## 3 Results and Discussion
### Metals  
The investigation of 16 elements (Al, As, Ba, Cd, Co, Cr, Cs, Cu, Fe, Mn, Ni, Pb, Sr, Tl, V and Zn), indoors and outdoors, depicted that their contribution to the total PM\({}_{10}\) was 22.28% in classroom C3 and 20.40% on the GRS, respectively. These results are comparable to those of [PERSON] et al. [94], which refer to similar measurements in Vienna, Austria. It is important to note that for both the indoor and outdoor environments, principal earth crust elements Fe and Al prevailed, in consistency with the studies of [PERSON] et al., 2017 [66] and [PERSON] et al. [95]. More specifically, in classroom C3, Fe and Al accounted for the 9.55% and 8.68% of the total PM\({}_{10}\), followed by Zn (2.8%), Cr (0.46%), Ba (0.43%), Cu (0.28%), Ni (0.16%), Mn (0.15%), Sr (0.13%), V (0.12%), Pb (0.09%), Co (0.03%), As (0.01%), Cd (0.003%), Cs (0.001%) and Tl (0.0009%). Similar results were found on the GRS (Fe: 13.08%, Al: 4.04%, Zn: 1.55%, Cu: 0.35%, Ba: 0.33%, V: 0.25%, Mn: 0.21%, Cr: 0.18%, Ni: 0.14%, Pb: 0.14% Sr: 0.1%, Co: 0.01%, As: 0.01%, Cd: 0.004%, Cs: 0.001% and Tl: 0.0007%).  
Among the investigated metals, Ni and Cd are regulated through WHO guidelines [96] with a maximum value of 1000 ng m\({}^{-3}\) for Ni and 5 ng m\({}^{-3}\) for Cd. Pb is regulated by the National Ambient Air Quality Standards (NAAQS) [97], with a value up to 150 ng m\({}^{-3}\) and by the 1999/30/EC European legislation directive [98], with a yearly limit of 500 ng m\({}^{-3}\). Because of their negative impact on humans, the 2004/107/EC directive [99] refers to annual limits of 20, 5 and 6 ng m\({}^{-3}\) for Ni, Cd and As, respectively (concerning PM\({}_{10}\)). Indoor concentrations of As, Cd, Ni and Pb measured in classroom C3 was not found to overpass the respective limit values.  
The calculation of EF (Equation (4)) for individual elements was implemented in order to investigate possible sources of indoor PM\({}_{10}\). Operationally, given the local variation in soil composition, an EF value exceeding 5 suggests that non-crustal sources contribute a significant fraction of the element [100]. An increase of the EF value provokes a subsequent increase to the contribution from non-crustal sources. Higher EF values implying anthropogenic activities of both the outdoor and indoor environment were determined sequentially for Cd, Zn, Cu, Pb, Ni and Cr, while the corresponding values for As and V were mainly of the outdoor environment (Table 6). For all trace metals studied, excluding Cr and Co, EF values were found to be higher for the outdoor compared to the indoor environment, indicating that air infiltration constituted a significant factor of the enrichment of the classroom in trace metals, masking the original differences of indoor characteristics. This trend was confirmed in a relevant study regarding urban office areas in China [101]. High abundance of Cd and Cu was associated with traffic-related dust, high levels of As and Pb originating from coal combustion, whereas Zn was linked with combustion and vehicular emissions (mainly tire and brake wear), and Ni is typically known as an element linked with oil combustion, according to previous studies [102; 103; 104; 105; 106; 107].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **EF\({}_{\text{indoor}}\)** & **EF\({}_{\text{outdoor}}\)** & **I/O Ratio** \\ \hline Al & - & - & 2.1 \\ As & 13 & 56 & 0.5 \\ Ba & 6.5 & 11 & 1.3 \\ Cd & 275 & 821 & 0.7 \\ Co & 17 & 14 & 2.5 \\ Cr & 47 & 39 & 2.6 \\ Cs & 2.4 & 4.0 & 1.3 \\ Cu & 95 & 253 & 0.8 \\ Fe & 2.2 & 6.5 & 0.7 \\ Mn & 1.5 & 4.4 & 0.7 \\ Ni & 32 & 62 & 1.1 \\ Pb & 51 & 170 & 0.6 \\ Sr & 3.7 & 6.4 & 1.2 \\ Tl & 9.4 & 17 & 1.2 \\ V & 12 & 52 & 0.5 \\ Zn & 307 & 470 & 1.4 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Enrichment factors (EFs) and indoor to outdoor concentration ratios (I/O) for the trace metals studied.