## 4 Discussion  
The general living and occupational ubiquitous nature of exposure to FA requires a deep understanding of the molecular mechanisms potentially underlying its induced toxicity and carcinogenicity. In this regard, although the limited number of studies and the lack of robust epidemiological data on occupational exposed populations still prevent the extrapolation of definite conclusions, preliminary evidences are emerging concerning the possibility that epigenetic changes may condition biological response to FA. Workers exposed to FA demonstrated a global increase in DNA methylation in whole blood samples, positively related to FA workplace levels, to the number of procedures involving such chemical exposure, as well as to have been directly employed in such specific job tasks [28]. Conflicting evidence in DNA methylation emerged when these human findings were compared with previously published in vitro results. In fact, these reported a significant decrease in global methylation in bronchial epithelial cells chronically treated with FA, or the absence of significant DNA methylation changes in acutely exposed human lymphoblastoid cells [29; 32]. This may be due to the different biological matrices investigated, i.e., the whole blood in humans [28] vs. bronchial and lymphoblastoid cells in the experimental in vitro settings [29; 32], which may be biased by cell-specific methylation patterns. Variable experimental designs, i.e., concerning doses and time periods of FA exposure, as well as the diverse analytical techniques adopted, i.e., high-performance liquid chromatography-diode array detector equipment [28], high-performance capillary electrophoresis assay [29], and tandem mass spectrometry [32], may all be responsible for quite different results. In this scenario, to define the role of DNA methylation changes as possible FA biomarkers of early effect, future in-field and epidemiological investigations seem absolutely necessary. However, although in vitro experimental settings may provide a limited representativeness of real human exposure contexts, such investigations may contribute to identify target genes whose expression changes may be involved in FA-induced methylation effects and toxic manifestations. This idea is supported by the specific patterns of DNMT1, DNMT3a, DNMT3b expression responses found in human bronchial epithelial cells that may suggest specific FA-induced alterations, in turn reflecting the methylation levels determined in the samples [42]. Such data, once confirmed in epidemiological investigations, may provide further support to the role of such specific patterns of changes as possible FA biomarkers of early effect to be considered in suitable risk assessment processes. Scientific evidence has been emerging concerning the role of histone post-translational modifications in a multitude of DNA related processes, such as DNA damage repair, DNA replication, and transcription of tumor-related genes [51; 52]. In particular, the dysregulation of histone H3 phosphorylation, reported following FA exposure, may perturb cellular pathways involved in human cancer development [30; 32]. In fact, it is known that in interphase cells, such phosphorylation phenomena are involved in the chromatin remodeling of the promoter regions of specific loci, i.e., FOS and JUN, implicated in tumorigenic processes, while in mitotic cells, they are responsible for the maintaining of chromosome stability and their accurate segregation [53]. Histone H3 phosphorylation was reported in aflatoxin B1-transformed hepatocytes, benzo(a)pyrene- and coke oven emissions-transformed human bronchial epithelial cells in which such modification performed a mediation role in cellular carcinogenic transformation [51]. The phosphorylation of H3S10 has been reported also to be induced by several environmental factors, such as arsenite [54], nickel [55], and UVB [56; 57]. Histone modification changes induced by FA exposure, included also the formation of FA-lysine histone adducts and the consequent reduction in lysine acetylation.