## 4 Discussion
Compared with this study, our results need further investigation to precisely describe the particle presence at <50 nm, where coarse sandpaper with grit P60 seems not to produce as many particles from potential combustion as P240 sandpaper. Moreover, it will be necessary to explain the behavior of particles in the size range of approximately 100 to 150 nm, where sandpaper grit P60 increased the number concentration above the level of both P120 and P240 grits. In our opinion, there is clear evidence that the surface area is an important metric for carcinogenic oak wood particles. Cancer of the upper respiratory tract develops after exposure to many kinds of wood dust. In addition, the wood dust of oak wood seems to be highly carcinogenic. It is assumed that exposure to wood dust can cause an increased incidence of other cancers, especially lung cancer and Hodgkin's disease [42]. In this regard, we consider oak wood sanding with an orbital sander to be a risky occupational activity, where the average LDSA for worker is approximately 120,000 \(\upmu\)m\({}^{2}\)/cm\({}^{3}\), for each investigated sanding grit. There is a lack of knowledge in the field of wood processing, especially in terms of occupational exposure to UFP. Comparing different occupational environments, [PERSON] et al. [43] detected LDSA concentrations ranging from 49 to 3200 \(\upmu\)m\({}^{2}\)/cm\({}^{3}\) for UFP generated by welding process in the automotive industry. [PERSON] et al. [44] found LDSA concentrations for welding process ranging from 8325 to 42,896 \(\upmu\)m\({}^{2}\)/cm\({}^{3}\) UFP, with a size of primarily 10-20 nm. According to [PERSON] et al. [45], LDSA in an underground mine ranged from 137 to 405 \(\upmu\)m\({}^{2}\)/cm\({}^{3}\). Our results underline the importance of future investigation of wood sanding in terms of LDSA, as it appears to be the most relevant physical metric for quantifying occupational exposure to smaller oak wood dust particles. Therefore, occupational exposure limits should also include LDSA values. Several studies have indicated a relation between the coarseness of the sanding paper and the mass concentration of the generated micro-sized aerosol [18; 27; 28]. The present study was designed to determine the effect of sandpaper grit size on airborne particle concentration and size distribution in UFP and CFP levels during oak wood sanding. The results of this study are in contradiction to results of study performed by [PERSON] et al. [46]. They concluded that the coarseness of the sanding paper did not influence the particle number concentration. However, they investigated the potential release of nano-objects, their agglomerates, and aggregates (NOAA) as a result of the sanding of wood surfaces treated with manufactured nano-object (MNO)-containing coatings. Exposure to wood dust at different wood working processes was studied by [PERSON] et al. [47] in the Netherlands. It was observed that during hand sanding of wood, the exposure limit of that time, set at 5 mg/m\({}^{3}\), was regularly exceeded. The sanding was identified as the activity which was responsible for all measured high concentrations. In contrast, our results showed that sanding of oak wood was always \(<\)5 mg/m\({}^{3}\) and it was confirmed by both used instruments, the DustTrak DRX and the OPS. However, when comparing the results from each instrument, differences were found in PM fraction, except the respirable. Mass concentrations were normalized to flowrate. Thus, the discrepancy was most likely caused by calibration. While the DustTrak DRX is calibrated to so-called Arizona test dust A2 particles [48], the OPS measures number sized distribution primarily and is calibrated for size with polystyrene latex (PSL) spheres per ISO 12501-1/4 at TSI [49]. To calculate the mass concentration obtained by the OPS, the standard tabular density of oak wood was set. In addition, the DustTrak DRX uses a cloud of particles to obtain a scattered signal, while the OPS uses single particle signal. There are several limitations to the scope of this study.