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The SCHER opinion states:
3. OPINION
3.1 Question 1
Are mercury releases caused by the use of dental amalgam a risk to the environment? The fate of mercury released from dental clinics as well as the fate of mercury released from air, water and soil from fillings placed in patients should be taken into account.
3.1.1 General comments
Over the past decade, the mercury releases to the environment due to dental amalgam use and disposal, and the potential consequences (risks) for human and environmental health has received increased attention. Although numerous publications are available describing one or more aspects of this issue, a comprehensive EU wide assessment of the human health and environmental risks of the Hg used in dental amalgam is – as far as could be established – not available. This type of risk assessment requires, next to extensive general information on the effects to humans and (various) environmental species, more detailed information on possible regional-specific differences in the use, release and fate of Hg originating from dental amalgam. This includes detailed quantitative information on the use and release pattern in all EU-27 countries, possible country-specific abatement measures, and differences in the fate of mercury due to regional-specific municipal wastewater treatment and sludge application practices.
As this type of information is not available to SCHER, a comprehensive risk assessment cannot be performed by the Committee. However, SCHER will attempt at addressing the question posed by the Commission based on individual reports made available and a screening level risk assessment performed according to EU Technical Guidance Document (TGD, 2003).
The only report known to SCHER which has assessed the environmental risk of Hg originating from dental amalgam is the RPA study (Floyd et al., 2002). The report concluded that the use of Hg in the products studied – including dental amalgam – were unlikely to pose significant risks to human health and the environment. These authors did, however, clearly state that the results of their study should be treated with caution as the model (EUSES) used for calculating the environmental Hg concentrations is not directly suitable for use with metals. The CSTEE was requested to evaluate this report and concluded in its opinion of 12 November 2003, that due to the high uncertainty associated with the model calculations and thus the high uncertainties regarding predicted environmental and human exposures, no science-based conclusions concerning the risk of the studied products can be made (CSTEE, 2003).
Several studies report on a mass flow analysis of Hg in the environment and have assessed the consumption and release of mercury used in dental amalgam.
The Danish Environmental Protection Agency (EPA) has performed a detailed analysis of the consumption – in relation to its various uses – of mercury in Denmark during 2002-2003 (Danish EPA, 2004). It is reported that the greatest intentional use of this metal is in dental fillings with a yearly use of 1200 (+/- 100) kg Hg. This corresponds to 34% of the total mercury consumption in Denmark. From this value, it is estimated that dental clinics discharge between 190 and 260 kg of Hg/y in their wastewater. The same study also estimated that a total of 120 to 180 kg Hg/y is disposed off in the form of extracted and lost teeth, 20-30% of which will enter the waste stream. Each year, approximately 170 kg of Hg is released to the atmosphere as a result of cremation and 70 kg enters the soil compartment as a result of burial. Finally it is reported that between 120 and 680 kg Hg is collected from strainers and extracted teeth, of that 50-140 kg enters the refuse stream and the remainder is collected and exported.
Very recently, the European Environmental Bureau (EEB) has published a detailed mass balance analysis of mercury used in dental applications (EEB, 2007). This report has examined – in a quantitative manner and across the EU-27 – all sources of amalgam Hg and the pathways by which it can enter the environment. It states that the EU-27, discharges 109 tonnes/y of mercury from dental practices and that mercury in the teeth of deceased persons contribute 14 tonnes Hg/y to the EU waste stream. The authors state that of this total of 123 tonnes, 77 tonnes will likely end up in various environmental media: i.e. 30 tonnes in soil, 23 tonnes in the atmosphere, 14 tonnes in surface water and 10 tonnes in ground water.
The RPA report estimates that approximately 70 tonnes Hg/year is released (into the environment) by the EU-15 (Floyd et al. 2002). The value given for Denmark is 1 ton/y which is comparable to the values reported in above mentioned report (Danish EPA, 2004). No further comparisons of the use quantities, release patterns and possible (predicted) environmental concentrations could be made as the type of information and calculations provided in the various reports is too diverse in nature.
As mentioned above, although this type of mass balance data contributes to the understanding of the magnitude and sources of mercury contamination caused by dental applications, it does not allow to asses the risks of Hg in amalgam in a quantitative manner. Hence, SCHER has attempted to perform a screening level risk assessment according to the EU TGD procedures (TGD, 2003).
3.1.2 Screening risk assessment
3.1.2.1 Direct risk for aquatic organisms: inorganic mercury
Information on emissions of mercury from dental practices can be assessed in a preliminary manner on the basis of Hg concentrations measured in dental clinic wastewaters. Two studies, covering current practice in Sweden and USA are available. The Swedish study (Hylander et al., 2006) presents measurements for several locations ranging from 0.77 to 74.1 mg Hg/l. It must be underlined that, due to legal requirements, Swedish clinics use amalgam separators prior to wastewater discharge. The efficiency of these systems was observed to be highly variable. Indeed, the wastewater concentrations measured after inspection and revision of these systems were about one order of magnitude lower, i.e. ranging from 0.23 to 6.6 mg Hg/l.
The concentrations measured in the USA (Stone et al., 2003) are similar to those reported for Sweden, ranging between 1.8 and 173 mg Hg/l for individual measurements. Site averages (5.4, 13.4 and 45.1 mg/l) were within the range observed for the Swedish locations. The presence of amalgam separators is not mentioned in the USA study, but the samples were collected from the liquid portion of the wastewater (avoiding settled material).
SCHER used the above information to estimate the releases of mercury to the wastewater system.
The Swedish study also reports annual release estimations, ranging from 0.32 to 83.8 g Hg per dental chair and per year, with a mean value of 14.5 g. Considering an average EU value of 80 dentists per 100,000 inhabitants (Eurostat web page, 2007), and the default values for a wastewater treatment plant (WWTP) described in the TGD (2003), the concentration of Hg in the WWTP inflow due to dental practice are estimated to be in the range of 3.5 to 918 µg Hg/l with and average value of 159 µg Hg/l. Assuming a retention at the WWTP of 96% due to sludge adsorption and a default dilution factor of 10, the expected Hg contribution from dental clinics in river waters receiving municipal effluents is calculated to range between 0.000014 and 0.0037 µg Hg/l, with an average value of 0.00064 µg/l or 0.64 ng/l.
It is clear that this contribution of Hg originating from dental amalgam use should be added to the natural and historical background concentrations as well as to the contribution from other Hg sources to fully assess the risks of Hg to the environment.
The comparison of these exposure estimations with the EC proposal for an Ecological Quality Standard (EQS) for direct effects of mercury on aquatic organisms (0.05 µg Hg/l as annual average and 0.07 µg Hg/l as maximum permissible concentration) indicates that the added risk to aquatic organisms from the contributions from dental clinics to the total mercury should be considered low.
3.1.2.2 Direct risk for soil organisms: inorganic mercury
A similar approach, using the generic TGD scenarios and default values, can be used for the preliminary assessment of the potential risk for soil dwelling organisms of mercury released from dental practice. Based on a default average production of 0.071 kg of sludge per person per day at the WWTP, the concentration of mercury in sludge as a consequence of releases from dental clinics is calculated to range between 0.001 and 2.4 mg Hg/kg dw with and average value of 0.42 mg/kg dw.
Considering that the reported EU average Hg concentration in sludge is 1.5 mg Hg/kg (http://ec.europa.eu/environment/chemicals/mercury
), it is suggested – based on this information – that the contribution of dental clinics represents about one third of the Hg total releases to the terrestrial compartment. From a risk assessment perspective these values are well below the current EU legal limits established under Directive 86/218/EEC. However, it should be mentioned that these limits have not been updated based on current knowledge. The added predicted environmental concentrations (PEC) soil resulting from the contribution of dental clinic emissions - following the TGD default values - range from 0.016 to 4.1 µg Hg/kg i.e. concentrations well below the reported NOECs for soil dwelling organisms (e.g. Verbruggen et al., 2001; de Vries et al., 2007). Thus, based on this screening risk assessment, a low direct risk to the soil compartment of dental Hg is expected.
The atmospheric emissions and further deposition of mercury from crematoria should be considered as an additional contribution of mercury from dental amalgams. The few measurements which are available indicate a large variability. The contribution from this source may be significant in some local scenarios, while the environmental relevance cannot be assessed without an in-depth analysis of the soil fate and ecotoxicology of mercury in soils based on recent developments concerning the environmental risk assessment of metals (e.g. SCHER opinions on the RAR for several metals).
3.1.2.3 Risks associated to the direct emissions of methylmercury from dental practice.
The concerns related to mercury in dental amalgams have been enhanced by the identification of methylmercury in wastewater from dental units in the USA. The measured concentrations where particularly high in tanks from large clinics (up to 0.2% of the total mercury) suggesting methylation within the tank. This maybe the result of the activity of sulphate reducing bacteria, which are present in the oral cavity of humans, and can therefore be released during the dental intervention. Methylation may also occur in the oral cavity but the methylmercury levels measured in the chair side wastewater where at least one order of magnitude lower that those measured in the tanks (Stone et al., 2003).
It should be noted that although the study was conducted in the USA, the levels of total mercury measured in the wastewater were similar to those reported for the EU.
Assuming 0.2% of the total mercury is released as methylmercury (Stone et al., 2003), and using similar exposure estimations as those conducted for inorganic mercury, the concentration of methylmercury in the WWTP inflow due to dental practice is estimated to be in the range of 0.000007-0.0018 MeHg µg/l with an average value of 0.0003 µg/l. Assuming a retention at the WWTP of 96% due to sludge adsorption and a default dilution factor of 10, the expected contribution from dental clinics in river waters receiving municipal effluents is estimated to range between 7 x 10-9 to 1.8 x 10-6 µg/l, with an average value of 3.2 x 10-7 µg/l. Also here, this value need to be added to the natural and historical background concentrations as well as to the contribution from any additional sources of methylmercury - including the methylation in the environment of the inorganic mercury released by the dental clinics - to assess the overall risk of methylmercury.
The main environmental concern for methylmercury is its potential for bioaccumulation and food web biomagnification resulting in a risk for secondary poisoning in ictivorous vertebrates. Thus, this screening risk assessment focused on secondary poisoning. It should be noted that the reported bioaccumulation factors (BAF) measured in the field for fish species collected at different locations range from about 20,000 to over 20,000,000. Using the larger values in this range, the releases of methylmercury (originating from dental amalgam) would exceed the EC proposal (within the Water Framework Directive (WFD)) of 20 µg methylmercury/kg in the prey of birds and mammals (EC, 2006a).
A preliminary risk estimation can be done by combining the TGD defaults with the individual values for each Swedish location and the field BAF (geometric means) reported in the WFD-EC document (EC, 2005). The Swedish values were considered as estimations of the expected releases to the WWTP. The default generic values of the TGD were used for estimating the PEC in water from these releases. The PECs were the multiplied by the BAF to estimate the expected concentration of mercury resulting from releases due to amalgam uses. Individual releases and BAF values were randomly combined using Monte Carlo analysis. The results are presented in Figure 1 and show that the risk of exceeding the EC proposal considering exclusively the direct emissions of methylmercury from dental facilities is of about 6%. If this contribution is assumed to represent about 10% of total anthropogenic contribution for methylmercury, the exceedance risk would rise to about 18%.
3.1.2.4 Risk associated to the environmental methylation of inorganic mercury: secondary poisoning, bioaccumulation and biomagnification potential for inorganic mercury releases.
The main concern related to the anthropogenic emissions of mercury into the environment is related to the well-known potential of this metal to bioaccumulate and biomagnify through the food chain resulting in high levels of exposure for top predators, including humans.
The bioaccumulation of inorganic mercury in biota - although significant and described even for the mercury present in dental amalgams (Kennedy, 2003) - is generally regarded to be of low relevance compared to that of organic forms of mercury and particularly methylmercury.
The potential for biomagnification is, therefore, related to the methylation of inorganic mercury which may result from both abiotic and biotic processes. The later seems to be the most relevant under environmental conditions.
The potential for bioconcentration of methylmercury in aquatic organisms is orders of magnitude higher than for inorganic mercury.
When the food-web bioaccumulation is considered, the overall bioaccumulation factor ratio between the concentration in the organisms and the concentration in water) may be well above one million (cf. above).
Although there are several models describing the bioaccumulation and biomagnification potential of mercury in different ecosystems, the variability - in terms of both the methylation potential and the overall biomagnification - is so high that no sound generic estimations can be done with the current level of knowledge.
In fact, the conclusion presented by the European Commission within the process of setting EQSs for mercury under the FWD was that “Due to the different site specific factors driving bioaccumulation of mercury in aquatic food webs, it seems on the basis of the current knowledge not appropriate to derive a general QSsecpois, water. An in depth assessment of the uncertainties associated with the bioaccumulation potential of (inorganic and organic) mercury and its toxicity to predators is required in order to derive reliable quality standards depending on site specific factors. Thus, it is suggested to set the QS for methylmercury for the time being for the concentration in biota only”.
SCHER supports this conclusion for both the aquatic and soil compartments and hence considers that it is not possible to conduct a quantitative assessment of the risk of inorganic mercury releases from dental amalgams for top predators. Nevertheless, the development of probabilistic risk estimations offers alternatives, and the possibility for conducting sensitivity analysis should be investigated (see question 4).
A preliminary assessment covering apparent methylation rates (overall result of the processes covering methylation, demethylation and transport from water column to sediment and vice versa) ranging from 0.0001 to 1% is presented in Figure 2. These results clearly show that assessing the methylation rate is a key element for a correct evaluation.
Source & ©: SCHER,
The SCHER opinion states:
3.3 Question 3
Comparison of environmental risks from use of mercury in dental amalgam and use of alternatives without mercury
Alternatives without amalgams for dental restoration often are resins generated by polymerisation processes. Data on toxic effects of resin monomers in animals and ecotoxicological data are not available from publicly accessible sources. However, since the materials used as a basis for resin generation are derivatives of methacrylic acids and glycidyl ethers, the well studied toxicology of methacrylate and its esters may be used as a basis for structure activity relationships to predict major toxicities.
Methylmethacrylate is rapidly absorbed after oral administration in experimental animals and is rapidly catabolized by physiological pathways to carbon dioxide. The major toxic effects of methylmethacrylate in animals are skin irritation and dermal sensitization. In repeated dose-inhalation studies, local effects on respiratory tissue were noted after methylmethacrylate inhalation. Neurotoxicity and liver toxicity were observed as systemic effects after inhalation of methylmethacrylate in rats and in mice to concentrations above 3000 ppm for 14 weeks. No developmental toxicity after methylmethacrylate with a NOAEC > 2000 ppm was observed. Methylmethacrylate is also clastogenic at toxic concentrations (EU-RAR 2002).
Regarding glycidyl ethers a detailed overview of the toxicity of these compounds based mainly on unpublished study reports is available (Gardiner et al. 1992). Based on this report, skin irritation and skin sensitization are the major toxicities observed. In addition, positive effects in genetic toxicity testing were seen with many glycidyl ethers at comparatively high concentrations.
Regarding the environmental risk, the available information is too limited for conducting a proper comparative assessment of amalgams and their alternatives. It should be noted that the assessment of environmental impacts of the substitute would require two complementary studies: a comparative risk assessment for the relevant environmental compartments, and a life-cycle assessment covering non ecotoxicological impacts such those related to energy and natural resources consumption, atmospheric emissions including greenhouse gases, waste production, etc.
Source & ©: SCHER,
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