## 4 Discussion  
Taken as a whole, the studies retrieved for this review suggest that polymorphisms in candidate genes may affect susceptibility for specific neurobehavioral functions associated with occupational Hg exposure. Occupational findings add to existing evidence indicating genetic determinants of mood and behavior that potentially increase susceptibility to Hg toxicity in human subjects. However, occupational studies have made only a modest contribution to understanding the adverse effects of elemental mercury. To date, no studies have investigated the role that genetic factors may play in the absorption, distribution, and excretion of mercury, and most genes are selected by authors based on criteria of biological plausibility and their role in neurological outcomes. The body burden of mercury in workers has been estimated on the basis of the duration and time of exposure, mainly by integrating these data with the Hg dosage in a single urine sample. No studies have measured mercury in tissues. The demonstration of an association between a polymorphism and neurobehavioral alterations, mood, or potentially neurotoxic symptoms cannot be considered evidence if it has not been confirmed by other studies replicating previous observations on the same genetic variant. Since the studies included in this review mostly attempted to identify new variants instead of examining previous findings from other candidate gene studies, it was not possible to pool results to categorically ascertain the influence of a genetic variant. Most of the studies were performed in Caucasian, or predominantly Caucasian populations, or did not report the ethnicity of participants. Consequently, the frequency of allele variations across ethnic groups could not be evaluated [58]. An evaluation of the findings was further limited due to the fact that the studies retrieved did not always consider the same confounding factors and included factors potentially influencing the results, such as ethnicity, alcohol, health status, as well as diet; this is a further limitation to the evaluation of the findings. Furthermore, published studies were not of high quality, particularly when specific methods for genetic studies were used for assessment [59]. Research findings demonstrate the difficulties encountered in reproducing behavioral or symptomatic results for very low exposures in different populations and with changing explanatory variables. Clearly, further studies are needed to confirm or disprove the associations observed and to investigate numerous other genetic factors that could be associated with the toxic effects of mercury in workers. However, the limited number of available studies that have been included in this systematic review indicate that adverse neurobehavioral effects may be associated with occupational elemental mercury exposures similar to those experienced by the general population due to mercury amalgam dental fillings, or other environmental pollution sources. Occupational studies therefore have an important significance for public health. Studies on occupational cohorts such as workers exposed to mercury in the same workplace could easily be carried out by an occupational health and safety service. The medical health surveillance of workers and their biological monitoring would be much more efficient if they could incorporate elements of genetics to promptly identify polymorphisms that increase the risk of neurological damage. Employers may have a legitimate interest in funding studies on the genetic status of their employees, in order to limit the exposure of hypersensitive individuals, who could be harmed even if exposed to levels not exceeding the permissible standard. Concern for both occupational and public health should encourage governments to intervene to fund or promote prevention in this area.