## 4 Discussion  
Wood processing has been known to emit a large amount of inhalable wood dust, but the emissions of particles with diameters smaller than 10 \(\upmu\)m and ultrafine particles, as well as their exposure levels, remain unclear [33]. The Institute for Occupational Safety and Health of Germany (IFA/DGUV) recommend two benchmark levels for airborne particles in the range of 1 nm to 100 nm, based on their density. For materials with a density exceeding 6 g/cm\({}^{3}\), the benchmark level is 20,000 \(\upmu\)/cm\({}^{3}\), and for materials with lower density, the benchmark level is twice as large in range of measurement, between 1 and 100 nm [31, 34]. The number concentration of 20,000 \(\upmu\)/cm\({}^{3}\) is recommended for metals, metal oxides and other bio-persistent granular nanomaterials. Determining the density of oak wood UFP is beyond the scope of this paper, nevertheless, based on the composition of oak wood and its standard tabular density at approximately 0.75 g/cm\({}^{3}\), a benchmark level of 40,000 \(\upmu\)/cm\({}^{3}\) would be more suitable. By comparing this benchmark level with the measurements of P60, P120 and P240 sanding paper grits at peak values, approximately 81% of this benchmark level was reached by sanding paper with grit P240. Sandpapers with grits P60 and P120 reached both approximately 72% of the benchmark level. Our experiment showed that the choice of grit size during sanding process requires serious attention in association with health impacts in the wood-processing industry. From the analysis of the time course of the measured data, we found a high degree of risk for workers after the end of sanding. The decrease in the concentration of airborne particles in the test room was relatively slow, the original background value from before the start of sanding was reached only after approximately 3 h, however the test room was not equipped with suction. Professional sanding commonly means that personal protective equipment (e.g., respirator) should be used. If so, the worker usually pulls it down immediately after finishing work, which leads to exposure to high concentrations of particles in the occupational environment. [PERSON] et al. [35] investigated the differential effects of coagulation, deposition, and ventilation on UFP size distribution indoor. They concluded that coagulation is a significant aerosol process for UFP dynamics and the primary cause for the shift of particle size distribution following an episodic high-concentration (>20,000 \(\upmu\)/cm\({}^{3}\)) UFP release with no fans operating. Previous research concluded that a variety of processes and phenomena can influence indoor particle levels and fates [36, 37]. However, when comparing our results to those of older studies, it must be pointed out that primary attention has been addressed to residential environment quality. Therefore, it remains unclear how the particle size distributions shown in Figure 4 were affected, and what causes distinct peaks at certain particle sizes. The obvious peak might be a result of particle formation due to combustion initiated by friction between sandpaper and wood. Combustion as a side effect of abrasion produces particles <50 nm scale [38, 39, 40]. These particles might be volatile organic compounds, semi-volatile organic compounds, elemental carbon, black carbon or any other products of combustion. There is also a less distinct peak in Figure 4, at approximately 30 nm. Our data are generally comparable with those reported in previous literatures, e.g., [PERSON] et al. [41] reported bimodal size distribution during the period from 0 to 10 min of wood combustion process, with a nucleation mode in the size range of 10-20 nm and an Aitken mode peaking at 40-50 nm. Subsequently, the bimodal distributions gradually became unimodal during the combustion period from 10 to 15 min.