## 3 Results and Discussion
### Quantitative VOC Differences between the Bio-Based Boards  
The total peak areas obtained in the SPME-GC-MS tests were used to semi-quantitatively compare the influence of the boards' feedstock, density, thickness, and lamination on the VOC emissions during 4 weeks. Figure 4 shows the total VOCs content, expressed as the area units from the chromatograms and the furfural fraction, as it was the most abundant individual compound determined for all binder-less boards, except the Commercial-PB with hexanal and \(\alpha\)-pinene as the main VOCs. The higher density (1200 or 1300 kg m\({}^{-3}\)) of the binder-less boards gave rise to a characteristically higher VOC level. The total VOCs from hep and wheat binder-less boards had comparable (within 5% difference) content, even though the number of components in the WS 1200 sample was significantly higher than in HS 1200-165 and 99, respectively (see numbers at the base of the bars in Figure 4A,B). There was a strong correlation between the density and the VOC emissions for all raw materials. Namely, the decrease in the density from 1300 to 700 kg m\({}^{-3}\) meant a decrease in VOCs more than two times. It could be explained that during the pressing at higher densities, VOCs could not escape from the PB matrix and, afterward, slowly evaporated from the sample to a greater extent and for a more extended period. For example, pellets from SE softwood with a density of 750 kg m\({}^{-3}\) exhibited a lower VOCs level compared to SE biomass immediately after the treatment [19]. The lower-density board samples contain a higher volume and number of pores through which the transportation of VOCs during manufacture is easier than from high-density samples that are highly compressed and, therefore, have few pores. However, as the high-density samples are highly compressed, they also contain a higher  
Figure 3: Examples of different chemical species detected in VOCs emitted from the bio-based particleboards. number of particles. In this context, in spite of the equal mass of the tested specimens, the lower-density samples contain less material than high-density samples, and, therefore, it also could be attributed to the explanation of the VOCs level that was also approved by another study [31]. The thicker binder-less boards (12 mm) had similar (within 10% difference) VOC emissions as their 6 mm thick counterparts. Furthermore, slightly higher emissions were released from HS-board samples than from WS-board samples with a significantly higher furfural concentration in 12 mm thick HS-based boards (Figure 4A). Regarding the overlaying effect by melamine-impregnated paper, the goal was achieved for the HS-based board by twice-reducing VOCs; however, the overlayed WS-based board-emitted VOCs increased by a factor of two. This could be explained by the different thermal effect on different lignocellulosics during the overlaying that obviously cause additional VOCs formation especially at elevated temperatures [2]. It was reported that the softening of HS (Uso 31) and WS pretreated by SE at 220 \({}^{\circ}\)C for 2 min occurs in the temperature ranges of 163-192 \({}^{\circ}\)C and 135-137\({}^{\circ}\)C, respectively [25]. Thus, the HS pretreated material is more thermally resistant than WS, which was also confirmed by the derived thermogravimetry analysis shown in Figure 5. This allows the conclusion to be made that the overlaying by melamine-impregnated paper at 145 \({}^{\circ}\)C resulted in increased VOCs emitted through the  
Figure 4: Semi-quantitative comparison of the total VOCs and the furfural fraction emitted from the bio-based boards after days 1, 14, and 28, depending on the raw material, density, thickness, and overlaying. Binder-less boards from (**A**) hemp shives, (**B**) wheat straw, (**C**) grey alder and birch wood, (**D**) suberinic acids-bonded and commercial particleboards. side surfaces only for WS-based board, as well as changed the determined VOCs proportion for both HS- and WS-based boards, as mentioned in a previous subsection.