An important factor that affects the amount of mercury in mothers is the consumption of marine products, especially fish. Mothers are environmentally exposed to mercury through the digestion of mercury-contaminated foods. There was a relationship statistically significant (between mercury concentration in hair and saliva and fish consumption. The average concentration of mercury in hair and saliva samples obtained from mothers with 1 serving of fish per week was significantly higher compared to mothers who consumed 1 serving of fish per month. The content of mercury in hair and saliva mainly reflects the absorption of organic mercury compounds through fish consumption. According to the findings, fish consumption had a significant effect on the amount of mercury in the mother's hair and saliva, and with more fish consumption, the mercury concentration increased.
In the present research, like the studies of Fekur et al. (2010), there is also a significant positive relationship between the amount of fish consumed and the amount of mercury in hair and saliva of mothers (Fakour et al., 2010). Schreier et al. (2015) also stated that the amount of mercury in hair is more influenced by the amount of mercury in food, especially fish (Schreier et al., 2015). As fish consumption increases, the amount of mercury increases (Aleksina et al., 2023). The amount and type of fish consumed is the reason for the significant difference between the groups in this factor. According to the questionnaire, mothers often consumed fish such as salmon, cod, grouper, and trout.
However, in current study, the amount of mercury in these fish was not measured. But only mercury pollution in this type of fish can lead to such a result. In a study in Italy, it was reported that people with high fish consumption had twice as much mercury as people who did not eat fish or consumed less fish (Dorea et al., 2003).
In our study there was a positive and significant correlation between the number of teeth filled with amalgam and the amount of hair mercury in the studied women (p = 0.002). The findings of the present research show that in addition to consuming fish in the diet, dental amalgam filling also contributed to accumulate mercury in the body. The amount of mercury absorbed in the human body through amalgam is about 1.2 to 1.10 of mercury absorbed through food (Feingold et al., 2020). Mercury vapor released from amalgam into the blood of pregnant women quickly crosses the placenta and accumulates in fetal blood, pituitary gland, and liver, as well as in amniotic fluid (Rebelo and Caldas, 2016). These findings show that in addition to getting mercury from the diet, dental amalgam also helps to accumulate mercury in the body. Cullen et al (2014) assessed hair mercury levels in 120 Irish mothers. In their study, the average concentration of mercury in mothers' hair was 0.262 micrograms/gram. It is worth noting that mothers who had higher education, consumed more fish, and had amalgam-filled surfaces had significantly higher levels of mercury in their bodies than others (Cullen et al., 2014). Norouzi et al. (2012) also found a positive and significant correlation between mercury levels in milk and hair of mothers with the number of teeth filled with dental amalgam. The average level of mercury in mothers who had four to eight teeth filled with amalgam increased from 5.47 micrograms to 13.33 micrograms per gram compared to mothers with one to three teeth filled with amalgam (Norouzi et al., 2012).
In the present study, the concentration of mercury levels in milk (p = 0.000) and saliva of mothers (p = 0.003) increased and were associated with a reduction in the length of their pregnancy. Gundacker et al. (2002) reported that the amount of mercury in the milk of mothers with a weight of less than 60 kg and a pregnancy period of less than 37 weeks, frequent consumption of grains and legumes, the mother's use of various vitamins, and residence in urban and industrial areas increases (Gundacker et al., 2002). Bjorklund et al. (2019) evaluated the amount of exposure to mercury in pregnant women and its effects on fertility and pregnancy outcomes, and concluded that in pregnant women, mercury passes through the placental membrane, which can cause spontaneous abortion, premature birth, congenital malformations, and fetal growth retardation (Bjørklund et al., 2019). Dudarev et al (2009) reported that the average level of mercury in the blood of mothers who had premature birth, low birth weight and spontaneous abortion was 30% higher than normal women. Their results also indicated when mercury concentrations exceeded 2 µg/L plasma, the relative risk of preterm delivery and low birth weight children and spontaneous abortions increased (Dudarev et al., 2009). The role of mercury as a toxicant in sexual reproduction and pregnancy has been completely neglected compared to the great attention that has been paid to reports on the neurotoxicology of this heavy metal. Despite some conflicting evidence, the role of mercury in human reproduction remains a major concern. The results of the present study indicate that nursing women and women of reproductive age should avoid minimal contact with mercury due to its potential adverse effects on fetal development. Unfavorable consequences on pregnancy and fetal development pathology (premature birth, low birth weight, miscarriage, stillbirth, congenital malformations) are the obvious exposure of the mother to mercury.
In the present study, the average concentration of mercury in the milk of mothers who had two children (first group) was 1.20 ± 0.73 µg/g, and mothers who had more than two children (second group) was 0.85 ± 0.71 µg/g. In a study of 100 nursing mothers in Madrid, Spain, the average concentration of mercury was 0.53 µg/L. Also, the results of the study showed that there was a significant relationship between the levels of mercury in breast milk, the age of the mother and the number of children, and a lower concentration of mercury was reported in older mothers with more children (García-Esquinas et al., 2011). In a study conducted by Bahmani and Maleki (2018) with the aim of investigating the amount of mercury, lead and cadmium in milk samples from 100 mothers in Kurdistan, the average concentration of mercury measured in the milk of mothers was 3.48 micrograms per liter. According to their findings, the average concentration of mercury in the milk of mothers with one child was higher than that of mothers with two or more children, which can be considered as a risk factor (Bahmani et al., 2018).
We found a negative correlation between fruit consumption of women and the concentration of mercury in their bodies (p = 0.004). This result is consistent with the results of Agah et al. (2010), Papadopoulou et al. (2019) and Walker et al. (2020), which showed a negative correlation between fruit and vegetable consumption and mercury accumulation in mothers' bodies (Agah et al., 2010; Papadopoulou et al., 2019; Walker et al., 2020). Several mechanisms cause the effect of fruit on mercury concentration. Photochemical substances may interact with toxic metals in the body in various ways, such as absorption and excretion, transport, binding to target proteins, metabolism and separation. A possible explanation is that the soluble fiber in the fruit can interfere with the absorption of mercury on the intestinal surface. Fruit consumption leads to an increase in antioxidants in the body. In fact, one of the properties of antioxidants in fruit is that it reacts with heavy metals or complexes and thus reduces their absorption in the body (Behrooz et al., 2013; Passos et al., 2007). Reduction of mercury due to the consumption of fruit due to the presence of soluble and insoluble dietary fiber, antioxidants of nutrients (vitamin C, E, selenium, beta-carotene), and other plant foods include polyphenols, flavonoids, anthocyanins and carotenoids (Feeney, 2004; Karimi et al., 2014). According to the results of our study consumption of fruits with high amounts of vitamin C such as oranges and kiwis is much more effective in reducing the level of mercury in the body. Passos et al. (2007) also reported an association between fruit consumption and low blood and hair mercury concentrations in Amazon coastal residents, suggesting a protective effect of fruit consumption against mercury exposure through dietary fish consumption (Passos et al., 2007). The most important findings of the present study are that fruit consumption seems to moderate the relationship between fish consumption and mercury exposure. The results show that for the same amount of fish consumption, eating fruit is associated with lower levels of hair mercury. In fact, women who had dental surfaces restored with amalgam and who ate large amounts of fish had higher levels of mercury than others. Therefore, fish consumption and dental amalgam restoration were among the most important routes of exposure to mercury in our study. Fruit and vegetable consumption, as well as the number of pregnancies, were effective in reducing mercury levels in the body of women in Barghi et al. (2012) studied (Barghi et al., 2012).
In the present study, a negative and significant relationship was observed between the weight of infants at birth and the amount of mercury in hair (p\(=\)0.002) and saliva of mothers (p\(=\)0.007). This means that the higher the mercury levels of mothers, the lower the birth weight of infants. Also, the results showed that mothers' mercury levels have no effect on the weight and height of five-month-old and two-year-old children. The result of this research is similar to the study of Dock et al. (1994), Grandjean et al. (2001), which showed a decrease in the birth weight of infants in mothers who had higher mercury levels (Dock et al., 1994; Grandjean et al., 2001). This can be due to the fact that a certain amount of this toxic metal accumulates in the fetal liver after the mother is exposed to mercury during pregnancy, and also because of the correlation between the level of toxic metals in the mother's blood and cord blood. The metal can freely pass through the paired barrier (Kim et al., 2017). A study found a significant positive association between mercury exposure of pregnant women in the United States and low birth weight in offspring (Zhu et al., 2018). In the present study, the average concentration of mercury in saliva (1.10 µg/g) was observed in Chabahar women. Based on the results, a positive correlation was observed between chewing gum (p= 0.001) and the amount of mercury in the mother's saliva. The average concentration of mercury in the saliva of mothers who chewed gum once to twice a week was 2.66±1.27 µg/g, compared to mothers who chewed gum less than once a month by 1.43±1.29 µg/g. In recent studies, it has been determined that amalgam continuously emits mercury in the oral cavity; This release increases with activities such as brushing teeth, chewing gum, and drinking hot liquids. In a study conducted by Saeed et al. (2011), they proved an increase in mercury levels in saliva after using chewing gum. According to their findings, people chewing gum had about 5 times more mercury than those who were exposed to occupational mercury (Saeed et al., 2011). Chewing gum continuously is associated with both increased oral mercury vapor levels and increased salivary total mercury levels. Behavioral features such as teeth grinding and chewing gum themselves accumulate mercury in large amounts in saliva (Tuček et al., 2020). WHO notes that exposure to mercury can be greatly increased by personal habits such as teeth grinding or chewing gum, and reports show that mercury levels in saliva after chewing gum, eating, or brushing teeth increases mercury 5 times (WHO, 2003).
Our analysis showed a positive relationship between mercury levels in saliva and mother's weight (p = 0.000). Mothers were divided into three groups based on their weight. Mothers weighing 45–60 kg were in the first group, mothers weighing 61–75 kg were in the second group, and mothers weighing 76–90 kg were in the third group. The average mercury in saliva was higher in mothers who weighed 76–90 kg than in the first and second groups. Gundacker et al. (2002) found that mothers with low weight (less than 60 kg) had significantly higher levels of mercury in their milk than others, while our study showed a positive correlation between body weight and mercury levels in maternal saliva (Gundacker et al., 2002). Vahter et al. (2000), Al-Sale et al. (2011), Cunha et al. (2013), also reported a result similar to our research, which indicates a significant correlation between the amount of mercury and the weight of the mother (Al-Saleh et al., 2011; Da Cunha et al., 2013; Vahter et al., 2000). In a study conducted by Fok et al. (2007) on the mercury concentration of 1057 mothers in Hong Kong, a decrease in height and weight gain of the mother had a significant effect on the increase in mercury concentration. They stated the reason for this is that the relationship between blood mercury concentration, height and weight of the mother can be explained, at least to some extent, by the difference in fat distribution (Fok et al., 2007).
To broader understand the toxicological effects of mercury in our study population, the concentration of mercury in milk, hair and saliva were compared with studies in other regions of the world (Tables 6, 7 and 8). Comparisons between our results for mercury levels in breast milk of mothers and those of other studies illustrated that the mean mercury in breast milk of mothers was lower than Hamadan (Vahidinia et al., 2019), Zimbabwe (Bose-O’Reilly et al., 2020), Ghana (Bansa et al., 2017), Moroccan (Cherkani-Hassani et al., 2021), Brazil (Rebelo et al., 2017) and Turkey (Yalçin et al., 2010) and higher than mothers in Norway (Vollset et al., 2019), Hungary (Ecsedi-Angyal et al., 2020) and Korea (Park et al., 2018).
Table 6
Comparison of the amount of mercury in milk in the present study with its values in other parts of the world
Reference | Range | Mean | Location |
This study | 0.21–1.7 | 1.23 | Chabahar |
Vollset et al, 2019 | 0.058–0.89 | 0.20 | Norway |
Vahidinia et al, 2019 | 1-28.7 | 2.8 | Hamadan |
Ecsedi-Angyal et al, 2020 | 0.1 -2 | 0.4 | Hungary |
Bose-O'Reilly et al, 2020 | 0.5–24.8 | 2.16 | Zimbabwe |
Bansa, et al., 2017 | 1.36–63.5 | 8.4 | Ghana |
Cherkani-Hassani, 2021 | 1.64–124 | 3.56 | Moroccan |
Rebelo et al, 2017 | 0.76–8.40 | 2.56 | Brazil |
Park et al, 2018 | 0.08–5.66 | 0.94 | Korea |
Yalcin et al, 2010 | 0.3–6.90 | 3.42 | Turkey |
Table 7
Comparison of the amount of mercury in hair in the present study with its values in other parts of the world
Reference | Range | Mean | Location |
This study | 0.67-3.00 | 1.81 | Chabahar |
Barghi et al, 2012 | 0.11–3.57 | 0.37 | Ahvaz |
Vieira et al, 2013 | 0.72–20.08 | 8.2 | Brazil |
Abdoel Wahid et al, 2023 | - | 7.87 | Suriname |
Chien et al, 2010 | 0.02–16.34 | 2.12 | Taiwan |
Nakamura et al, 2023 | 0.40–19.16 | 2.65 | Japan |
Al-Saleh et al,2015 | - | 1.67 | Saudi |
Sekovanic et al, 2020 | 0.02–2.38 | 0.55 | Croatian Coast |
Fakour et al, 2010 | 6.320 − 0.150 | 1.281 | Mashhad |
Grandjean et al, 2014 | 2.63–7.85 | 4.35 | Faroe Islands |
Cerbino et al, 2018 | 0.13–20.48 | 5.24 | Brazil |
Gonzalez et al, 2019 | 0.01–30.12 | 2.54 | Peru |
Table 8
Comparison of mercury levels in saliva in the present study with its values in other parts of the world
Reference | Range | Mean | Location |
This study | 0.20–1.99 | 1.10 | Chabahar |
Fakour et al, 2010 | 0.001–40.5 | 4.14 | Mashhad |
Ganss et al, 2000 | 0.10–45.46 | 2.97 | Germany |
Agha Hosseini and Vaziri, 2004 | - | 0 .16 | Tehran |
Sousa et al, 2022 | 2.0–11.1 | 3.8 | Switzerland |
Zimmer et al, 2002 | 6.7–406 | 76.4 | Germany |
Oner,et al, 2020 | 0.50- 0.60 | 0.13 | Turkey |
Pigatto et al, 2013 | 0.1–168 | 38.1 | Italy |
Comparing maternal hair mercury in the present study and other regions of the world incicated that the mean mercury in hair of mothers in the present study is lower than Brazil (Cerbino et al., 2018; Vieira et al., 2013), Suriname (Abdoel Wahid et al., 2023), Taiwan (Chien et al., 2010), Japan (Nakamura et al.), Faroe Islands (Grandjean et al., 2014), Peru (Gonzalez et al., 2019) and higher than Ahvaz (Barghi et al., 2012), Saudi (Al-Saleh et al., 2015), Croatian Coast (Sekovanić et al., 2020) and Mashhad (Fakour et al., 2010).
In this paper we have compared the saliva mercury values of mothers with several recent studies and shown that the mean mercury of saliva of mothers was found to be lower than Mashhad (Fakour et al., 2010), Germany (Ganss et al., 2000; Zimmer et al., 2002), Italy (Pigatto et al., 2013), and Switzerland (Sousa et al., 2022) and higher than Tehran (AGHA and BEGIANIAN, 2004) and Turkey (Öner et al., 2020). Considering the high amount of fish consumption of residents in the coast of the Oman Gulf, the fish consumption limits should be applied to nursing women and sensitive groups. It is also necessary to monitor and implement environmental standards to prevent the entry of pollutants into the marine environment.
The estimated mean of daily intake in current study was attained 0.2 (µg/kg body wt/day), which was less than the values recommended by the World Health Organization (0.5 µg/g body weight) (WHO/FAO, 1989) and US EPA a reference value of inorganic mercury (0.3 mg /kg body weight/day) (USEPA, 1997). The study by Costa et al. (2005) in the Brazilian provided the mean and maximum mercury intake of 0.86 and 3.46 mg/kg bw/day, respectively (Da Costa et al., 2005). The authors expressed that 56% of infants demonstrate daily intake higher than the WHO recommended value (0.5 mg/kg bw/ day).
The average amount of mercury in milk, hair and saliva was measured in this study. According to the correlation between the concentration of mercury in saliva and hair of mothers, there was found a significant correlation and a weak correlation between the concentration of mercury in milk and hair and there was also milk and saliva). WHO has announced the mercury level of 1.4–1.7 µg/g as a normal level in breast milk. In this study, the average level of mercury in breast milk samples was lower than the recommended standard of WHO (WHO, 2003). The USEPA guideline limit is 5 µg/g (USEPA, 1997) and the WHO normal level and WHO threshold level are 2 and 5 µg/g respectively for hair mercury (Bratel et al., 1997). The NOAEL limit for Hg in female hair is 10 µg/g (Al-Majed and Preston, 2000), and the Canadian Health Guide has identified 6 µg/g as the increased risk level (Wheatley, 1979). In the present study, the average concentration of mercury in mothers' hair is lower than the standard values by different organizations. The concentration of mercury in milk exceeded the WHO normal level in 8.5% and hair mercury in 12.5% of mothers. Unfortunately, no normal limit for mercury in saliva has been reported in the WHO guidelines, although some studies have reported 5 µg/L as the recommended level for removing amalgam restorations, and other studies have reported a concentration of 25 µg/L as the threshold for the concentration. Salivary mercury (Ganss et al., 2000). The amount of total mercury in the saliva of the mothers of this study was lower than the recommended limit for removing amalgam restorations. 10% of the saliva samples also had mercury above the recommended critical limit for removing amalgam restorations (5 µg/g).