## 4 Discussion  
The uniqueness of this research can be recognized in two key aspects. First, human embryonic lung fibroblasts were used as the cellular model. These are a rarely employed model in PM toxicity studies, which more commonly rely on immortalized epithelial or tumorigenic cell lines. In a recent study, [PERSON] et al. emphasized the importance of using normal human cell lines to better assess toxicological effects in a physiologically relevant context [30,31]. Second, unlike most studies, which focus on urban PM, this research examined industrial PM, specifically that derived from biomass treatment plants. These facilities are widespread and expanding across Europe and beyond, and the composition of the PM they generate is markedly different from urban PM, being primarily biological in nature. Organic waste composting plants exhibit the highest and most significant levels of airborne PM and endotoxins. This is expected due to the intrinsic characteristics of these facilities, both in terms of the quantity and composition of the processed material and the type of treatment, which involves the continuous movement and overturning of organic matter. This process generates a substantial amount of dust, including coarse particles that are visibly detectable by personnel involved in sampling. Additionally, composting plants are predominantly indoor, whereas other plant types are semi-outdoor or outdoor,  
Figure 6: Gene Expression Analysis in Relation to Plant Type. The graph represents only the genes whose expression differed significantly among the plants (ANOVA), \(p<0.05\) (*) and \(p<0.01\) (**). leading to differences in ventilation and contaminant dispersion. In these same plants, the cytotoxic activity of PM was significantly higher compared to other facilities, such as agro-zootechnical or lignocellulosic matrix plants. This effect is likely attributable to both the higher PM concentration--given the significant direct correlation between these two variables--and the composition of the airborne mixture in these plants. In most cases during the sampling period, the occupational exposure limits for respirable and inhalable dust, as well as endotoxins [29], were not exceeded. However, one composting plant significantly surpassed these thresholds, while a second similar facility exceeded the EU/m\({}^{3}\) limits during one sampling day. Additionally, data indicate higher contamination levels (PM and endotoxins) and greater cytotoxicity in the biomass entry, storage, and treatment areas compared to the product exit areas. The most polluted areas were identified as the refinement zone and the composting tunnel, which aligns with the greater biomass movement occurring during the early treatment stages. A key biological finding emerged from the XTT assay. To assess potential biological modulation without excessive cell death, cells were exposed to 1% organic PM extract, a sub-lethal dose. Relative to the control (cells exposed only to culture medium), in most cases, a mild cytotoxic effect was observed, with cell viability decreasing between 0% and 20%. Interestingly, in some cases, PM exposure appeared to increase mitochondrial dehydrogenase activity, as indicated by negative delta values, with a maximum induction of 27.5%. This effect was exclusively observed in eight samples, all derived from the finest PM fraction (PM\({}_{0.49}\)). The mechanisms by which PM exerts its toxicity are closely linked to mitochondrial activity, as mitochondria are the primary producers of ROS and among the most common cellular targets for oxidative damage (e.g., mtDNA damage, lipid peroxidation, and protein oxidation). Evidence from in vitro studies shows that PM generally has an inhibitory effect on mitochondrial activity, particularly reducing the functionality of certain enzymes, including succinate dehydrogenase, which is responsible for the reduction of the salt in the XTT assay, as observed for most of the samples evaluated.