagnetic Field Exposure in Wearable Communications: A Review <s> V. ANALYSIS OF SAR DATA FROM COMMERCIAL WEARABLE COMMUNICATIONS DEVICES <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> BIB002
We understand the widespread curiosity on the safety of using the commercial wearable communications devices. Motivated from the interest, we provide an extensive investigation on the SAR levels of the wearable communications devices that are in the market, which distinguishes this work from prior surveys  [16] BIB001 . The SAR values that are discussed in this section are either (i) revealed in each product's data sheet or (ii) in case not disclosed, calculated based on the analysis and measurement methods discussed in Sections III and IV, respectively. A. Apple AirPod, Fitbit, and Snap Wearable Video Camera Table I lists the SARs for Apple AirPod, Fitbit, and Snap wearable video camera. Notice that for accurate understanding, along with the SAR, other technical details are also providedi.e., the carrier frequency, physical spacing from the skin, conducted power, and duty cycle of a signal. It is also noteworthy that the amount of mass for calculation of a SAR differs according to the body part on which the device is worn-i.e., averaged over 1 g and 10 g for head and body, respectively. For instance, the first two rows of Table I indicate Apple AirPod A2031. The SAR averaging mass for SAR evaluation is 1 g and 10 g when worn (i) next to mouth and (ii) on body, respectfully. Devices that are designed to be worn on the wrist may operate in speaker mode for voice communication, with the device worn on the wrist and positioned next to the mouth. When next-to-mouth SAR evaluation is required, the device is positioned at 10 mm from a flat phantom filled with head tissue-equivalent medium. The device is evaluated with wrist bands strapped together to represent normal use conditions. Devices that are designed or intended for use on extremities or mainly operated in extremity only exposure conditions; i.e., hands, wrists, feet, and ankles, may require extremity SAR evaluation. In this case, the device is evaluated with the back of the device touching the flat phantom, which is filled with a body tissue-equivalent medium. The devices that are listed in Table I operate based on two wireless technologies-namely, IEEE 802.11b and Bluetooth. The carrier frequency is a key factor that dominates the level of SAR BIB002 . The rationale is inferred from Eq. (3). A wave with a higher frequency penetrates shallower to the human skin, which in turn lowers δ and hence increases the SAR. Since 802.11b and Bluetooth operate at the same frequency of 2.4 GHz, we defer the discussion on the impact of carrier frequency for now. AirPod shows higher SARs for operating based on Bluetooth. According to Table I , the SAR for AirPods is 0.581 W/kg for the left earbud and 0.501 W/kg for the right ear. That makes for a combined 1.082 W/kg when worn in both ears. In comparison, the SAR for an iPhone XS is 1.19 W/kg, or just 10% more than that of the AirPods  . During testing, Apple Watch radios were set to their highest transmission levels and placed in positions that simulate a normal use with 10 mm separation, for a use against the head, and no separation, for one against the wrist  . Looking across the Apple Watch Series 4 models, the highest SARs are found as 0.37 W/kg, 0.17 W/kg, and 0.13 W/kg, in cellular transmission, Wi-Fi, and Bluetooth, respectively. The model also supports simultaneous transmission (i.e., cellular plus Wi-Fi), which yields the SAR of 0.50 W/kg. Notice that all of these four values fulfill the current safety guidelines. (Recall 1.6 W/Kg and 2.0 W/Kg by the FCC  and ICNIRP  , respectively.) As an example, Figs. 2a and 2b illustrate the SAR of Apple A1889 and A1891 versus the carrier frequency. Commonly from the two figures, one can observe a general pattern showing that the SAR increases as the frequency gets higher. The rationale can be recalled from the definition of a SAR, which has the 'penetration depth' in its denominator: a higher carrier frequency yields a smaller penetration depth. In other words, although the EMF with a higher frequency travels to a shallower point into human skin, a greater amount of EMF energy is absorbed.