## 2 Materials and Methods  
The EMF measurements were conducted during an experimental campaign of the Telecommunications and Electromagnetic Applications Laboratory of the Department of Electronic Engineering of the Hellenic Mediterranean University, on RF exposure levels estimation in the vicinity of nursery and primary schools at the broad area of the municipality of Chania (population about 110,000), which is located in the northwest part of Crete island in Greece. The time period of the measurement campaign was during May, June, and September 2018. The selective radiation meter SRM-3006 (Narda Safety Test Solutions, Pfullingen, Germany) at 9 kHz-6 GHz was used. The Narda 3501/03 three-axis (isotropic) E-field antenna (frequency range: 27 MHz-3 GHz, dynamic range: 0.2 mV/m-200 V/m) was adapted to the basic unit. The expanded measurement uncertainty for the electric field strength, in conjunction with the SRM basic unit and 1.5 m RF cable, valid for the temperature range of \(+15\)\({}^{\circ}\)C to \(+30\)\({}^{\circ}\)C, is \(+3.3/-5.3\) dB (worst case) according to the Narda datasheet [17]. However, the estimated sampling uncertainty should also be taken into account by summing the squares [18]. Thus, the end result of the calculations for the uncertainty (worst case) is \(+4.5/-6.1\) dB (confidence level 95%). The measurements were conducted around 69 nursery and primary schools, named simply as schools hereafter. Calibration was performed at the beginning of the measurement set, in the vicinity of each school. The points of interest were selected in the vicinity of each school by surveying the area and avoiding proximity to objects as indicated in [19]. Moreover, the sweep method [19] was used in order to determine the most appropriate measurement locations. The radiation meter SRM-3006, in connection with the three-axis E-field antenna, has the capability to perform broadband and frequency-selective measurements simultaneously. The result of any broadband measurement, as given by the radiation meter, is a single value of the electric field strength that corresponds to the entire frequency range (i.e., 27 MHz-3 GHz) and is denoted as \"total\" in the last line of Table 1. The frequency-selective measurements comprise recordings of the electric field strength from 7 distinct frequency bands; the latter are listed in the first column of Table 1.  
A total of 396 in situ audits were performed. Each audit comprised both broadband and frequency-selective recordings, as explained in the previous paragraph. The results were grouped per environment type; 168 audits were conducted around 29 schools in urban areas, whereas the remaining 228 audits were carried out in the vicinity of 40 schools in suburban environments. During each audit, measurements were performed with the radiation meter attached onto a wooden tripod at three different heights: 1.1, 1.5, and 1.7 m [20]. Figure 1 depicts the experimental setup used in this study, i.e., the selective radiation meter with the isotropic antenna, attached onto the wooden tripod at a height of 1.1 m.  
At each height \(i\) (\(i\) = 1, 2, 3 for 1.1, 1.5, and 1.7 m, respectively) and frequency band \(f\), the measured value of the electric field strength (V/m), averaged over 6 min, was recorded and is denoted by \(E_{i,f}^{j}\); the superscript \(j\) is used to identify the specific measurement points; thus, \(j\) = 1, 2,..., 396. Hereafter, the mean value of \(E_{i,f}^{j}\), \(i\) = 1, 2, 3 is taken into consideration, for the frequency band \(f\), i.e., \(E_{f}^{j}=(1/3)\sum_{i=1}^{3}E_{i,f}^{j}\). In the case of exceptionally high values of the measured electric field strength, an effort was made to pinpoint possible radiation sources in the area and determine whether the measurement location was along the direction of the maximum radiation intensity of the source.