Dental chair a possible source of neurotoxic mercury waste
Published: Sunday, 30-Mar-2008
News release from: http://www.news-medical.net/print_article.asp?id=36719
Mercury is a large component of dental fillings, but it is not believed to pose immediate health risks in that form.
When exposed to sulfate-reducing bacteria, however, mercury undergoes a chemical change and becomes methylated, making it a potent, ingestible neurotoxin.
While the major source of neurotoxic mercury comes from coal-fired electric power plants, researchers at the University of Illinois at Chicago and at Urbana-Champaign say mercury entering drain water from dental clinics and offices is also a source.
"We found the highest levels of methyl mercury ever reported in any environmental water sample," said Karl Rockne, associate professor of environmental engineering at UIC and corresponding author of the study that appeared online March 12 in the journal Environmental Science and Technology.
Working with James Drummond, UIC professor of restorative dentistry, Rockne gathered waste water samples in collection tanks generated from both a single-chair dentist's office and a 12-chair dental clinic to check for methyl mercury.
Water collected was allowed to settle. Clear layers above the settled particles were then analyzed for presence of methyl mercury. Fine, slow-settling particles of mercury get into the waste water mostly after dentists use high-speed drills to remove old amalgam fillings. The numerous fine particles the drilling produces provide an ample source of exposed mercury surfaces, making them prime targets for sulfur-reducing bacteria that commonly live in anaerobic conditions and are known to methylate mercury.
"It appears to be produced partially, if not fully in the waste water, and it's being produced very rapidly," said Rockne, adding that it was significant this was happening before the particles were getting into sewers, where sulfur-reducing bacteria thrive.
The finding raised the question whether the culprit bacteria were living in the mouths of dental patients. "We don't have the answer," Rockne said.
Based on their sample studies, the researchers estimate that 2-5 kilograms, or up to 11 pounds, of methyl mercury could be entering the public water supply of the United States each year from dental waste water. While this may not seem like much, methyl mercury is highly toxic in minute amounts.
When in waterways, methyl mercury tends to get biomagnified up the food chain, moving from algae and phytoplankton to fish and, ultimately, to humans.
While surprised by the level of contaminants found in the study, Rockne says follow-up research is necessary -- then, possibly, some basic engineering.
"Amalgam separators are a good first step, but maybe something else is necessary downstream to prevent further methylation and prevent further soluble mercury from getting through the system," he said.
"We have to take more steps to prevent the problem from occurring in the first place," he said. "We're dealing with a pipe -- a control point. As an engineer, I see this as a problem that is tractable -- something we can definitely do something about."
The abstract and a link for the whole article can be found at: http://pubs.acs.org/cgi-bin/abstract.cgi/esthag/asap/abs/es7027058.html
ASAP Environ. Sci. Technol., ASAP Article, 10.1021/es7027058
Web Release Date: March 12, 2008
Characterization of Methyl Mercury in Dental Wastewater and Correlation with Sulfate-Reducing Bacterial DNA
Xiuhong Zhao,† Karl J. Rockne,*† James L. Drummond,‡ Ryan K. Hurley,‡ Christopher W. Shade,§ and Robert J. M. Hudson§
Department of Civil and Materials Engineering and Department of Restorative Dentistry, University of Illinois at Chicago, Chicago, Illinois, and Department of Natural Resources and Environmental Sciences, University of Illinois Urbana–Champaign, Urbana, Illinois
Accepted February 7, 2008
Abstract:
Dental wastewater (DWW) was collected over two months from a 12-chair clinic and a single-chair office to identify conditions that may affect Hg methylation. DWW was settled for 24 h and samples were collected from the top and bottom of the supernatant to simulate a range of particles that may escape in-line traps. Total Hg spanned 5 orders of magnitude (0.02–5000 µM), following a log-normal distribution with p10, p50, and p90 concentration values of 0.24, 31 and 4000 µM, respectively; typically well in excess of free aqueous Hg solubility. Methyl Hg was present in high levels (2–270 nM), also following a log-normal distribution with p10, p50, and p90 concentration values of 2.8, 17, and 100 nM, respectively. There were no statistically significant differences (90% CI) in p50 methyl Hg or total Hg between the clinic and office. Methyl Hg was predicted from total Hg data by (±95% CI): Log(Me-Hg) = 0.33(±0.06) × Log(T-Hg) − 2.27(±0.13). Total methyl Hg from DWW to U.S. wastewater collection systems is estimated to be 2–5 kg yr−1. Equilibrium speciation modeling predicted that DWW Hg was primarily in sulfide−Hg complexes, except at high total Hg levels where organo−Hg complexes become significant. DNA extracts amplified by quantitative polymerase chain reaction with primers for total eubacteria and sulfate-reducing bacteria (SRB) indicated that the total eubacterial DNA was composed primarily of SRB, and highly significant correlations were found between methyl Hg and both amplified Desulfobacteraceae (p < 0.0001) and Desulfovibrionacaea DNA (p < 0.00001). Both are known Hg methylators. In marked contrast, there was no significant correlation between methyl Hg and amplified Desulfobulbus DNA, a genus generally not known to methylate Hg at high rates. These results strongly suggest that SRB are implicated in DWW Hg methylation.
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