## 3 Results and Discussion
(2018) found a linear relationship between HF concentration (>5 ppm) in the occupational environment and mean urinary fluoride levels in electronics industry workers. The wide variation of fluoride levels in urine has been ascribed to the consumption of tea and seafood (water intake was not considered because only <1% of the water supply is fluoridated in Japan). In agreement with [PERSON] et al. [PERSON] et al. (2018), we also rejected the fluoride intake by potable water in Ziar nad Hronom which met hygienic standards. In the occupational environment, fluoride intake via inhalation can reach 16.8 mg per day; nevertheless, daily intake of fluoride by optimally fluoridated potable water is about 1.4-2.4 mg [PERSON] et al. (2018). During our research, participants were asked to avoid tea consumption and use of toothpastes enriched with fluoride for at least 48 h prior to urine collection; however, an apparent limitation of this procedure is its verification. [PERSON] et al. [PERSON] et al. (2018) and [PERSON] et al. [PERSON] et al. (2018) concluded that tea is a significant source of fluoride intake. A major source of limitation in this study is fluoride intake effected by the composition of diet. With low water fluoride, the urinary fluoride concentration is much more influenced by eating habits [PERSON] et al. (2018). According to [PERSON] (2018), vegetarian diet leads to more alkaline urine and in addition increases the urinary fluoride  
Figure 3: Correlation graph between total (particulate + gas-phase) fluoride concentrations in the occupational environment and post-shift urinary levels. excretion. Assessment of fluoride intake by food is quite challenging due to exposure to multiple dietary sources and supplements. Some authors in other studies used diet standardization to reduce this limitation, however, it is quite difficult to conduct [7]. Urinary fluoride concentrations of 0.8-1.2 mg/L are regarded as indicating optimal exposure to fluoride in population [31]. Figure 4 compares pre- and post-shift urinary fluoride concentrations for all study participants. According to Wilcoxon signed-rank test, mean post-shift urinary fluoride concentration was significantly (\(p<0.001\)) higher compared to mean pre-shift urinary fluoride concentration for all investigated participants. From this, it can be concluded that there is an increase in fluoride concentration in primary aluminum workers during their work performance, although the relationship between occupational exposure and urinary fluoride levels was not confirmed. Overall, these findings are in accordance with previous findings reported by [PERSON] et al. [32] who found significant (\(p<0.05\)) difference between mean pre- and post-shift concentrations in workers exposed above OSHA PEL (2.5 mg/m\({}^{3}\)) to calcium fluoride (CaF\({}_{2}\)), and a weak correlation between intensity of occupational exposure to calcium fluoride and post-shift urinary fluoride concentration. Mean fluoride exposure in the dustiest environment was 24.3 mg/m\({}^{3}\) (40.6% respirable fraction) which is approximately 10 times OSHA PEL. Only one urinary concentration exceeded the recommended limit value of 7 mg fluoride per liter. It is important to highlight the fact that potroom of aluminum smelter is a high-temperature workplace; therefore, the relating of the urinary excretion of fluoride to g-creatinine is necessary. According to the World Health Organization (WHO) [31], the mean 24-h urinary creatinine value is 15 mg/kg of body weight per day. Several previous studies have not reported creatinine correction. [PERSON] et al. [33] reported the mean urinary fluoride concentration of 1.49 (\(\pm 0.63\)) mg/g creatinine in children. The biological exposure index (BEI) for fluoride exposure is 3 mg/g creatinine prior to shift and 10 mg/g creatinine at end of shift [34]. The BEI were not exceeded in the monitored spot urine samples. We found that maximum pre- and post-shift urinary fluoride concentrations were 2.42 mg/g creatinine and 3.73 mg/g creatinine.