## 4 Discussion
On the other hand, it is important to consider the presence of a strong biological component within the mixture used in this study. As recent studies have shown, this biological component significantly influences the toxicity and oxidative potential of PM, with opposing effects depending on the microbial species present. For example, [PERSON] et al. [32] observed, in cell-free systems, that the oxidative potential of certain chemical species in PM decreased gradually with increasing concentrations of _S. epidermidis_ and other bacterial species, likely due to the ability of the bacterial cell wall to chelate these chemical components and inhibit their reactivity. In contrast, high concentrations of _A. fumigatus_ spores and other fungal species increased the oxidative potential of PM. However, it is also known that cytotoxicity assays using tetrazolium salts (e.g., XTT, MTT) are influenced by numerous factors that can perturb cellular and mitochondrial homeostasis, potentially leading to partial reprogramming of cellular metabolism, which may sometimes cause confounding effects in interpreting results. Among these factors, plant extracts (which contain polyphenols, flavonoids, etc.) are potential contributors, demonstrating the ability to directly reduce XTT salt through the presence of free -SH groups, thus overestimating cell viability. It cannot be ignored that the PM samples used in our study, due to their derivation from plants treating organic biomass, may contain plant-derived components that contributed to these effects. However, this effect was observed only in a few PM\({}_{0.49}\) samples, so it cannot be generalized to all of the extracts. Another study evaluated the oxidative effects of tert-butylhydroquinone (t-BHQ) on Madin-Darby Bovine Kidney cells using the XTT assay [33]. The authors explain that pro-oxidant agents, such as t-BHQ, at low doses, can induce the transcription of phase II detoxifying enzymes through the Nrf-ARE pathway (nuclear factor E2-related factor;antioxidant responsive elements), promoting cytoprotection against oxidative damage. In such studies, exposure of MDBK cells to t-BHQ at low doses induces an apparent increase in cell viability, which was not observed with the crystal violet (CV) staining. Numerous studies have shown that the toxicity mechanisms of PM are also closely linked to the activity of the Nrf transcription factor mentioned above, which plays a key role in protecting the cell from oxidative damage and inflammation. There is in vitro and in vivo evidence demonstrating that organic chemical components of PM\({}_{2.5}\) significantly induce activation of this pathway [34; 35]. The predominant component of the PM under study is of biological origin. It cannot be ignored that organic components such as PAHs (polycyclic aromatic hydrocarbons), which may also originate from the use of transport vehicles within the plants, are present in trace amounts and, along with the biological components, enhance the oxidizing nature of the sample. These effects were, furthermore, observed only in PM\({}_{0.49}\) samples, the finest fraction, which carries these organic components in higher amounts compared to coarse PM. The gene expression analysis revealed that PM had a quite modulatory effect on all of the genes examined, but it had a highly significant impact on the expression of ERCC1 and IL-8, with transcript increases of 25% and 53%, respectively. In contrast, XRCC1, XPF, and XPA showed a mild increase in expression, but the biological response was highly variable. This suggests that the high heterogeneity of the pollutant compound mixture in PM\({}_{10}\) airborne particles may lead to inconsistent biological effects, sometimes even opposing ones. The XRCC1 protein plays a crucial role as a scaffold in various enzymatic complexes for DNA repair, particularly in the processes of NER (Nucleotide Excision Repair) and BER (Base Excision Repair).