Alex Cain of the EPA discusses the findings of his study which evaluated the substance flow analysis of dental mercury releases into the environment.
In 2007, Alex Cain of the EPA published a study in the Journal of Industrial Ecology that evaluated the Substance Flow Analysis of Dental Mercury Releases Into The Environment. His comprehensive analysis evaluated several release pathways of dental mercury that had not been investigated before. The findings of the study show that the combined total of all dental mercury releases are much more significant than previously thought and warrant being addressed by our government. Of note: while the study examined all sources of mercury released into the environment, this article focuses solely on the dental mercury releases.
Substance Flow Analysis of Dental Mercury Releases Into The Environment
Modeling Mercury Flow
Estimates of mercury disposed of annually were derived in a number of different ways, depending on the product. For dental amalgam, the model estimates three types of mercury.
- First, mercury that is placed into the teeth of dental patients.
- Second, it estimates mercury in “contact amalgam” (that comes into contact with teeth, but is left over after the tooth is filled).
- Third, it estimates mercury in “noncontact amalgam” that is prepared in the dental office but that does not come into contact with patients.
The model separately estimates mercury removed from living patients’ teeth, based on estimates of the number of removals that dentists conduct, and adds the result to the “contact amalgam” total.
Distribution factors are applied to contact and noncontact amalgam to determine the amount of mercury that flows to the solid, infectious or hazardous waste management system, to mercury recycling, or to the dental office wastewater system.
Once in the dental wastewater system, the mercury can be captured by a chairside trap, where again it can flow to the solid, infectious, or hazardous waste management system, to mercury recycling, or back into the dental office wastewater system. Mercury that re-enters the dental office wastewater system can then be captured by a vacuum system filter or an amalgam separator, or be discharged to a sewage treatment plant.
Distribution factors indicate the likelihood that a product, or mercury in a product, will follow a given pathway. We used distribution factors to predict the fate of mercury after it is disposed of as municipal solid waste (MSW), hazardous waste, infectious waste, or wastewater sludge. The distribution factors indicate how much of each category is incinerated versus landfilled, composted, or spread on land. In many cases, we used product- pecific distribution factors to estimate the fraction of the inventory of a product that will be disposed of every year. In addition, we used product-specific factors for the percentage of a product that is recycled rather than discarded as waste, or that ends up in the air and steel recycling system.
For example, dental amalgam distribution factors account for the fraction of amalgam waste caught in a chair-side trap or vacuum filter and for the amount of amalgam in bodies that get cremated or buried.
Whereas distribution factors indicate the amount of mercury expected to follow a given pathway (e.g., landfilling), emission factors indicate the amount of mercury that each pathway releases to the environment.We developed emission factors for the potential mercury pathways in a product’s lifecycle, including releases associated with product use and transport as well as disposal.
For dental amalgam, we considered additional pathways, including human waste, tooth loss, cremation, and infectious waste disposal.Dental amalgam contains approximately 50% mercury, which is slowly released via wastewater systems to the environment through human wastes (Association of Metropolitan Sewerage Agencies 2000).
Some emissions estimates are not based strictly on applying an emissions factor to mercury flow, but rather are independently derived and then plugged into the model. For instance, estimates of mercury releases from dental office vacuum systems are derived not from the amount of mercury used, but from an emissions factor that is based on the number of dentists using amalgam.
Estimates of dental mercury in feces and urine are based on measurements that are not calibrated to the amount of mercury in use. Estimates of mercury in the teeth of corpses are based on mortality statistics and on estimates of the average number of fillings that people have at the end of life.
Results and Discussion
In 2005, switches and relays, dental amalgam, andmeasurement and control devices are responsible for most of the total mercury releases caused by products and most of the releases to land. Switches and relays account for more than half of estimated 2005 product-related air releases,
Pathways of Product Mercury Releases
Other significant pathways include disposal of solid waste, a variety of pathways unique to dental amalgam (most notably human cremation), and disposal of sewage sludge.
The model also indicates that the use of dental amalgam may result in significant mercury releases through several little-studied pathways.
The most important of these pathways is cremation of corpses, a source that is growing as cremation becomes more common. The model estimates air releases from this source at more than 2 tonnes in 2005. Dental office vacuum systems are also significant sources of mercury according to the model—more than 1 tonne of air releases in 2005. In addition, the model calculates that approximately 150 kg of mercury are released annually in exhaled breath as a result of dental amalgam fillings.
…whereas dental amalgam is responsible for more than half of estimated product-related water releases.
Atmospheric releases associated with dental amalgam use have remained relatively stable, with reduced releases caused by controls on incineration of infectious waste partially offset by increased releases from cremation of corpses.
The model shows that dental amalgam is a major cause of mercury discharges to wastewater.
Dental office discharges to sewage treatment plants are estimated at 5.5 tonnes, consistent with a recent estimate of 6 tonnes (Vandeven and McGinnis 2005). An additional estimated 1.2 tonnes is discharged to sewage treatment in human wastes as a result of gradual erosion of dental amalgam fillings.
More unexpectedly, the model indicates that dental amalgam is also an important source of mercury releases to the air.
In addition to releases from sludge incineration and land application of wastewater treatment plant biosolids, which are related primarily to amalgam fillings, mercury is emitted from crematories as a result of amalgam fillings in corpses. Dental offices also release mercury to the air through their vacuum systems, and people exhale mercury as a result of a gradual release of mercury from dental fillings. According to this analysis, dental amalgam is responsible for more than 4.5 tonnes of mercury releases to air, making it a significantly larger contributor of air releases than mercury-containing lamps.
Opportunities and Obstacles in Reducing Product-Related Mercury Releases
The use of mercury in dental amalgam is also expected to continue. Therefore, releases caused by excretion of amalgam by human bodies will continue. Even with continuing use of dental amalgam, though, there is a potential for reducing releases through better waste management and through the use of amalgam separators.
Table 7 shows the estimated reductions in mercury releases to air, water, and land that would result if dentists were to fully implement best management practices (BMPs) to recycle all scrap amalgam and amalgam captured by existing chair-side traps and vacuum filters.
In addition, if dentists were to universally install amalgam separators capable of removing 95% of amalgam from dental office wastewater, then there would be an even bigger reduction in mercury releases. Projected reductions of air releases from dental amalgam are relatively small because a significant share of estimated air releases caused by dental amalgam results from cremation of corpses, dental office vacuum systems, and exhalation of mercury vapor, none of which would be affected by these measures.
Releases from cremations are forecast to increase due to increased rates of cremation and higher levels of tooth retention due to better dental care.
These estimates of potential mercury reductions from amalgam installation are also based on the assumption that sewage treatment plants capture mercury discharges from dental offices at the same efficiency rate as they capture other mercury discharges. It might be the case, however, that treatment plants are more effective at capturing mercury from dental offices than from other sources, because the mercury from dental offices is more likely to be in particulate form.
If sewage treatment plants are more effective at capturing dental amalgam discharges than we assume, then total mercury discharges to water caused by amalgam would be less than we estimate, and the impact of amalgam separators on water discharges would also be less (Larry Walker Associates 2002).
Cain, A., Disch, S., Twaroski, C., Reindl, J. and Case, C. R. (2007), Substance Flow Analysis of Mercury Intentionally Used in Products in the United States. Journal of Industrial Ecology, 11: 61–75. doi: 10.1162/jiec.2007.1214