Occupational mercury exposure in association with prevalence of multiple sclerosis and tremor among US dentists

ORIGINAL CONTRIBUTIONS

ARTICLE 1

COVER STORY









































Occupational mercury exposure in association with prevalence of multiple sclerosis and tremor among US dentists Julia Anglen, MS; Stephen E. Gruninger, MS; Hwai-Nan Chou, MS; Jennifer Weuve, ScD; Mary Ellen Turyk, PhD; Sally Freels, PhD; Leslie Thomas Stayner, PhD

I

nvestigators in many studies using the general population have reported no association between exposure to elemental mercury (Hg0) from dental amalgam and neurologic diseases.1-4 Workers occupationally exposed to higher concentrations of Hg0 (> 500 micrograms per liter urinary Hg0), however, have been reported to experience tremors, memory deficits, blurred vision, paresthesia, nerve conduction deficits, and other neurotoxic effects.5-8 The US Environmental Protection Agency has set the lowest observed adverse effect level for ambient Hg0 exposure at 0.025 mg per cubic meter, approximately equivalent to 25 mg/L urinary Hg0 concentration.9-11 Investigators in studies of low-level Hg0 exposure examining this threshold for neurologic deficits have found both null7,12-14 and positive findings.7,8,12,13,15 Dentists’ occupational exposure to Hg0 has decreased over the past decades because of the development of encapsulated amalgam technology, the increasing use of composite resin restorations, and an overall heightened awareness of occupational Hg0 hygiene. In 1980, dentists’ urinary Hg0 concentrations averaged 20 mg/L, in contrast to more recent averages of 2.04 mg/L urinary Hg0 (Hwai-Nan Chou, MS, American Dental Association This article has an accompanying online continuing education activity available at: http://jada.ada.org/ce/home.

ABSTRACT Background. The effects of chronic occupational exposure to elemental mercury (Hg0) are largely unknown. The objective was to evaluate the association of occupational Hg0 exposure with multiple sclerosis (MS) and tremor. Methods. The study included 13,906 dentists who attended the American Dental Association’s annual meeting over 24 years (1986-2007 and 2011-2012). Participants reported MS and tremor and provided urine specimens for Hg0 analysis. The authors estimated mean Hg0 exposures over time and used logistic regression to estimate the associations of 3 Hg0 exposure measures with MS or tremor. Results. Among participants, 0.18% reported MS and 1.24% reported tremor. Hg0 exposure was not associated with MS (odds ratio [OR] per 191 micrograms per liter in cumulative Hg0 exposure, 0.85; 95% confidence interval [CI], 0.39-1.85). Increased prevalent risk of tremor was found with exposure to both urinary Hg0 exposure (OR, 1.10 [95% CI, 1.00-1.22]) and cumulative Hg0 exposure among younger dentists (< 51 years; OR, 1.13 [95% CI, 1.05-1.22]). Conclusions. Occupational Hg0 exposure in US dentists decreased over time and now is approaching that of the general population. Our results suggest a positive association between Hg0 exposure and tremor. Practical Implications. Studies with more sophisticated outcome and exposure measures, and including more retired dentists, would provide critical information toward understanding the relation of Hg0 exposures to MS and tremor risk. Key Words. Elemental mercury; dentists; occupational health; multiple sclerosis; tremor; epidemiology. JADA 2015:146(9):659-668 http://dx.doi.org/10.1016/j.adaj.2015.05.016

Copyright ª 2015 American Dental Association. All rights reserved.

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ORIGINAL CONTRIBUTIONS

Dentists from health screening program years 1976-1984, 1986-2007, and 2011-2012 (n = 34,040)

Dentists with available urinary mercury data (n = 23,582) (Figure 2)

Dentists with available multiple sclerosis and tremor data (years 1986-2007 and 2011-2012) (n = 16,486)

Unique observations (n = 13,906)

Figure 1. Diagram of study sample illustrating how participants were selected for the study. Dentists were selected from years the health screening program was offered (1976-1984, 1986-2007, and 20112012). We selected dentists with urinary mercury data from that sample and used their measurements for Figure 2. From those with urinary mercury data, participants who also had outcome data (multiple sclerosis and tremor; years 1986-2007 and 2011-2012) were selected for the study.

[ADA], unpublished data, 2012).16 Approximately 80% of inhaled Hg0 vapor is absorbed by rapid diffusion through the lungs and distributed to all parts of the body via the circulatory system.3 Hg0 can cross the blood-brain barrier, where it is oxidized to inorganic mercury, which has strong affinity for thiolcontaining proteins that are prevalent in the brain.17 The half-life of inorganic mercury in the brain is not well characterized, ranging from months to decades.18 Multiple sclerosis (MS) is a chronic autoimmune and neurodegenerative disease of the central nervous system.19 In rabbit and rat models, exposure to Hg0 causes autoimmune disease, similar to the immune cell activity observed in patients with MS.20,21 Investigators in studies of Amazonian gold miners have reported associations of low-level urinary Hg0 concentrations (3.7 mg/L) with both elevated cytokines and antinuclear antibodies, which are biomarkers associated with immune response and autoimmune disease, respectively.22-24 Tremor is an involuntary movement characterized by oscillatory motion, occurring during rest or activity. The prevalence of tremor in people with MS ranges from 25% to 60%,25-27 and this tremor is characterized by demyelinating lesions in the brain that produce

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intention tremor, the most common type of cerebellar tremor.28 There are various classifications of tremor unrelated to MS, including but not limited to accentuated physiological, parkinsonian, and essential tremor.29,30 Investigators in 3 studies reported associations of lowlevel occupational exposure to Hg0 vapor with increased risk of tremor,7,31,32 but 2 others did not.12,33 Investigators in a few studies have reported associations of Hg with an increase in memory disturbance, a symptom of MS, in groups with low-level exposures.7,31,34,35 Although the results from 2 studies reported a weak, positive association between exposure to dental amalgam and MS in the general population,36,37 to our knowledge, no study has evaluated MS risk among groups occupationally exposed to Hg0. The objectives of this study were to estimate the extent of occupational Hg0 exposure in a sample of US dentists participating in the health screening program at the ADA’s annual meeting and compare it with Hg0 exposures in the US general population; determine the prevalence of MS and tremor among dentists; and examine whether there is an association between occupational Hg0 exposure and these neurologic outcomes in this sample of dentists. METHODS

ADA survey. Since 1964, the ADA has collected personal health information with informed consent by using a questionnaire and biological specimens from volunteer dentists during the ADA’s annual meeting held in various cities across the United States (Appendix, available online at the end of this article). Urinary Hg0 measurements were offered in the years 1976-1984, 1986-2007, and 2011-2012. Data from each year’s questionnaire and biological measurements of Hg0 exposure were compiled to build a database with 34,040 observations from 1976 to 2012. The ADA’s Institutional Review Board approved this research, and data were deidentified before sharing with outside researchers. Hg0 exposure assessment. Hg0 was quantified in spot urine samples from a single void. Hg0 analysis was conducted at the ADA Laboratory in Chicago, IL, by using cold vapor atomic absorption spectrophotometry according to a method previously described.38 The lower limit of detection (LOD) for urinary Hg0 analyses was 0.2 mg/L. Urine Hg0 concentrations below the LOD were imputed as LOD/O2 (272 of 23,582 total measurements ¼ 1%). Urinary Hg0 concentrations formed the basis of 2 Hg0 exposure measures used in our primary analyses. The first was simply the urinary Hg0 concentration at the ABBREVIATION KEY. ADA: American Dental Association. Hg: Mercury. Hg0: Elemental mercury. HSP: Health screening program. LOD: Limit of detection. MeHg: Methyl mercury. NHANES: National Health and Nutrition Examination Survey.

ORIGINAL CONTRIBUTIONS

MEAN URINARY MERCURY CONCENTRATION (μg/L OR ppb)

25

20

15

10

5

0 1976

1980

1986

1990

1995

2000

2005

2012

CALENDAR YEAR Mean Urinary Hg0

Polynomial Curve (Mean Urinary Hg0)

Figure 2. Arithmetic mean urinary mercury (Hg) concentrations in micrograms per liter in US dentists (standard errors shown) were calculated between the years 1976 and 2012 when the health screening program was offered (that is, excluding the years 1985, 2008, 2009, and 2010) (n ¼ 23,582). Urinary Hg0 concentrations were measured via atomic absorption spectrophotometry at the American Dental Association laboratories. ppb: Parts per billion.

time of the questionnaire. For the second, we estimated cumulative Hg0 exposure for each dentist by summing the mean urinary Hg0 concentration from our data for each corresponding year the dentist practiced (as reported via questionnaire). We used the average of the first 7 years of the survey (1976-1982) to represent the average in years before 1976. To account for the high interindividual variability, we adjusted the average by using the ratio of the participant’s urinary Hg0 concentration to the total average urinary Hg0 for the year the participant was tested: 0 0 Hg0 ratio ¼ UHgparticipant =average UHgyear

A cumulative exposure for each dentist was estimated using the following equation: Cumulative Hg exposure Xtotal years practiced ¼ ðHg ratioÞ  i¼1 ðaverage urinary Hg from HSP sampleÞi in which i indicates the initial year practiced and HSP indicates health screening program. For example, a dentist had their urinary Hg0 measured in the year 2000 and had been practicing

for 20 years. Hypothetically, the dentist’s urinary Hg0 concentration in 2000 was 2.0 mg/L, whereas the average urinary Hg0 for that year’s health screening program was 1.0 mg/L. The ratio (2:1) would be applied to the average of each previous year that dentist worked (20 years) to calculate the cumulative dose exposed until that point: Cumulative Hg0 exposure X ¼ ð2=1Þ  ðaverage urinary Hg0 from 1980 þ 1981 þ 1982. þ 2000Þ For dentists with 3 or more measurements (with available outcome data; n ¼ 1,542/13,906), the mean (standard deviation [SD]) number of years between all 3 measurements was 6.0 (10.6) years. We did not find substantial variability in urinary Hg0 measurements across visits for most of this sample and, thus, calculated cumulative Hg0 exposure ratio by using a participant’s last measurement. For the third measure of Hg0, dentists reported the number of amalgam restorations they placed and removed per week via questionnaire. Assessment of neurologic outcomes. Neurologic information was available only for MS and tremor and in surveys conducted in the years 1986-2007 and 2011-2012.

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ORIGINAL CONTRIBUTIONS

URINARY MERCURY (μg/L)

URINARY MERCURY (μg/L)

Hg0 concentrations in 92 13 participants were below 13 the LOD, which was 0.05 12 12 nanograms per milliliter. 11 11 Measurements below the 10 LOD were imputed in 10 NHANES as the LOD/O2. 9 9 To obtain estimates more 8 8 comparable with our sample of dentists ac7 7 cording to age and in6 6 come, we excluded 5 5 NHANES participants younger than 25 years 4 4 (n ¼ 976) and those from 3 3 income levels lower than 2 2 $75,000 (n ¼ 1,167). We calculated arithmetic 1 1 means (with 95% confi0 0 dence intervals [CIs]) categorized according to 150 100 50 0 50 100 150 age group (< 40 years, NO. OF PARTICIPANTS 40-59 years, and $ 60 years) and stratified NHANES ADA NHANES data according to high or low levels of seafood consumption Figure 3. The distribution of urinary mercury concentrations in micrograms per liter was compared between American Dental Association (ADA) dentists in 2011 and 2012 (n ¼ 780) and the general population by using a (n ¼ 336) by using subsample of National Health and Nutrition Examination Survey (NHANES) participants (aged > 25 years and the mean for seafood income > $75,000) from 2011 to 2012 (n ¼ 364). consumed per month for each category. The The question for both neurologic outcomes was posed as NHANES data were weighted by the survey sampling follows: “Please indicate if you have or have ever had the weights and design variables. We inspected distributions of the 3 measures of Hg0 following diseases or conditions .” exposure for outliers. We computed bivariate statistics of Sample population for analysis. All 23,582 urinary the crude associations of key characteristics with the 2 Hg0 measurements were used to calculate the extent of neurologic outcomes and Hg0 exposure measures. We Hg0 exposure in US dentists (Figure 1). We restricted 0 used t tests and analysis of variance to compare unadanalyses of associations between occupational Hg exposure and the neurologic outcomes to dentists who justed differences in means and Pearson c2 tests for differences in proportions. We used Wilcoxon rank sum participated during these years, had urinary Hg0 measurements, and answered the neurologic outcome ques- tests for analyses of amalgam restorations placed and tions. For those who participated in more than 1 annual removed per week. Lastly, we used logistic regression to estimate the association of Hg0 exposure with MS or screening, we used the last observation. However, if a tremor, fitting separate models for each measure of Hg0 participant ever reported the diagnosis of interest, we exposure. We first assessed dose-response relationships used the earliest record in which tremor or MS was reported to be as close to the age of diagnosis as possible by using quartiles of exposure and then used continuous and to avoid misestimating Hg0 exposure. These analyses forms of the variables because dose-response estimates were unstable from the small number of cases. For included data from 13,906 unique dentists (Figure 1). reporting, we transformed all results to odds ratios (ORs) Statistical analysis. First, we compared the distribucorresponding to a mean increment in Hg0 exposure. tions of urinary Hg0 measurements in our sample with These ORs approximate risk ratios because the outcomes that in the US general population by using data from the 2011-2012 cycle of the National Health and Nutrition are rare. We adjusted all models for sex, age (continuous), and calendar year (continuous). Examination Survey (NHANES),39 a nationally repreTo explore whether these associations varied according sentative sample of noninstitutionalized civilians living in the United States, in which investigators measured to age, we also fit models stratified by age (first with quintiles of age and narrowed to report 2 age groups of urinary Hg0 concentration in 2,507 men and women.

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ORIGINAL CONTRIBUTIONS

RESULTS

2.5

MEAN URINARY MERCURY CONCENTRATION (95% CI)

equal size: those 50 years or younger and those older than 50 years), as well as models containing a cross product between age and the exposure measure. We included dentists who reported both MS and tremor in the analysis of MS, and we first excluded and then later included them in a sensitivity analysis for tremor. Final tables for tremor are data from participants with only tremor. We excluded diabetic participants with tremor from tremor analyses (n ¼ 16). We considered all statistical tests statistically significant at P < .05.

2.0

1.5

1.0

0.5

0 < 40

≥ 60

40-59

AGE (YEARS) ADA

NHANES Low Fish

NHANES High Fish

Figure 4. Arithmetic mean urinary mercury concentrations (micrograms per liter; 95% confidence interval [CI]) were compared according to age group (< 40 years, 40-59 years, and $ 60 years) between American Dental Association (ADA) dentists in 2011 and 2012 (n ¼ 780) and the general population by using a subsample of National Health and Nutrition Examination Survey (NHANES) participants (aged > 25 years and income > $75,000) from 2011-2012. NHANES participants were stratified further according to high or low levels of seafood consumption (n ¼ 336) according to the mean for seafood consumed per month for each category. Study results have shown methyl mercury from fish consumption is able to be demethylated and, thus, contributes to a substantial proportion of elemental mercury measured in urine.40

Figure 2 presents the distribution of average urinary Hg0 concentrations for dentists according to calendar year. From an initial peak arithmetic mean of 20.1 mg/L (95% CI, 14.0-26.2) in 1976, urinary Hg0 concentrations decreased by 90% over the subsequent 32 years of the ADA’s health screening program. In 2012, the arithmetic mean concentration was 2.04 mg/L (95% CI, 1.87-2.22). Despite the reduction in urinary Hg0 among dentists since 1976, it was still significantly higher (1.69 mg/L [95% CI, 1.581.81]) than among the NHANES population (0.66 mg/L [95% CI, 0.54-0.78]) in the years 2011 and 2012 (Figure 3). Furthermore, dentists had significantly higher urinary Hg0 concentrations in every age group than did the subset of NHANES participants, even compared with those with high levels of seafood consumption, a common source of Hg exposure in the population (Figure 4). Cumulative Hg0 exposure increased with age and was higher in men (Table 1), but this finding may be related to the higher proportion of women in the younger age group than in the older age group (24.6% versus 8.0% [not shown in tables]). Urinary Hg0 concentrations were higher among those reporting using amalgam, but even those reporting no use had detectable Hg0 concentrations. In addition, those with more than 10 amalgam restorations in their mouth had higher urinary Hg0 concentrations than did those with 10 or fewer amalgam restorations. In these unadjusted analyses, the only exposure measure that was associated statistically with tremor was number of amalgam restorations placed and

removed per week. No exposure measure was associated significantly with MS. Urinary Hg0 concentrations and cumulative Hg0 exposure were higher among those not reporting MS than among those reporting MS, and they were higher among those reporting tremor than among those without tremor. Twenty-three (0.18%) and 154 (1.24%) dentists reported having MS or tremor, respectively (Table 2). Dentists from the sample were, on average, aged between 48 and 54 years, mostly male (83-88%), nonsmokers (83-94%), worked approximately 34 hours per week, reported having used dental amalgam (93-95%), and removed or placed between 15 and 19 dental amalgam restorations per week. Among the 23 participants with MS, 9 indicated experiencing tremor (39.1% [not shown in tables]). None of the measures of Hg0 exposure was associated with an increased risk of prevalent MS (OR per 191 mg/L in cumulative Hg0 exposure, 0.85; 95% CI, 0.40-1.84) (Table 3). Higher number of amalgam restorations placed and removed per week was associated with a decreased risk of prevalent tremor (Table 4). Both urinary Hg0 and cumulative Hg0 exposure were associated with increased tremor risk (OR corresponding to 4.7 mg/L urinary Hg increment, 1.10 [95% CI, 1.00-1.22]; for cumulative Hg0 exposure among 50-year-old or younger dentists, OR corresponding to 92 mg/L increment ¼ 1.13 [95% CI, 1.05-1.22]). Results from analyses involving unit

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ORIGINAL CONTRIBUTIONS

TABLE 1

Descriptive statistics for covariates of interest according to mercury exposure. CHARACTERISTIC

AMALGAM RESTORATIONS PLACED AND REMOVED PER WEEK (n [ 8,566*) No. of Participants

Mean (SD†)

< 40

1,965

21 (14)

40-49

2,188

21 (14)

50-59

2,242

> 59

P Value

URINARY MERCURY CONCENTRATION (MICROGRAMS PER LITER) (n [ 7,632) No. of Participants

Mean (SD)

1,697

4.6 (6.4)

2,038

4.4 (5.1)

18 (14)

2,137

2,171

17 (14)

Female

1,527

17 (14)

Male

7,039

20 (14)

P Value

CUMULATIVE MERCURY EXPOSURE (mg/L) (n [ 6,480) No. of Participants

Mean (SD)

P Value

Age, y < .01

1,552

51.9 (107.1)

1,760

118.7 (165.8)

4.7 (6.9)

1,806

217.0 (354.8)

1,760

4.8 (6.7)

1,362

368.8 (522.3)

1,091

3.3 (4.3)

6,541

4.9 (6.5)

.17

< .01

Sex < .01

< .01

918

63.3 (148.4)

5,562

202.4 (357.9)

< .01

Hours Worked per Week < 30

931

14 (12)

30-39

3,187

20 (13)

685

3.5 (4.0)

2,761

4.4 (6.2)

> 40

1,663

Missing

2,785

19 (14)

1,467

20 (14)

2,719

< .01

588

196.1 (303.5)

2,410

179.4 (322.9)

4.9 (6.0)

1,312

161.4 (362.4)

5.0 (6.9)

2,170

195.5 (352.3)

< .01

.08

Tobacco Use Yes

355

21 (14)

No

5,560

19 (14)

Missing

2,651

19 (14)

7,752

21 (13)

.01

344

5.3 (8.7)

5,013

4.9 (6.9)

2,275

3.9 (4.0)

6,976

4.8 (6.3)

286

2.5 (2.9)

370

4.1 (8.2)

.33

303

187.1 (272.8)

4,275

186.6 (351.8)

1,902

173.1 (321.0)

.98

Amalgam Use Any None

621

0 (0)

Missing

193

0 (0)

< .01

< .01

6,032

184.0 (328.8)

196

133.5 (345.1)

252

188.8 (535.8)

.03

Number of Amalgam Restorations in Mouth # 10

3,775

20 (14)

3,145

4.4 (5.4)

2,584

175.9 (306.8)

> 10

375

22 (14)

407

5.3 (5.5)

352

238.6 (337.2)

4,416

19 (14)

4,080

4.7 (7.0)

3,544

182.0 (361.7)

14

17 (12)

14

3.9 (2.7)

10

112.6 (189.9)

8,552

19 (14)

7,618

4.6 (6.3)

6,470

182.8 (339.9)

Missing Self-Reported Diagnosis of Multiple Sclerosis Yes No

< .01

.47

< .01

.64

< .01

.51

Self-Reported Diagnosis of Tremors Yes No Missing

88

14 (12)

80

243.9 (394.9)

8,433

19 (14)

7,492

4.6 (6.1)

6,375

181.0 (337.2)

45

16 (14)

36

4.7 (3.7)

25

416.0 (614.9)

< .01

104

5.7 (13.8)

.08

.10

* Number of participants who answered the question. † SD: Standard deviation.

increase in all exposures and outcomes can be found in supplemental tables (eTable 1 and eTable 2, available online at the end of this article). DISCUSSION

Occupational Hg0 exposure in US dentists has declined substantially over the past few decades. These remarkable reductions in exposure reflect heightened Hg0 hygiene awareness, the development of precapsulated amalgam material, and the increased use of composite resin

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for dental restorations. Dentists’ urinary Hg0 concentrations are slightly higher than those of the general population reported in NHANES for each age group, even when compared with consumers of high levels of seafood. Our findings on cumulative Hg0 exposure and tremor varied according to age. Prevalent tremor was more likely with higher cumulative Hg0 exposure in younger dentists (# 50 years of age). Nearly all participants were practicing dentistry, raising the possibility that this study underrepresented dentists who have retired or are not

ORIGINAL CONTRIBUTIONS

TABLE 2 healthy enough to work; this force of se- Descriptive statistics* for covariates of interest by outcome lection may have been status. less prominent in younger dentists than CHARACTERISTIC MULTIPLE SCLEROSIS (23/12,550) TREMOR (154/12,527) among older dentists Yes No P Value Yes No P Value (who also would have Total, Prevalence (%) 23 (0.2) 12,527 (99.8) NA† 154 (1.2) 12,373 (98.8) NA accrued higher cumu- Sex, No. (%) 0 lative Hg expoMale 20 (87) 10,226 (83) .62 135 (88) 10,085 (83) .09 sures).41 Similarly, Female 3 (13) 2,076 (17) 18 (12) 2,064 (17) dentists with tremor Amalgam Use, No. (%) might reduce their Yes 20 (95) 10,995 (94) .75 129 (93) 10,858 (93) .74 engagement in more No 1 (5) 764 (6) 10 (7) 755 (7) physically demanding Tobacco Use, No. (%)‡ tasks such as perYes 3 (17) 556 (6) .07 7 (7) 549 (6) .65 forming restorations; No 15 (83) 8,310 (94) 88 (93) 8,236 (94) this reverse causation Age, y, Mean (Standard Deviation) 48.1 (13.6) 49.5 (12.1) .61 53.5 (11.3) 49.4 (12.1) < .001 pathway could under- Hours Worked per Week, Mean 34.3 (12.8) 34.5 (8.3) .93 34.1 (8.1) 34.5 (8.3) .82 lie the inverse associ(Standard Deviation)§ ation between the * The number of participants for each characteristic may not equal the overall total number of participants because of missing data. number of amalgam † NA: Not applicable. restorations placed ‡ The question was asked for survey years 1986-1995 and 2006-2012 (multiple sclerosis: 18 of 9,322; tremor: 95 of and removed per 8,880). § The question was asked for survey years 1986-1988 and 1995-2012 (multiple sclerosis: 11 of 7,883; tremor: 82 week and tremor of 7,863). prevalence. Also, cumulative TABLE 3 Hg0 exposure is a function of the total Adjusted* association of multiple sclerosis with occupational years practiced and mercury exposure. urinary Hg0 concenEXPOSURE CASES/TOTAL NO. ODDS RATIO FOR MEAN UNIT INCREASE P tration. If a younger OF PARTICIPANTS (95% CONFIDENCE INTERVAL)† VALUE dentist were to have Number of Amalgam Restorations 14/8,566 0.70 (0.33-1.50) .36 the same cumulative Placed and Removed per Week Hg0 exposure as an Urinary Mercury Concentration 14/7,632 0.80 (0.43-1.49) .48 older dentist, the (Micrograms per Liter) younger dentist would 10/6,446 0.85 (0.40-1.84) .69 Cumulative Mercury Exposure (mg/L) have a higher dose of Hg0 over a shorter * Adjusted for age, sex, and calendar year. † Means are as follows: number of amalgam restorations placed and removed per week, 19.1; urinary mercury time, which might inconcentration, 4.7 mg/L; cumulative mercury exposure, 191 mg/L. crease the likelihood of tremor. We did not observe an increased risk of MS with any 100,000. We would expect MS to be even lower in our of the measures of occupational Hg0 exposure examined study population than the reported numbers, given that in this study. Our analyses had limited power because MS is more common in women, although most of our sample was male. Furthermore, in contrast to our findof the small number of cases (n ¼ 23) in our sample. ings, the findings from recent studies showed a positive Although there was no evidence of an association with Hg0 exposure, the prevalence of MS in our sample was association between Hg0 exposure and increased antihigher than the prevalence reported in the US population. nuclear antibodies,44 as well as Hg0 exposure and MS.45 Investigators in some studies have shown that the The most recent study from the Centers for Disease number of amalgam restorations in participants’ mouths Control and Prevention in 2010 showed an estimated prevalence of MS ranging from 47.2 per 100,000 in Texas contributes to urinary Hg0 concentrations and is correlated with the Hg0 content of brain tissue at autopsy.36,46,47 In to 86.3 per 100,000 in Missouri and 109.5 per 100,000 42 our sample, urinary Hg0 concentrations varied between population in Ohio. In 2009, the National Society for Multiple Sclerosis reported 400,000 prevalent cases of MS those with high versus those with low numbers of amalgam in the United States, or 130 per 100,000.43 The prevalence restorations in the mouth (> 10 or # 10 amalgam restorations)40 (P < .01) as seen in Table 1; however, the of MS in our sample is substantially higher at 183 per

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ORIGINAL CONTRIBUTIONS

TABLE 4

both harmful and protective effects, conditional on the amount of MeHg and omega-3 fatty acids in EXPOSURE CASES/TOTAL NO. ODDS RATIO FOR MEAN UNIT INCREASE P OF PARTICIPANTS (95% CONFIDENCE INTERVAL)† VALUE the fish consumed.48,49 Number of Amalgam Restorations 94/8,550 0.63 (0.46-0.86) .003 Furthermore, without Placed and Removed per Week proper adjustment, Urinary Mercury Concentration 109/7,616 1.10 (1.00-1.22) .05 fatty acids in fish can (Micrograms per Liter) mask the harmful efMean Cumulative Mercury fects of MeHg on the Exposure (mg/L) nervous system.50 InAge 23-50 y 34/3,448 1.13 (1.05-1.22) .002 vestigators in some Age 51 y or older 51/2,994 1.00 (0.94-1.06) .97 studies have found * Adjusted for age, sex, and calendar year. † Means are as follows: number of amalgam restorations placed and removed per week, 19.1; urinary mercury associations of high concentration, 4.7 mg/L; cumulative mercury exposure, 92 mg/L. MeHg exposure from fish consumption magnitude of these differences was small, and standard and decreased performance on psychomotor tests, which deviations were relatively large (mean, 5.3 [SD, 5.5] versus are considered preclinical indicators for some types of mean, 4.4 [SD, 5.4] mg/L). tremor.51-53 To our knowledge, this is the largest study to date Information on tobacco use was limited and missing describing Hg0 exposure among practicing dentists. The for a large percentage (33.0%) of the study participants. study participants were fairly homogeneous with respect However, smoking is not likely to be a confounding to education—all participants were dentists—thus, we variable in this study. Mean Hg0 measurements did not vary according to tobacco use in our study population would expect less confounding by socioeconomic status than in studies that draw samples from the general pop- (P ¼ .33). Although smoking is a risk factor for MS and is ulation. MS is rare in the population, and even though it both positively associated as well as inversely related to different types of tremor,54-56 only a trace amount of Hg0 was more common in our study, the small number of cases limited our study’s statistical power and our ability is present in cigarettes and does not contribute subto conduct detailed analyses of this outcome. The volun- stantially to the body burden of Hg0.57 tary nature of participation in the ADA’s health screening Underdiagnosed MS is probably unlikely in this program could bias our findings in either direction. For study population of health care workers who, if they example, dentists who had neurologic illness or had been had MS, were likely aware of their diagnosis. However, using amalgam more frequently than usual may have more specific information on the subtype of MS could been more interested in participating. In contrast, dentists be important if Hg0 influences the pathways that unwith MS or severe tremor might have been less likely to derlie 1 or more of these subtypes.58 visit the annual session or be practicing dentistry. Misdiagnosis of tremor also could be a limitation. Results from a 2013 study among dentists showed that Tremor is a sign of multiple neurologic diseases, methyl mercury (MeHg) exposure via fish consumption including MS and Parkinson disease, cerebellar contributes to urinary Hg0 measurements more than problems, effects of drugs, and even physiological previously thought.40 Sherman and colleagues40 used a variations.29,30,59 novel approach with Hg isotopes to measure the type of More specific information regarding the type of Hg present in urinary Hg. They found MeHg could be tremor would have been useful. Additional tests also demethylated and, thus, contributed to a substantial may be advantageous, including an electromyogram proportion of Hg0 measured in urine. Although we and blood and urine tests to detect drug interaction, lacked complete information on the dentists’ seafood chronic alcoholism, or another condition that may consumption, our data demonstrate that urinary Hg0 cause tremor.59 concentrations were higher with occupational amalgam use (urinary Hg0, 4.8 [6.3] versus 2.5 [2.9] mg/L). CONCLUSIONS Furthermore, dentists have a higher urinary Hg0 concentration than do consumers of high levels of seafood in Occupational Hg0 exposure in US dentists has declined the general population (Figure 4). dramatically over the past few decades and is now Aside from exposure misclassification, fish conslightly higher on average than that in the general popsumption also could confound the association of Hg0 ulation. Our study results indicate that the occupational exposure and tremor. Investigators in studies involving exposure to Hg0 from amalgam may increase the risk of fish consumption and the nervous system have found tremor in practicing dentists. More studies of tremor, as

Adjusted* association of tremor and occupational mercury exposure.

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ORIGINAL CONTRIBUTIONS

well as MS among dentists, are needed to confirm these findings. Future studies should include retired dentists and better characterization of exposures, potential confounding variables, and neurologic outcomes. n SUPPLEMENTAL DATA

Supplemental data related to this article can be found at http://dx.doi.org/10.1016/j.adaj.2015.05.016. Ms. Anglen is a research assistant, Research and Laboratories, Science Institute, American Dental Association, 211 E. Chicago Ave., Chicago, IL 60611, e-mail [email protected]. Address correspondence to Ms. Anglen. Mr. Gruninger is a senior research fellow, Research and Laboratories, Science Institute, American Dental Association, Chicago, IL. Mr. Chou was a manager, Chemistry, Research and Laboratories, Science Institute, American Dental Association, Chicago, when this article was written. He now is retired. Dr. Weuve is an associate professor of medicine, Rush Institute for Healthy Aging, Rush University Medical Center, Chicago, IL. Dr. Turyk is an associate professor, Division of Epidemiology and Biostatistics, School of Public Health, University of Illinois at Chicago, Chicago, IL. Dr. Freels is an associate professor, Division of Epidemiology and Biostatistics, School of Public Health, University of Illinois at Chicago, Chicago, IL. Dr. Stayner is a professor, Division of Epidemiology and Biostatistics, School of Public Health, University of Illinois at Chicago, Chicago, IL. Disclosure. None of the authors reported any disclosures. The authors acknowledge all of the American Dental Association member dentists who participated in the health screening program during the American Dental Association’s annual meetings. 1. Bjorkman L, Pedersen N, Lichtenstein P. Physical and mental health related to dental amalgam fillings in Swedish twins. Community Dent Oral Epidemiol. 1996;24(4):260-267. 2. Saxe S, Wekstein M, Kryscio R, et al. Alzheimer’s disease, dental amalgam and mercury. JADA. 1999;130(2):191-199. 3. Clarkson TW. The three modern faces of mercury. Environ Health Perspect. 2002;110 Suppl 1:11-23. 4. Dodes J. The amalgam controversy: an evidence-based analysis. JADA. 2001;132(3):348-356. 5. Albers JW, Kallenbach LR, Fine LJ, et al. Neurological abnormalities associated with remote occupational elemental mercury exposure. Ann Neurol. 1988;24(5):651-659. 6. Levine SP, Cavender GD, Langolf GD, Albers JW. Elemental mercury exposure: peripheral neurotoxicity. Occup Environ Med. 1982;39(2):136-139. 7. Langworth S, Almkvist O, Soderman E, Wikstrom BO. Effects of occupational exposure to mercury on the central nervous system. Br J Ind Med. 1992;49(8):545-555. 8. Piikivi L, Hänninen H, Martelin T, Mantere P. Psychological performance and long-term exposure to mercury vapors. Scand J Work Environ Health. 1984;10(1):35-41. 9. US Environmental Protection Agency. Integrated Risk Information System: Mercury, elemental (CASRN 7439-97-6). Available at: http://www. epa.gov/iris/subst/0370.htm. Accessed February 23, 2015. 10. Liang YX, Sun RK, Sun Y, Chen ZQ, Li LH. Psychological effects of low exposure to mercury vapor: application of a computer-administered neurobehavioral evaluation system. Environ Res. 1993;60(2):320-327. 11. Roels H, Abdeladim S, Ceulemans E, Lauwreys R. Relationships between the concentrations of mercury in air and in blood or urine in workers exposed to mercury vapour. Ann Occup Hyg. 1987;31(2):135-145. 12. Boogaard PJ, Houtsma AT, Journée HL, Van Sittert NJ. Effects of exposure to elemental mercury on the nervous system and the kidneys of workers producing natural gas. Arch Environ Health. 1996;51(2):108-115. 13. Piikivi L, Hanninen H. Subjective symptoms and psychological performance of chlor-alkali workers. Scand J Work Environ Health. 1989; 15(1):69-74.

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50. Choi AL, Mogensen UB, Bjerve KS, et al. Negative confounding by essential fatty acids in methylmercury neurotoxicity associations. Neurotoxicol Teratol. 2014;42:85-92. 51. Dolbec J, Mergler D, Passos CJS, De Morais SS, Lebel J. Methylmercury exposure affects motor performance of a riverine population of the Tapajos River, Brazilian Amazon. Int Arch Occup Environ Health. 2000;73(3):195-203. 52. Lebel J, Mergler D, Branches F, et al. Neurotoxic effects of low-level methylmercury contamination in the Amazonian Basin. Environ Res. 1998; 79(1):20-32. 53. Roels H, Abdeladim S, Braun M, Malchaire J, Lauwerys R. Detection of hand tremor in workers exposed to mercury vapor: a comparative study of three methods. Environ Res. 1989;49(2):152-165. 54. Salzer J, Hallmans G, Nyström M, Stenlund H, Wadell G, Sundström P. Smoking as a risk factor for multiple sclerosis. Mult Scler. 2013;19(8):1022-1027. 55. Louis ED, Benito-León J, Bermejo-Pareja F. Population-based casecontrol study of cigarette smoking and essential tremor. Mov Disord. 2008; 23(2):246-252. 56. Hernán MA, Takkouche B, Caamaño-Isorna F, Gestal-Otero JJ. A meta-analysis of coffee drinking, cigarette smoking, and the risk of Parkinson’s disease. Ann Neurol. 2002;52(3):276-284. 57. Chiba M, Masironi R. Toxic and trace elements in tobacco and tobacco smoke. Bull World Health Organ. 1992;70(2):269-275. 58. Mayo Clinic. Diseases and conditions: multiple sclerosis—tests and diagnosis. Available at: http://www.mayoclinic.org/diseases-conditions/ multiple-sclerosis/basics/tests-diagnosis/con-20026689. Accessed February 8, 2015. 59. National Institute of Neurological Disorders and Stroke. Tremor fact sheet. Available at: http://www.ninds.nih.gov/disorders/tremor/detail_ tremor.htm. Accessed June 3, 2015.

ORIGINAL CONTRIBUTIONS

eTABLE 1

Adjusted* association of multiple sclerosis with occupational mercury exposure. CASES/TOTAL NO. OF PARTICIPANTS

ODDS RATIO FOR UNIT INCREASE (95% CONFIDENCE INTERVAL)

P VALUE

Number of Amalgam Restorations Placed and Removed per Week

14/8,566

0.98 (0.94-1.02)

.36

Urinary Mercury Concentration (Micrograms per Liter)

14/7,632

0.95 (0.83-1.09)

.48

Cumulative Mercury Exposure (mg/L)

10/6,446

1.00 (1.00-1.00)

.69

EXPOSURE

* Adjusted for age, sex, and calendar year.

eTABLE 2

Adjusted* association of tremor and occupational mercury exposure. CASES/TOTAL NO. OF PARTICIPANTS

ODDS RATIO FOR UNIT INCREASE (95% CONFIDENCE INTERVAL)

P VALUE

No. of Amalgam Restorations Placed and Removed per Week

94/8,550

0.97 (0.96-0.99)

.003

Urinary Mercury Concentration (Micrograms per Liter)

109/7,616

1.02 (1.00-1.04)

.05

Age 23-50 y

34/3,480

1.001 (1.000-1.002)

.002

Age 51 y or older

51/2,992

1.00 (1.00-1.00)

.97

EXPOSURE

Cumulative Mercury Exposure (mg/L)

* Adjusted for age, sex, and calendar year.

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