## 2 Technical Overview
### Safety Guidelines on Electromagnetics  
Safety guidelines outline two different categories of restrictions for what concerns electromagnetic exposures, respectively named: (i) basic restrictions; and (ii) reference levels or maximum permissible exposure. The basic restrictions, that also include a proper safety factor, define the threshold values above which specific biological effects can be expected and thus occur. Effects can be tissue heating from RF energy absorption above 100 kHz or nerve stimulation, from contact currents or induced currents or fields in the body below 10 MHz. The RF energy absorption is limited by the specific absorption rate (SAR) averaged over the full body to prevent thermal stress, and over any 10 or 1 g tissue mass to prevent local thermal injuries. Below 10 MHz, the induced fields are additionally limited to prevent undesirable and hazardous excitation of nerve tissue. In other words, basic restrictions are defined in terms of the appropriate biologically effective quantities, and are set below the threshold for the appropriate critical effects. Since the direct measurement of the basic restrictions is technically difficult or often impractical in many daily situations, reference levels are derived from basic restrictions and they are derived and reported in terms of a directly measurable parameter of theexternal electromagnetic exposures; such a procedure makes the exposure guidelines more practical and versatile. Indeed, while the basic restrictions are closely related to the biological effect, the reference levels are directly related to the emission levels of the different electromagnetic sources and are thus easier to be evaluated [10,11].  
Different scientific committees from national and international organizations develop the safety guidelines for electromagnetic field exposure. The most relevant of these organizations are the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the Standards Coordinating Committee 28 (SCC28) of the IEEE. These committees constantly monitor scientific literature and reports in order to tune exposure limits based on the effects that the scientific community has established. These limits, revised every few years, are issued in guidelines that are also revised every few years [12, 13, 14].  
In the European Union, the 1998 ICNIRP Guidelines yield the basis for the national regulators to endorse appropriate legislation. In 2004, the European Union distributed a directive with the aim to guard workers from possible dangerous effects caused by electromagnetic fields [15]. The United States considers parts of both the guidelines of the National Council on Radiation Protection [16] and the 1992 issue of the guidelines of the IEEE C95.1 standard [14]. In Switzerland, national regulation uses ICNIRP Guidelines 1998, and was the first country to introduce precautionary limits below the ICNIRP guidelines for fixed installations in sensitive areas (e.g., schools, living areas, hospitals) which are 20 dB [17]. In the Commonwealth of Independent States (CIS), the exposure to electromagnetic fields is limited according to the Sanitary and Epidemiologic Rules and Regulations (the SanPiN [18,19]).