## 1 Introduction
### The Metrics of Thermal Effects of EMF Exposure and Evaluation  
It is well known that exposure to a high intensity electromagnetic field (EMF) produces biological effects in human tissues, potentially leading to adverse health consequences [20; 21; 22; 23; 24]. There are multiple factors to consider with respect to the biophysical effects of the influence of EMF, as well as their significance with respect to the safety and health of the population. The behaviour of human tissues varies with the frequency of the EMF and bioelectromagnetic interactions may be thermal from tissue being heated by absorbed electromagnetic energy, or more complex non-thermal [21; 23; 25; 26; 27; 28; 29]. The most commonly-accepted measure of direct biophysical thermal effects is the specific energy absorption rate (SAR, in watts per kilogram, W/kg--body-averaged or local SAR values). The relevant SAR limits for the protection against electromagnetic hazards to the general public or workers (at a level five times higher) are considered in the evaluation of exposure to EMF with respect to the minimum safety requirements [22; 23; 24]. General public limits of time averaged SAR are 0.08 W/kg--body-averaged SAR (WBSAR); 2 W/kg--local head and torso SAR and 4 W/kg--local limb SAR, both averaged over 10 g cubic mass (SAR10g). They are applicable for EMF exposure at a frequency from 100 kHz up to 6 GHz (International Commission on Non-Ionizing Radiation Protection (ICNIRP) 2020, IEEE C95.1-2019) or up to 10 GHz (ICNIRP 1998) [22; 23; 24]. In the reported studies, the SAR averaged over the whole head (WHSAR) was determined due to the limitation of the human body model to the head model only. It should be noted that ICNIRP 1998 limits are still the most important because they were referenced in various binding legislative documents, such as for example European Directive 2013/35/EU [30].  
Compliance with SAR limits may be evaluated by compliance with the exposure limits: incident electric field strength (E, in V/m) and incident magnetic field strength (H, in A/m), or alternatively incident power density (S\({}_{\text{inc}}\), in (W/m\({}^{2}\)) [22; 24]. The newest guidelines suggest using E or H for frequencies up to 30 MHz, E and H or S\({}_{\text{inc}}\) for frequencies from 30 MHz up to 2 GHz, and using S\({}_{\text{inc}}\) for higher frequencies [23].  
Wearable IoT devices are often used in close proximity to the human body; in such cases, in accordance with international safety guidelines, a SAR compliance assessment is required, even when the incidental EMF has already been evaluated [23; 24]. The application of the aforementioned higher SAR limits of workers' EMF exposure also triggers the application of specific EMF safety programsincluding various safety measures required by labour law for workers exposed to strong EMF (such as EMF exposure evaluation at a particular workplace and its limitation, safety training of exposed workers, etc.) [30]. Because workers using wearable IoT devices are usually not covered by such EMF safety programs in the work environment and would not be suitably trained to mitigate hazards from EMF exposure, it is appropriate to carry out SAR compliance assessments against general public limits, when it is expected that only weak EMF exposure is found.