## 4 Discussion  
This study showed that occupational exposure, even to very low doses, of IR can cause an increased frequency of spontaneous chromosomal breaks and induce an increased frequency of spontaneous MN in subjects with at least one allelic variant of the _XRCCI_ gene. Moreover, analysis of the individual mutagen sensitivity demonstrated a dependency of BLM-induced MN frequency negative on IR exposure, and positive on the null _GSTM1_ genotype and on alcohol consumption. Occupational exposure to IR has progressively reduced in the last 30 years, as safer radiogenic machines emitting less IR have replaced earlier, less safe equipment. Italian law for the health and safety of workers exposed to IR is detailed in legislative decree n. 230 of 1995 and subsequent modifications and integrations derived from European Union norms and those of the International Commission on Radiological Protection. The limit value of the annual effective whole body dose for workers exposed to IR is set at 20 mSv, and for the general population at 1 mSv. The cumulative occupational exposure found in our examined workers showed a median value of 5.31 mSv, with a range of 0.20-54.43 mSv, so apparently very low. Although the workers were exposed to very low doses of IR, an increased frequency of chromosome but not of chromatid breaks was observed in the exposed workers group, similar to that observed by [PERSON] et al. [32] and [PERSON] et al. [6] in workers exposed to higher doses of IR. The increased chromosome breaks in the workers examined, but without the presence of ring or dicentric chromosomes, can be explained by the fact that very low doses of IR may not be sufficient to produce the two independent chromosomal breaks necessary to form more complex alterations. Moreover, it should be considered that the DNA double-strand breaks induced by exposure to IR could trigger a recombinogenic exchange that, if incomplete, appears in metaphase as a chromatid break [33]. However, this mechanism is activated only at exposure to higher doses than those found in our study [6]. Previous studies showed an increased frequency of spontaneous MN in health care workers exposed to mean cumulative doses of IR ranging from 19.49 to 40.61 mSv, but without observing any relation to the duration of exposure and the IR dose [4; 34]. In our study, instead, in workers exposed to much lower doses of IR, no increase in the frequency of spontaneous MN was observed, as also reported by [PERSON] et al. [35], even if their workers were exposed to a much higher mean cumulative dose than that observed in our study, equal to 82.3 mSv. The absence of a greater frequency of MN in the examined workers could depend on their exposure only to very low doses of IR. The genotoxic damage determined by such doses would, in fact, be neutralized by the activation of apoptosis mechanisms of the cells damaged during replication in vivo and by the induction of DNA repair enzymes transcription until the saturation of the receptor sites occurs [35; 36; 37]. These mechanisms would be more evident for MNs than for chromosomal breaks, because the former are to be regarded as unstable aberrations and so they undergo a clear decline thanks to the activation of the mechanisms previously reported [38]. The Comet assay parameters did not reveal higher values in the workers exposed to IR than the controls. Previous studies of the effects of IR by the Comet assay, in any case using biomarkers other than TI, elicited discordant results whereby some showed an increase [15; 39; 40] and others no effect [7; 14; 32]. Because the Comet assay reveals recent DNA damage occurring in lymphocytes after exposure to IR, the fact that this was not observed in the exposed workers seems to indicate that the very low doses to which they were chronically exposed are not able to cause an effect detectable by this test, also because of a possible short term up-regulation of the DNA repair capacity induced by the very low IR doses [7; 41].