Sci Total Environ (2011)
Mercury (Hg) burden in children: The impact of dental amalgam
Al-Saleh I, Al-Sedairi A
The risks and benefits of using mercury (Hg) in dental amalgam have long been debated. This study was designed to estimate Hg body burden and its association with dental amalgam fillings in 182 children (ages: 5–15 years) living in Taif City.
Hg was measured in urine (UHg), hair (HHg) and toenails (NHg) by the Atomic Absorption Spectrophotometer with Vapor Generator Accessory system. Urinary Hg levels were calculated as both micrograms per gram creatinine (μg/g creatinine) and micrograms per liter (μg/L). We found that children with amalgam fillings (N=106) had significantly higher UHg-C levels than children without (N=76), with means of 3.763 μg/g creatinine versus 3.457 μg/g creatinine, respectively (P=0.019). The results were similar for UHg (P=0.01). A similar pattern was also seen for HHg, with means of 0.614 μg/g (N=97) for children with amalgam versus 0.242 μg/g (N=74) for those without amalgam fillings (P=0).
Although the mean NHg was higher in children without amalgam (0.222 μg/g, N=61) versus those with (0.163 μg/g, N=101), the relationship was not significant (P=0.069). After adjusting for many confounders, the multiple logistic regression model revealed that the levels of UHg-C and HHg were 2.047 and 5.396 times higher, respectively, in children with dental amalgam compared to those without (Pb0.01). In contrast, a significant inverse relationship was seen between NHg levels and dental amalgam fillings (P=0.003). Despite the controversy surrounding the health impact of dental amalgam, this study showed some evidence that amalgam-associated Hg exposure might be related with symptoms of oral health, such as aphthous ulcer, white patches, and a burning-mouth sensation. Further studies are needed to reproduce these findings.
The present study showed that significant numbers of children with or without amalgam had Hg levels exceeding the acceptable reference limits. The detrimental neurobehavioral and/or nephrotoxic effects of such an increased Hg on children should be a cause of concern, and further investigation is warranted.
Our results are alarming and indicate an urgent need for biomonitoring and assessment of exposure. Changes in dental practices involving amalgam, especially for children, are highly recommended in order to avoid unnecessary exposure to Hg.
Mercury (Hg) is a naturally occurring metal that exists in three chemical forms: organic, inorganic and elemental. Each form has its own profile of toxicity and source of exposure. While diet, especially fish and other seafood, is the main source of exposure to organic Hg, dental amalgam is an important source of elemental Hg vapor (Clarkson and Magos, 2006). Amalgam consists of approximately 50% Hg and other toxic metals (WHO, World Health Organization., 2003). Rigorous chewing and brushing of the teeth stimulate the release of Hg vapor from amalgam surfaces (Barregard et al., 1995; Isacsson et al., 1997; Ganss et al., 2000). All forms of Hg have adverse health effects at a high level of exposure. Allegations of the role of Hg in dental amalgam and its effects upon the immune system, renal system, oral and intestinal bacteria, reproductive system and the central nervous system have been ongoing for many decades (Bates, 2006). Many calls to continue, reduce, or ban its use have been issued, while some suggest that patients should be informed of the recognized benefits and risks (Spencer, 2000; Mitchell et al., 2005; Martin and Woods, 2006). Despite the debate over the safety of dental amalgam fillings, amalgam has also been widely used to restore posterior teeth in pediatric dentistry (Fuks, 2002). Few studies have assessed the safety of dental amalgam restorations in children, and the majority of studies on children have found no significant associations between dental amalgam fillings and neuropsychological or renal effects or immune functions (Bellinger et al., 2006; DeRouen et al., 2006; Woods et al., 2008; Shenker et al., 2008; Ye et al., 2009).
However, most authors still believe, as a precaution that future use of amalgam should be avoided since it does involve some level of exposure to Hg. Oken and Bellinger (2008) suggest that even though dental amalgam seems safe and free of adverse neurocognitive effects in children, one cannot exclude the possibility that exposure at younger ages, or follow-up for a longer duration, might reveal some evidence of harm.
Although most dentists and health professionals have accepted its safety, low cost, and durability for more than 150 years (Newman, 1991), amalgam is still surrounded by some unsettled controversies about its consequences on health. Few restrictions limit the use of amalgam worldwide. Sweden may become the first country to entirely eliminate the use of amalgam (Gelband, 1998). Germany has recommended the restriction of its use in young children, pregnant women, and patients with severe kidney problems (Harhammer, 2001). Its use has likewise seen a decreasing trend in the USA, Australia, Scandinavia, and to a lesser extent in the UK (Burke, 2004). A study by Beazoglou et al. (2007) estimated the financial impact of a ban on amalgam restorations for selected groups of the population. Such a ban would produce substantial short- and long-term increases in expenditures for dental care, decreases in use, and increases in untreated diseases. The authors recommended that governments should seriously consider these effects when contemplating possible restrictions on the use of amalgam restorations. The US Food and Drug Administration (FDA) has recently designated a special regulatory control by classifying dental amalgam into class II to provide reasonable assurance of its safety and effectiveness (FDA, 2009). The FDA's website now states that "Dental amalgams contain Hg which may have neurotoxic effects on the nervous systems of developing children and fetus." The FDA is advising pregnant women, people who may be more sensitive to Hg exposure, and individuals with existing high levels of Hg to discuss options for dental filling with their health practitioner.
Due to limited research on this issue in the Saudi environment, dentists openly disagree on the benefits of amalgam. Opinions expressed in local (newspaper) media or the internet vary. The scientific evidence is obviously not overwhelming. The Saudi Dental Society clearly supports the use of amalgam on the grounds of safety and consistency with the practices of international dental associations particularly those of the USA, Canada, and EU countries, and the approval of the World Health Organization. The last meeting of the Saudi Dental Society recommended further field studies on the subject. Dental caries may affect more than 90% of children (Al Dosari et al., 2004; Al-Malik and Rehbini, 2006) which offers a good argument for its persistent use. It might, however, pose potential dangers. Given the number of children with dental caries, the demand for the use of dental amalgam is expected to increase. Although data on the use of amalgam in children are still not available, Mahmood et al. (2004) conducted a cross-sectional study of 10 polyclinics within the metropolitan area of Riyadh and found that amalgam was the most commonly used restorative material (53%). Al-Saleh and Shinwari (1997) reported concentrations of urinary Hg in females with amalgam fillings of 12.04 μg/g creatinine, compared to those without of 8.66 μg/g creatinine. The authors also found that such exposure may be associated with a deterioration of renal function.
The toxicokinetics of Hg, such as absorption, distribution, metabolism, and excretion, is highly dependent on the form of Hg (Pavlogeorgatos and Kikilias, 2002). Dental amalgam fillings are the primary source of inorganic Hg, which is predominately excreted through urine (WHO, 2003). Organic Hg exposure can be assessed in hair or whole blood (Goldman and Shannon, 2001) because only 4% of the dose is excreted in the urine (Smith et al., 1994). Urinary Hg is not considered a good index of organic Hg body burden (WHO, 1990). Many studies have used urinary Hg to estimate exposure to amalgam fillings in children (Pesch et al., 2002; Levy et al., 2004; Trepka et al., 1997). However, Hg measurement in other non-invasive biological material such as hair, nails, and saliva has also been used in some epidemiological studies (Zimmer et al., 2002; Ohno et al., 2007; Esteban and Castaño, 2009; Fakour et al., 2010a). Large variability is often observed in biological monitoring, which suggests uncertainties in the reliability of the biomarkers for estimating Hg exposure and its health effects (Berglund et al., 2005). A recent review by Esteban and Castaño (2009) revealed that choice of a matrix depends on various factors such as the target chemical, toxicokinetics of the chemical, limit of detection, and available amounts.
Based on the ongoing controversy over the safety of dental amalgam, we conducted this study to examine the extent of Hg exposure in children 5–15 years of age with and without dental amalgam fillings. We also measured Hg in various matrices (urine, hair and toenails) to assess precisely their sensitivity, specificity, and reliability as biomarkers of Hg exposure from dental amalgam fillings.
4.1. Influence of dental amalgam on Hg burden
This study is the first in Saudi Arabia to evaluate the levels of Hg in children measured in various matrices and to assess Hg's relation to dental amalgam fillings. Our results have clearly demonstrated the association between dental amalgam fillings and the levels of Hg in the urine and hair of children. These observations support previous studies (Levy et al., 2004; Woods et al., 2007). In general, our overall means of urinary Hg measured as UHg (3.749 μg/L) and UHg-C (3.763 μg/g creatinine) in children with dental amalgam fillings were much higher than those reported in other studies, as shown in Table 7. Although, the World Health Organization established an upper limit of normal UHg for unexposed adults of 20 μg/L, respective levels for children have not yet been established (WHO, 2003). Background levels of UHg and UHg-C in an unexposed population are generally expected to be b10 μg/L (ATSDR, 1999) and 5 μg/g creatinine respectively (IPCS, 2003). In this study, the levels of UHg in children without amalgam were in the range of general background levels (b10 μg/L) in unexposed populations. UHg-C levels were over 5 μg/g creatinine, with an average of 9.163 μg/g creatinine (range: 5.355– 19.242 μg/g), in 19.7% of children without amalgam. In children with dental amalgam, 2.8% had UHgN10 μg/L, and 22.6% had UHg-CN5 μg/g creatinine. Our results should be interpreted with caution because WHO and ATSDR estimates are only applied to healthy adults and not to children. In general, very limited information is available on background Hg exposure in children. Many published data indicate that the vast majority of unexposed children should have urinary Hg levels b5 μg/L (Ozuah et al., 2003; Bose-O'Reilly et al., 2010).
However, comparing our results to the recently defined reference value for UHg by the German Commission of Human Biomonitoring for UHg in children (3–14 year olds), 90.8% of children's urinary Hg concentrations without amalgam were above the reference value of 0.4 μg/L (Schulz et al., 2009). In contrast, the authors reported that UHg for children with more than two dental amalgams was 3.1 μg/L at the 95th percentile. In this study, children with dental amalgam had a considerably higher 95th percentile of 8.538 μg/L, and more than 47% had UHg levels over 3.1 μg/L, with a range of 3.129 to 15.575 μg/L.
Surprisingly, 51.3% of children without amalgam also had UHg levels over 3.1 μg/L, with a range of 3.113 to 7.946 μg/L and a 95th percentile of 5.73 μg/L. Few authors have proposed background levels of UHg-C for children. Batáriová et al. (2006) considered 0.37 μg/g creatinine as a background level of UHg-C which corresponds to a level of UHg of 0.42 μg/L. Using this reference value from Batáriová et al. (2006), 99.1% and 88.2% of children in our study with and without dental amalgams, respectively, had UHg-C levels above 0.37 μg/g creatinine. Although, the results presented here generally support the contribution of dental amalgam filling to urinary Hg levels, we also notably, found high urinary Hg among children that did not have dental amalgam. Our findings likely suggest the possible contribution of other sources.
Hair has been widely used as a bioindicator to evaluate human exposure because of its growth rate (1 cm per month) and the tendency of Hg to accumulate in the hair follicles over long-periods of time (WHO, 1990). Many studies have used hair to assess Hg exposure from the consumption of fish (Srogi, 2007). To our knowledge, few studies have examined the relation of HHg to dental amalgam. Interestingly, our study revealed that HHg was also highly associated with dental amalgam fillings, and children with dental amalgam had HHgN2.5 times higher than those in children without. The finding was in accordance with the levels reported in adults (Babi et al., 2000; Lindow et al., 2003; Fakour et al., 2010a,b; Ryo et al., 2010) but not in children (Batista et al., 1996; Pesch et al., 2002;Dunn et al., 2008; Kruzikova et al., 2009). In children, 62.6%with dentalamalgamate fish and 37.4%did not. Multiple logistic regression modeling confirmed that the presence of amalgam fillings leads to an increased risk of high HHg in children, even after adjusting for seafood consumption (OR=5.396). As shown in Table 7, the levels of HHg found in children with amalgam fillings (0.614 μg/g dry wt.) were higher than those reported by Dunn et al. (2008) and Barregard et al. (2008) of 0.3 and 0.4 μg/g dry wt., respectively. Comparable studies are very limited, and further research is required to confirm our observations. Hair is generally the preferred matrix to determine exposure to methylmercury, but hair may also be used an alternative to urine as an indicator of long-term Hg exposure from dental amalgam fillings. Hair is a non-invasive matrix especially useful in studies involving children.
The mean HHg level for all children in this study was 0.453± 1.155 μg/g dry wt., which was notably higher than levels found in other studies: 0.18 μg/g in the Czech Republic (Puklová et al., 2010); 0.23 μg/g in Germany (Pesch et al., 2002), and 0.31 μg/g in the USA (Surkan et al., 2009). Our levels, however, were lower than some others: 0.74 μg/g Seoul, Korea (Kim et al., 2008); 0.94 μg/g in Barcelona, Spain (Diez et al., 2009); 0.938 μg/g in Italy (Montuori et al., 2006); 2.2 μg/g South China (Ip et al., 2004); and 0.96 μg/g in Granada, Spain (Freire et al., 2010). Both the US Environmental Protection Agency and the National Academy of Sciences recommend to keep hair Hg levels at b1.0 μg/g, corresponding to a reference dose (RfD) of 0.1 μg/kg body weight per day (US EPA, 1997). This reference dose was not based on a direct adverse effect of HHg but was derived from maternal HHg levels that correspond to the Hg levels in cord blood, which over a lifetime of exposure should not result in adverse effects. Recent studies by both Surkan et al. (2009) and (Freire et al., 2010) reported that HHg levels"1 μg/g (EPA reference dose) in children from consumption of fish were associated with delays in neuropsychological development. In our study, hair Hg levels exceeded the US EPA reference dose of 1 μg/g in 10 children (5.8%), with a range of 1.003 to 14.304 μg/g dry wt. Of these, only one child had a level of HHg higher than the tolerance limit of 10 μg/g set by the WHO (1990). Nine children had dental amalgam fillings, and eight of these consumed fish. These children were apparently concurrently exposed to various forms of Hg. We know that inorganic Hg does not incorporate into hair in a fashion similar to organic Hg (Dunn et al., 2008; Diez et al., 2008; Kruzikova et al., 2009). A study by Magos and Clarkson (2008) revealed that hair accounts for only a small fraction, less than 10%, of the total elimination of organic Hg from the body if this form of Hg is the main source of exposure. Other species of Hg are eliminated by hair but at a lower rate. Further studies on the chemical forms of Hg (organic and inorganic) in hair samples are needed in order to identify the sources of Hg in the hair of Saudi children.
The levels of Hg in toenails in this study are generally very small for both groups of children,with amalgam(0.163±0.339 μg/g drywt.) and without amalgam (0.222±0.212 μg/g dry wt.). Interestingly, NHg was inversely associated with dental amalgam fillings, which might reflect another chemical form of Hg. One of the possible explanations for this finding seems related to the association between recent vaccination (b1 year) and dental amalgam fillings. As shown in Table 3, 79.3% of the children with amalgam were vaccinated. After excluding children without amalgam fillings,we stratified NHg levels by the use of vaccine. Non-vaccinated children tend to have significantly higher NHg (0.27± 0.534 μg/g dry wt., N=37) than vaccinated children (0.101± 0.095 μg/g dry wt., N=64), with a P-value of 0. In contrast, when we excluded children with amalgam, we found the reverse.
Non-vaccinated children had lower NHg (0.212±0.230 μg/g dry wt., N=45) than vaccinated children (0.252±0.156 μg/g dry wt., N=16) with a moderate level of significance (P=0.065). Although we cannot rule out a chance finding, the excretion pattern of Hg appears to differ in vaccinated and nonvaccinated children with amalgam. Thimerosal, a preservative in vaccines, contains 49.6% Hg by weight and is metabolized to ethylmercury and thiosalicylateethyl (American Academy of Pediatrics, 1999). Haley (2005) reported a synergistic mechanism between thimerosal and other vaccine components that affects Hg toxicity. This might let us think that if some synergistic effect between amalgam fillings and thimerosal may be responsible for reduced Hg excretion in these children.
The overall Hg mean level in toenails found in this study was 0.185±0.298 μg/g dry wt. The data from this study is difficult to compare to data from others, only carried out in adults (Ohno et al., 2007; Rees et al., 2007; Joshi et al., 2003). A threshold of 0.33 μg/g for Hg in toenails has been suggested to be equivalent to the EPA reference dose of 1 μg/g of Hg in hair (US EPA, 1997; Choi et al., 2009).
Of all tested children, 11.7% (N=19) had levels of NHgN0.33 μg/g, with a range of 0.332–3.337 μg/g. Of these seven had dental amalgam fillings. Guallar et al. (2002) found an increase in the risk of myocardial infarction among men with toenail HgN0.66 μg/g. In our study, four children (2.5%) had toenail Hg greater than 0.66 μg/g, in a range of 0.778 to 3.337 μg/g. Of these four, only one child had four dental amalgam fillings.
4.2. Relationship between hair and nail Hg
Like hair, nails have also been recommended as a biomarker of long-term Hg exposure. Rodushkin and Axelsson (2000), Ohno et al. (2007), and Zolfaghari et al. (2007) reported correlation coefficients between HHg and NHg of 0.9, 0.858, and 0.57, respectively. Hair and nail matrices are assumed to have unique properties of keratin compositions (Kitahara and Ogawa, 1991).
Our study has shown that the overall level of HHg (0.453 μg/g dry wt.) was more than two times higher than Hg levels in toenails (0.185 μg/g dry wt.), with a weak but significant correlation (Pearson's r=0.25). These values suggest that hair and nails may share similar sources and patterns of exposure, but a previous study revealed that the higher HHg concentration might be attributable to the differences in chemical composition, particularly the sulfur content, and in blood flow during growth (Suzuki et al., 1989). Rodushkin and Axelsson (2000) suggested that differences in mode of deposition (endogenous and exogenous) could account for higher levels of Hg in hair. Ohno et al. (2007) estimated the strength of the correlation between HHg and NHg using a reciprocal-x model of regression analysis. Applying their approach, we found that the 95% CIs of the regression line of HHg (Y-axis) on NHg (X-axis) ranged from 0.583 to 1.728 for the slope and from 0.397 to 0.806 for the intercept.
The ratio between HHg to NHg had a mean±SD of 5.047±10.427, ranging from 0.2 to 92.37. Our ratio is higher than the ratios of 2.38 and 2.56 reported by Ritchie et al. (2002) and Ohno et al. (2007), respectively, but these differences might be due to differences between children and adults. Budtz-Jørgensen et al. (2004) suggested that strands of hair in children tend to retain more Hg than adults. Also, our ratio does not fall within the 95% CI of the slope. The exclusion of two outliers did not change the results. The two matrices, therefore, might not fully share sources and/or times of exposure, and the forms of Hg in hair and nails are likely different. When we reexamined our multiple logistic regression model by testing the influence of dental amalgam fillings on hair-to-nail Hg ratio (HHg/NHg) instead of each separately, after adjusting for confounders as described in Table 4, the OR of HHg/NHg for children with dental amalgam was 2.988 (95% CI: 1.824–4.896) significantly higher than for those without (P=0). UHg-C was also predictive of dental amalgam fillings (OR: 2.034, 95% CI: 1.306–3.165). Nails, like, hair, reflect past exposure but have the potential to integrate multiple routes of exposure over long periods of time because of the slow rate of growth of toenails (Yaemsiri et al., 2010). The reliability of nails to determine associations with dental amalgam fillings may thus be limited, especially if the levels of Hg in nails are low. The hair-to-nail Hg ratio (HHg/NHg), though, was influenced by the presence of dental amalgam fillings, which might suggest that the proportion of inorganic Hg in hair was higher than organic Hg. Organic Hg mainly incorporates in hair but usually transforms into inorganic Hg as it enters the hair follicle and/or as the hair grows (Clarkson, 1997; Dolbec et al., 2001). Without knowledge of the species present, this mechanism cannot be ruled out.
4.3. Risk factors associated with Hg exposure in dental amalgam group
Using multiple regression analyses, the main risk factors associated with urinary Hg body burden in children with dental amalgam fillings were gender, aphthous ulcer, and brushing once a day (Table 6a,b). Girls with dental amalgam had higher urinary Hg levels (both UHg and UHg-C) in comparison to boys, in agreement with Woods et al. (2007). A recent experimental study revealed that differences in whole-body retention and accumulation of Hg in organs are regulated by genetic factors and gender (Ekstrand et al., 2010) with higher Hg renal retention in males than in females. We also noticed that an increase in HHg was associated with the feeling of a burning mouth-sensation. An immediate hypersensitivity reaction associated with the Hg in amalgam restorations resulted in erythematous lesions, severe burning, and itching, and difficulty in breathing (Kal et al., 2008).
Children living in the western part of Taif had significantly lower levels of HHg than those living in the northern area. Interestingly, the levels of Hg in the hair of subjects with dental amalgam were not influenced by seafood consumption. This might be due to the small size of children who do not eat seafoods (N=8) versus fish eaters (N=97). In contrast, HHg in children without dental amalgam who ate seafood (0.275±0.191, N=56) was significantly higher than in those without amalgam (0.141±0.084, N=18), with P=0.001. Our study revealed that children attending B and C dental clinics had higher NHg levels than those attending A dental clinics. This finding indicates the existence of potential Hg source(s) in these particular dental clinics. The model also reinforced our earlier observation (Table 4) that vaccinated children had lower NHg levels than nonvaccinated children, which should be examined further. Our study also revealed some association between white patches and NHg levels. Further research on this issue is needed, especially for children with hypersensitivity to Hg amalgam (Kazantzis, 2002). These findings are of interest and indicate a need for further research with much larger sample sizes for understanding the possible influences of these factors on Hg levels. Lastly and in contrast to many studies, our results found no association between Hg levels in any matrix and the number of dental amalgam fillings (Pesch et al., 2002; Woods et al., 2007; Dunn et al., 2008). Khordi-Mood et al. (2001) also found that the increase in UHg in children 9–12 days after receiving dental amalgam fillings did not correlate with the number of fillings. Mortda et al. (2002) reported similar results for HHg and NHg. According to Maserejian et al. (2008), determining the number of dental amalgam fillings is insufficient if only a rough estimate of current Hg exposure due to amalgam is needed. However, more detailed information on surfaces can provide a better estimate of the association, and simple counts of the number of dental fillings are unlikely to capture the full influence of amalgams in children over time. Finally, some limitations of this study need consideration for the interpretation of our findings:
(1) subjects were limited to the children of military staff, which might affect the ability to generalize to other sectors;
(2) no information on socioeconomic status was collected;
(3) no data on the size of amalgam restoration and surface area were collected; and
(4) some children (17%) had had their amalgam fillings for more than 12 months.
In conclusion, the results of our study clearly demonstrate the influence of dental amalgam fillings on the levels of Hg in urine and hair samples of Saudi children.
Our data also revealed that nails are not an appropriate bioindicator of Hg from dental amalgam fillings, at least not for low Hg levels. The hair-to-nail Hg ratio was influenced by dental amalgam fillings, which might indicate differences in the proportion of organic to inorganic Hg.
Despite the controversy surrounding the health impact of dental amalgam, this study provides some evidence that amalgam- associated Hg exposure might be related with symptoms of oral health, such as aphthous ulcer, white patches, and burning-mouth sensation.
Further studies are needed to reproduce these findings. By and large, the present study showed that significant numbers of children, whether with or without amalgam, had Hg levels exceeding the acceptable reference limits. These levels should be a cause for concern about the detrimental neurobehavioral and/or nephrotoxic effects on children, and further investigation is warranted.
Our results are alarming and indicate an urgent need for more biomonitoring and assessment of exposure. Changes in dental pactices involving amalgam, especially for children, are highly recommended in order to avoid unnecessary risks of exposure to Hg.