Amalgam toxicity—environmental and occupationalhazards

Preben Horsted-Bindslev*

Department of Dental Pathology, Operative Dentistry and Endodontics, Faculty of Health Sciences, RoyalDental College, University of Aarhus, Vennelyst Boulevard, DK-8000 Aarhus C, Denmark

Received 2 December 2003; accepted 3 February 2004

KEYWORDSAmalgam; Mercury;

Environment; Waste;

Body burden;

Neuropathology;

Reproduction

Summary Objectives. To discuss briefly the recent developments in mercuryproduction, consumption and waste handling especially in relation to the use ofmercury in dentistry. Furthermore, to discuss the toxicological and reproductiveaspects of the mercury body burden of dental personnel.

Data, sources and study selection. The data discussed are primarily based onpublished scientific studies and on publications and reviews from governmental andother official authorities which have been published within the last 10 years,References have been traced manually or by MEDLINEw.

Conclusions. Global production and consumption of mercury is decreasing, as is theproduction of amalgam fillings in some countries. By proper measures it is possible tofurther reduce the environmental burden of mercury from dental clinics. In general,the mercury body burden of the dental personnel can be kept below the normallyaccepted toxicological limits and reproductive effects have not been proven provideda proper mercury hygiene regimen is adopted.q 2004 Elsevier Ltd. All rights reserved.

The use of mercury for production of dental silveramalgam restorations and the later release ofmercury from the restorations has been a matterfor concern especially during the last 30 years.1

Passions have run high on the risk of health hazardsfrom mercury in individuals with amalgam fillings.The dentists, who are the persons, most highlyexposed to mercury, have had a more relaxedattitude towards their own exposure and to someextent to the possibility of causing environmentalproblems. In recent years, the dental personnelhave been aware of their environmental responsi-bility and for some years especially the female

personnel have been aware of the possible riskof reproductive disturbances caused by injudicioushandling of mercury.

The aim of this review article is to discuss brieflythe recent developments in the environmental andoccupational aspects of dental treatment withamalgam.

Environment

Consumption

Mercury occurs naturally in areas of previous highvolcanic activity and is also today produced from

0300-5712/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.jdent.2004.02.002

Journal of Dentistry (2004) 32, 359–365

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*Tel.: þ45-8942-4140; fax: þ45-8620-2202.E-mail address: ph-b@odont.au.dk

mining in areas, such as Algeria, Spain, Kyrgyz-stan, China, Mexico and Peru. Mercury is releasednaturally to the environment by erosion ofmineral deposits, volcanic eruptions and geysers.Mercury is moreover distributed in the environ-ment by human activities such as metal smeltingand coal production and as uncontrolled wastedisposal. By natural means it has been estimatedthat between 2700 and 6000 t of elementalmercury is released to the biosphere throughdegassing from the earth’s crust and oceans. Afurther 2000–3000 t from industrial wastes andthe combination of fossil fuels add up theenvironmental load.2

Mercury is known to be neurotoxic and nephro-toxic. Toxic effects on the respiratory, cardiovas-cular and gastrointestinal systems have been shownfollowing acute exposure to elementary mercury.3

Fetuses and newborn babies are more sensitive tomercury than adults and there seems to be greatdifferences in sensibility among individuals.Environmental mercury is accumulated in foodchains, particularly in the aquatic milieu where ahigh degree of biomagnification occurs. The foodchain seems to be the predominant route of humanexposure to methyl mercury, which is the mosttoxic form of mercury.2

Because of the toxicity of mercury and theresulting environmental and occupational problemsseveral countries have adopted regulations toreduce or ban the sale and use of mercury products.The global production from mining of mercury hasdecreased in the last 15–20 years from an estimateof about 6200 t per year in the period 1981–1985 to1800 t in 2000.4 The use of mercury in the chlor-alkali production, which has been a prime consumerof mercury, is about to be phased out in severalcountries. Thus, in the USA consumption of mercuryin this industry dropped from 247 t in 1990 to 136 tin 1996.4 However, a decrease in consumption ofmercury in this industry does not automaticallymean that less mercury come onto the market.Substitution and closing of a mercury based chlor-alkali plan renders a large inventory of mercuryavailable for recycling.

In dentistry, a comparable reduction of mercuryconsumption has occurred. In the USA the con-sumption dropped from 44 t in 1990 to 31 t in 1996.4

In Denmark, the consumption dropped from 3.1 t in1982–1983 to 1.2 t in 2001.5 Similarly, the pro-duction of amalgam fillings reported to the DanishNational Health Service dropped from about 3.0million items in 1982 to 1.1 million items in 2001(Fig. 1). The tendency may reflect the decrease incaries prevalence, but also a substitution ofamalgam with alternative materials.

Even though the use of mercury in dentistry hasbeen reduced in countries where regulatorymeasures have been taken, the local governmentalauthorities may press for a further reduction. Theuse in dentistry may be strongly marked becausethe proportion used for amalgam makes up agreater percentage of the total amount of mercurywhen mercury used for other purposes as batteriesand medical devices has been totally banned. Thus,even though consumption of mercury in dentistry inDenmark has dropped from 3.1 to 1.2 t thepercentage of the total consumption doubled from17 to 34% in the same period.5

Waste handling

As long as amalgam fillings are produced inrestorative dentistry and patients have amalgamfillings in their teeth, the dental profession has anobligation to minimize or, preferably totally toeliminate release of mercury to the environment.

The mercury cycle in dentistry is illustrated inFig. 2. Surplus of triturated and carved amalgamshould be collected and stored in tight boxes undercover of for example used X-ray fixer for laterrecycling.6 Units must be equipped with filters and/or separators, which catch particles occurring fromremoval of old amalgam fillings. Installation ofeffective separators have shown to eliminate up to91–99% of the mercury in the waste water from thedental clinic.7,8 It has further been shown that themercury concentration in the sludge from wastewater treatment plants declined 29–80% in areaswhere separators were installed in the dentaloffices.9,10

If a significant reduction is not obtained afterinstallation of separators the reason may besediments of mercury present in the sewerage.Thus, installation of filters is not a guarantee forlow concentration of mercury in the wastewater.

Figure 1 Number of amalgam fillings (left) in relation toconsumption of mercury in dentistry in Denmark (right).

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The waste water pipes of old clinics may containsediments of mercury, which is graduallyreleased. Further, it is of utmost importancethat the dental personnel act according to localregulations and are properly educated to handleand maintain filters and store scrap until laterdeposition.11 Extracted teeth with amalgamfillings should be handled as risk waste insteadof being thrown in a waste basket which oftenhappens in daily practice.11 In some countries,extracted teeth are donated to dental schools forteaching purposes.

Mercury may slowly be released to the soil fromburied persons with amalgam fillings and to the airby cremation. However, analyses from soil anddrain water samples from a Danish cemeteryshowed no detectable amounts of mercury.7

In Sweden, crematories performing more than1000 cremations per year must be equipped withfilters to catch emitted mercury from amalgamfillings. In Denmark, the Danish EnvironmentalProtection Agency5 has estimated that about170 kg mercury from an annual cremation of about

41,000 persons is emitted to the air. The Danishauthorities have in line with most countries notdecided in favour of combustion filters.

Occupational hazards

Body burden

The mercury body burden of dental personnel hasoften shown higher levels than the general popu-lation because dental personnel handle mercury inthe clinic, remove or assist in removing amalgamfillings and they may also have amalgam fillingsthemselves. Urine mercury level is used for deter-mining long time exposure to inorganic mercury. Aconcentration at 1–5 mg Hg/l urine is considered tobe within normal range for non-occupationalgroups.12 –15 The level at which symptoms of chronicintoxication are recognized vary greatly betweenindividuals partly because of differences in individ-ual sensibility. Subtle and non-specific symptoms ofmercury intoxication have been discussed in theliterature at concentrations above 25–50 mg Hg/lurine.3,16 –19

Because of the diffuse nature of symptomsdiagnosis of chronic mercury intoxication is alwaysbased on history of exposure. Weakness, fatigue,loss of appetite and gastrointestinal disturbancesare described as symptoms occurring after longtime exposure to low level exposure. Classicalsymptoms after high exposures are tremor anderethism.12

The urine mercury level of dental personnel hasbeen determined for many years and examples aregiven in Table 1. In the USA measurements havebeen published in 1968 at 40 mg/l in 1987 at 12 mg/land in 1995 at 5 mg/l showing a dramatic decline

Figure 2 Mercury cycle in dentistry.

Table 1 Mercury concentration in urine from dentalpersonnel.

Mean Range

USA 196820 40 mg/lUSA 198521 15 mg/l 5% . 50 mg/lSweden 198622 4 mg/l

Control 3 mg/lUSA 198723 12 mg/l 13% . 20 mg/lNorway 199024 8 mg/l 0–55 mg/lUSA 199525 5 mg/l 2% . 20 mg/lSweden 199726 5 mg/l 2–27 mg/l

Control 4 mg/l 0–23 mg/lVenezuela 200127 22 mg/lMexico 200228 3 mg/l 0.2–12 mg/lHolland 200315 11 mg/l 5–22 mg/l

Amalgam toxicity—environmental and occupational hazards 361

during the period (Table 1). The decrease maymainly reflect better operatory mercury hygiene,but the 1995 level may also reflect a decrease incaries prevalence and to some extent use ofalternative materials. In Sweden, the figures from1986 to 1997 are almost similar and not differentfrom a control group reflecting a high operatorystandard (Table 1). Still in some countries the meanand range HgU level is rather high and approachesconcentration where subtle symptoms have beenobserved in sensitive individuals. The high concen-tration in some dentists may indicate that a changein operatory hygiene is required (Table 1). Thus, inthe latest study from the USA more than 20% of thedentists still used squeeze cloths.25

Mercury hygiene measures comprise proper lay-out of the surgery, removal of amalgam restora-tions, methods of trituration and condensation,cleaning and sterilization of instruments andstorage of amalgam waste.6,29 Also spillage mustbe considered although it should be possible toavoid this by use of modern trituration methodssuch as disposable capsules.

If proper mercury hygiene is adopted it ispossible to confine the mercury body burden ofthe dental personnel to concentrations which donot differ significantly from non-occupationalgroups.

Neurobehavioral studies

A method to evaluate occupational exposure is tomeasure the air mercury level at the workplace.WHO has decided an exposure limit of50 mg Hg/m3 air (TWA: Time weighted average)corresponding to an estimated urine concen-tration of about 80 mg Hg/l30 which with today’sknowledge of mercury toxicology seems to be toohigh. Some countries have therefore adopted alower concentration of 25 or 30 mg Hg/m3 as theupper limit.

In recent years, focus has been directed againstneurobehavioral diagnosis of subtle symptomscaused by long time occupational exposure of lowdose inorganic mercury. As discussed in the pre-vious paragraph the mercury body burden of dentalpersonnel may be slightly higher than non-exposedcontrols, but most often below the normallyaccepted toxicological concentrations and unlessacute symptoms occur after spillage the personnelin general do not exhibit the classical symptoms ofmercury intoxications.

In order to disclose subtle differences from non-exposed groups which may not be visible in the dailypractice, various neurobehavioural and other tests

have been used. Some examples of the testbatteries are given in the following.

Finger tapping requires the subject to tap alever as many times as possible with the index fingerin 10 s.31 Intentional hand steadiness test requiresthe subject to insert and hold a metal stylus in aseries of six increasingly smaller holes for 15 sintervals.31 Also reaction time, hand tremor,electro neurography, simple reaction time, wordrecognition, word recall and various other memorytests together with questionnaires on previousmaladies and mood state personality have beenused.26,31 –33

How do dental personnel perform compared toother groups occupationally exposed to inorganicmercury?

Electrophysiological and neurological examin-ations were performed at a group of dentistschlor-alkali workers and miners from the CzechRepublic exposed to a level of Hg concentration inthe air of the workplace of 20, 36 and 77 mg Hg/ m3,respectively.32 The mean HgU/l concentration was13, 129 and 840 mg, respectively, and 1 mg in acontrol group. Electroneurography did not showreduced velocity on motor fibres in peripheralnerves in the dental and chlor-alkali groups, but atendency to subtle changes in latency was observedwith a visual evoked potential examination in thesegroups. Most of the miners showed classical signs ofmercury intoxication.32

A few studies have found that dentists performworse in some neurobehavioral tests compared tounexposed controls.34 –35 The mean HgU/l of thedentists in one of the studies was 36 mg Hg/l35 andsignificantly higher than in a study from Swedenwhere the mean TWA was 3.0 mg pr. m3 correspond-ing to a mean HgU of 5 mg Hg/l and no neuropatho-logical problems were recorded.26 A study fromScotland33 with a similar low HgU concentration of4 mg among the dentists, but a rather high TWAfrom the area of the mixing device of 30 mg Hg/m3

did show some changes in psychomotor responsecompared to controls, but without significantassociation between changes and mercury concen-tration in urine. Neither did self-reported kidneydisturbances and memory disorders, which weremore pronounced among the dentists compared tocontrols, have any relation to the mercury concen-tration in urine. Also there was no associationbetween number of amalgam surfaces in thedentists and controls and their scores in any of thepsychomotor tests.

A recent meta analysis for neurobehaviouralresults due to occupational mercury exposureconcluded that a significant performance effect

P. Horsted-Bindslev362

was shown for mean urinary concentrationsbetween 18 and 34 mg Hg/g creatinin.36

Based on the rather heterogeneous picture ofresults from neurological studies it seems justifiedto conclude that with our present knowledge a riskof subtle neurotoxic changes may occur in dentalpersonnel showing a urine concentration of mercurybelow what can be shown when operating withinthe accepted threshold limit. However, it must bestressed that other factors such as the dailyexposure to high frequency vibrations37 and stressmay be equally important for the subtle behaviouralchanges. Also it should be stressed that none of thestudies referred to has shown the dental personnelto suffer the classical signs of mercury intoxication.

Reproductive hazards

Although the placenta acts as a barrier to preventtoxic substances to the fetus it has been shown thata substantial fraction of maternal blood containinginorganic mercury can reach the fetus.38 However,major studies from the USA, Sweden and Denmarkhave not found differences in spontaneous abor-tions, infant survival, birth weight and congenitalabnormalities between dental personnel and con-trol groups39–41 and Schuurs42 concluded in a reviewpaper that negative reproductive effects fromexposure to mercury in the dental office areunproven. In a study from the USA, mercury hygienefactors were evaluated and augmented with thenumber of amalgams prepared each week, deter-mination of mercury concentration in operatory airwas not made. It was shown that dental assistantswith a high occupational exposure to mercury wereless fertile than unexposed controls.29 The authorsconclude that the fecundability of women whoprepared more than 30 amalgams per week and whohad five or more poor mercury hygiene factors wasonly 63% of that for unexposed women (Table 2).The authors emphasize that reduced fertility couldreflect other exposures found among women

working in offices with poor hygiene andmany amalgams. It was also found that womenwith low exposure were more fertile than unex-posed controls.29

In a recent major study from Norway nodifference in fertility was found between highschool teachers and dental surgeons of whom 1/3placed more than 50 fillings a week.43

The Norwegian study showed that it is possible tomake a considerable number of amalgams withoutfertility problems whereas the USA study suggeststhat establishing a proper hygiene regimen is ofprime importance.

Conclusions

Amalgam is about to be replaced by alternativerestorative materials. The risk of environmentalproblems from disposal of mercury containingwaste from dental clinics will therefore decreaseover time. All forms of mercury have adverseeffects on health at high doses. However, theevidence that exposure to very low doses ofmercury has adverse effects is open to wideinterpretation.1 Based on our present knowledgeon risk of environmental and occupational hazardsfrom use of mercury in dentistry the followingconclusions can be drawn:

† Use of mercury in dentistry has decreasedsubstantially, especially during the last decade.

† Amalgam separators and proper collection,handling and storage of waste have shown tosignificantly reduce mercury discharge from thedental clinics.

† Given a proper mercury hygiene the mercurybody burden of dental personnel may not differfrom the general population.

† A subtle neurobehavioural effect from long timework in dentistry cannot be ruled out. Sucheffect may be caused by a variety of factors suchas various chemicals, high-frequency vibrationsand stress.

† Provided a good mercury hygiene is establishedthere is no proven negative reproductive effectfrom exposure to mercury among dentalpersonnel.

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