## 3 Results and Discussion
Figure 4: Comparison of pre-shift and post-shift urinary fluoride concentration for all participants. In agreement with WHO [31], the time of urine collection was approximately equal during the whole study period, which is important for correct interpretation of the results, because urine that has accumulated in the body over a shorter time period may manifest a short-lived peak of the fluoride concentration. The findings of the present study demonstrate variations in urinary fluoride levels within participants. We assume that improper use of a personal respiratory protection might contribute to these differences. According to [PERSON] et al. [9], mean urinary fluoride concentrations were 1.3 and 3.0 mg/g creatinine in pre-shift and post-shift urine samples. The results of our study showed markedly lower mean concentrations of particulate fluoride and HF in the occupational environment. Our results also showed lower pre- and post-shift urinary fluoride concentrations. Research by [PERSON] et al. [9] was performed on a sample of 32 workers in a shorter period (days 1 and 3 of a three-day workweek), making it more sensitive to local and short-term increases in fluoride concentration in the occupational environment. On the other hand, our study provided information obtained from a longer time period and various operational situations that may have occurred in aluminum smelter. A frequency distribution in Figure 5 shows that 89% of all study participants had urinary fluoride difference (post-shift minus pre-shift concentration) below 1 mg/g creatinine, with the maximum value at 3.38 mg/g creatinine and the minimum was at a negative value of \(-0.82\) mg/g creatinine (a negative result of urinary fluoride difference was found in nine workers). According to [PERSON] et al. [35], the difference between post- and pre-shift urinary fluoride concentration should not exceed 3 to 4 mg/g creatinine, respectively, pre-shift values should be <3 mg/g creatinine. Tobacco smoking was the main confounding factor of fluoride exposure we decided to investigate. In total, 35 study participants were regular smokers consuming at least 10 cigarettes per day. Other 39 participants declared they were strict non-smokers and they were not aware of the regular secondhand smoke exposure. There was a separate smoking area in smelter. Workers had the right to one uninterrupted 30-min rest break during their working day, if they worked 8 h per day. Majority of smokers took this break also as smoking break. Further, each of the smokers confirmed that they smoked at least one cigarette before their shift. Figure 5: Frequency distribution of urinary fluoride difference for all study participants. Figure 6 shows a comparison of mean pre- and post-shift urinary fluoride concentrations according to smoking habits of investigated primary aluminum workers. The results in the present study showed no statistically significant difference of the urinary fluoride level among group of smokers and non-smokers before the shift (\(p\) = 0.62) and after the shift (\(p\) = 0.11) by K-W test. This suggests that under the given research conditions, tobacco smoking does not affect the fluoride content of workers in primary aluminum smelter. It remains unclear why non-smokers show higher or equal pre-shift mean urinary fluoride concentration (0.469 \(\pm\) 0.379 mg/g creatinine) than smokers (0.441 \(\pm\) 0.338 mg/g creatinine). We assume that pre-shift samples reflected the worker's total burden of fluoride and although smokers consumed a cigarette before shift, this was not reflected despite to rapid kinetics and the biological half-life of fluoride in human body. From the difference between pre- and post-shift urinary fluoride values, a decreasing trend of urinary fluoride concentration in smokers was found (r = \(-\)0.133, \(p\) = 0.447). The group of non-smokers was characterized by a slightly increasing trend (r = 0.03, \(p\) = 0.868). The research on the interaction between tobacco smoking and fluoride absorption and, subsequently, excretion is limited.