Mercury in dental amalgam: Are our health care workers at risk?

ABSTRACT

Dental amalgam in fillings exposes workers to mercury. The exposure to mercury was investigated among 1871 dental health care workers. The aim of the study was to evaluate the risk of mercury exposure among dental compared to nondental health care workers and to determine other risk factors for mercury exposure. Respondents answered questionnaires to obtain demographic, personal, professional, and workplace information and were examined for their own amalgam fillings. Chronic mercury exposure was assessed through urinary mercury levels. In total, 1409 dental and 462 nondental health care workers participated in the study. Median urine mercury levels for dental and nondental health care workers were 2.75 μg/L (interquartile range [IQR] = 3.0175) and 2.66 μg/L (IQR = 3.04) respectively. For mercury exposure, there were no significant risk factor found among the workers involved within the dental care. The Mann–Whitney test showed that urine mercury levels were significantly different between respondents who eat seafood more than 5 times per week compared to those who eat it less frequently or not at all (p = 0.003). The urinary mercury levels indicated significant difference between dental workers in their practice using squeeze cloths (Mann–Whitney test, p = 0.03). Multiple logistic regression showed that only the usage of cosmetic products that might contain mercury was found to be significantly associated with the urinary mercury levels (odds ratio [OR] = 15.237; CI: 3.612–64.276). Therefore, mean urinary mercury levels of health care workers were low. Exposure to dental amalgam is not associated with high mercury exposure. However, usage of cosmetic products containing mercury and high seafood consumption may lead to the increase of exposure to mercury.

Implications: Exposure to the high levels of mercury from dental amalgam can lead to serious health effects among the dental health care workers. Nationwide chronic mercury exposure among dental personnel was assessed through urinary mercury levels. Findings suggest low urinary mercury levels of these health care workers. Exposure to dental amalgam is not associated with high mercury exposure. However, the usage of cosmetic products containing mercury and high seafood consumption may lead to the increase of exposure to mercury.

Introduction

Dental amalgam contains mercury, combined with other materials such as tin, silver, copper, and zinc (Brown, 1988). It has been used as a dental restorative material for more than 150 years (Kilpatrick and Neumann, 2007; Lee Pair et al., 2004; Rugg-Gunn et al., 2001). However, the use of amalgam by dental health care workers exposes them to the toxic effects of mercury, if it is not handled properly (Martin et al., 1995; Rave et al., 2015). Exposure to high levels of mercury can lead to serious health effects in dental health care workers (Richardson et al., 2011; Atesagaoglu et al., 2006; Boyd et al., 1991; Cook and Yates, 1969; Gronka et al., 1970; Iyer et al., 1976;; Mantyla and Wright, 1976; Merfield et al., 1976). Mercury vapor releases in the air in the form of elemental mercury. Furthermore, a study by Mutter (2011) reported that mercury vapor is about 10 times more toxic than lead to human neurons and has synergistic toxicity with other metals. This is a major concern in dental health care workers, as 80% of such inhaled vapor will enter the bloodstream (Schuurs, 1999). In acute exposure, where mercury vapor is inhaled at high levels, fatalities may result from corrosive bronchitis and interstitial pneumonitis. With lesser severity, it can affect the central nervous system and cause hyperexcitability and tremors (Amdur et al., 1993; Echeverria et al., 1995).

A major cause of mercury exposure is spillage, which releases mercury vapor. Dental staff may also be exposed to mercury from improper mercury hygiene, manual mulling to express excess mercury from freshly mixed amalgam, ultrasonic amalgam condensers, mechanical amalgamators, heating the amalgam carrier to dislodge set particles, failure to use high-vacuum suction while removing old amalgam restorations, and improper dry heat sterilization of amalgam-contaminated instruments (Naleway et al., 1985). Dental health care workers handling mercury were shown in a number of studies to have higher levels of mercury than those who did not handle or had less exposure to amalgam (Harakeh et al., 2002; Holland et al., 1994; Naleway et al., 1985; Ritchie et al., 2004; Tezel et al., 2001). A study by Rathore et al. (2012) observed that the main exposure to mercury from dental amalgam occurs during placement or removal of restoration in the tooth and that mercury hygiene procedures need to be followed to minimize the risks of adverse health effects. The Malaysian Dental Council (MDC) has formulated a “Code of Practice for Dental Mercury Hygiene,” which should be strictly followed by dental health care workers to reduce this potential problem (MDC, 2002). In addition to this practice, it is critical to determine mercury level in occupationally exposed personnel and its harmful effects on the body. The objective of this present study is to evaluate the risk of mercury exposure among the dental health care workers of Malaysia.

Methods

This nationwide study was carried out from the end of July 2004 to the end of December 2005 within the Ministry of Health of Malaysia, for a period of 16 months. Sample size was calculated using Epi Info 7.1.4 (Epi, 2015), and participants were selected through stratified random sampling. Ethical approval was granted by the Medical Research Ethics Committee (MREC). In total, 1409 dental and 462 nondental health care workers participated in the study. After informed consent was obtained, questionnaires were administered by a physician to obtain information on demography, personal habits, and professional and workplace characteristics. The questionnaire also investigated other possible exposures to mercury, including from seafood consumption, smoking, hobbies, recreational activities, and use of skin-lightening cosmetics and alternative medicine. Participants were also examined by dentists for the number of amalgam fillings they had.

To assess chronic mercury exposure, spot urine samples were examined for urinary total mercury levels of participants. Urine samples were collected in an acid-washed plastic containers. Approximately, 20-mL samples of mid-stream urine were obtained from participants and labeled accordingly. Samples were stored at 4°C and analysis was performed within a month. Cold vapor atomic absorption spectrometry using a flow injection mercury system (FIMS-400) was used to determine urinary mercury (Tahir et al., 2012; Md. Noh et al., 1998). The detection limit of this method based on three standard deviations of the blank was 0.05 μg/L in urine when a 500-μL sample loop was used. The relative standard deviation of the signal at a level 5 μg/L Hg(II) in urine was about 2% (five measurements). Blanks, calibration standards, and calibration check standards were included in the assays as quality controls. The health care workers were considered as having high exposure if urine mercury levels were above 20 μg/L (Pohanish, 2012).

The statistical package SPSS (version 19.0 for Windows; SPSS, Inc.) was used to analyze data. Data were cleaned and checked for discrepancies before the analysis. The main variable of urine mercury level was found to be skewed; therefore, the normality assumption was not fulfilled. Median and interquartile range (IQR) are presented to replace mean and standard deviation (SD). Median values were compared using the Mann–Whitney and Kruskal–Wallis (KW) tests. Relationship between variables was tested by Spearman correlation. Multiple logistic regression analysis was carried out for personal, professional, and workplace factors that may influence urinary mercury levels among healthcare workers. For all tests, a p value ≤0.05 denoted statistical significance.

Results

In total, 1406 dental and 462 nondental health care workers participated in the nationwide study. The participants were 77.0% Malays, 13.5% other Bumiputras, 5.0% Chinese, and 4.4% Indians. The group comprised 78.9% female and 21.1% male participants. The highest number of participants (14.5%) was from the largest state, Sarawak, and the lowest number from Perlis (0.5%). The mean ages of dental and nondental health care workers were 36.52 (SD = 9.65) and 38.3 (SD = 9.62) years, respectively. Most of the dental health care workers were dental nurses, followed by attendants, dental surgical nurses, and dental officers. The dental personnel were matched with nondental health care workers, who consisted mostly of nurses, assistant nurses, attendants, and medical officers (Table 1). The dental and nondental health care workers were matched according to locality and similar educational level for each job category, with the assumption that matched respondents had a higher probability of similar diets (such as consumption of seafood) and lifestyles.

Table 1. Distribution of staff categories in study.

Staff categories	Dental (n = 1409), n (%)	Nondental (n = 462), n (%)
Dental officers/medical officers	141 (10.0%)	65 (14.1%)
Dental Nurses/health nurses, medical assistants, assistant health officers	499 (35.4%)	161 (34.8%)
Dental surgical assistants (DSA)/assistant nurses and midwives	365 (25.9%)	148 (32.2%)
Attendants	403 (28.6%)	85 (18.5%)
Other supporting staff	1 (0.1%)	3 (0.6%)

It was found that 97.8% of all health care workers (dental and nondental) sampled had urinary mercury levels of 10 μg/L and below, and 99.6% of participants had urinary mercury levels of ≤20 μg/L; see Table 2. The urine mercury levels were found to be not normally distributed based on the Kolmogorov–Smirnov test (p < 0.05). Median urine mercury levels for dental and nondental health care workers were 2.75 μg/L (IQR = 3.0175) and 2.66 μg/L (IQR = 3.04), respectively, with no statistically significant difference using the Mann–Whitney test between the two groups (p = 0.202). The prevalence rate of high mercury levels (>20 μg/L) was 0.4% for both dental and nondental groups.

Table 2. Urine mercury levels among health care workers.

Urine mercury level (μg/L)	Dental (n = 1409), n (% of dental respondents)	Nondental (n = 462), n (% of nondental respondents)
0–5.00 μg/L	1192 (84.6%)	396 (85.7%)
5.01–10.00 μg/L	179 (12.7%)	59 (12.8%)
10.01–20.00 μg/L	29 (2.1%)	5 (1.1%)
20.01–35.00 μg/L	2 (0.1%)	1 (0.2%)
35.01–50 .00 μg/L	1 (0.1%)	0 (0%)
50.01 μg/L and above	3 (0.2%)	1 (0.2%)

Personal factors were looked at to see whether there were any significant differences of urinary mercury levels by gender, smoking behavior, taking alternative treatment and the usage of cosmetic products that might contain mercury, number of amalgam fillings, and seafood consumption (Tables 3 and 4). The results of the Mann–Whitney test showed that the urine mercury levels were significantly different between respondents who eat seafood more than five times per week of compared to those who eat it less frequently or have no consumption at all (p = 0.003). However, gender (p = 0.898), smoking behavior (p = 0.170), taking alternative treatment (p = 0.717), and usage of cosmetic products with mercury (p = 0.962) showed no significant differences in urinary mercury levels. The mean of the number of amalgam fillings and amalgams removed was 9.44 and 0.73 per week, respectively. Correlations between urinary mercury levels with number of amalgam fillings per week and number of amalgams removed per week were established. None of the Spearman correlations was found to be significantly correlated (p > 0.05), although a slight positive correlation was shown.

Table 3. Personal characteristics of health care workers.

Personal characteristics	Dental (n = 1409), n (%)		Nondental (n = 462), n (%)
Mean (SD) age in years	36.52 (9.65)		38.29 (9.62)
Gender (n = 1871)	Male	Female	Male	Female
	302 (21.4)	1107 (78.6)	91 (19.7)	371 (8.03)
Mean (SD) amalgam fillings in mouth	4.22 (3.65)		3.36 (3.41)
Smoking (n = 1871)	Yes	No	Yes	No
	118 (8.37)	1291 (91.6)	31 (6.7)	431 (93.2)
Skin-lightening cosmetic use	Yes	No	Yes	No
(n = 1870)	136 (9.65)	1273 (9.03)	53 (11.47)	408 (88.3)

Table 4. Average seafood consumption.

Average seafood consumption	Dental respondents (n = 1409), n (%)	Nondental respondents (n = 462), n (%)
More than 10 times per week	436 (30.9%)	149 (32.3%)
5 to 10 times per week	603 (42.8%)	183 (39.7%)
Less than 5 times per week	253 (18.0%)	89 (19.3%)
Rarely	107 (7.6%)	37 (8.0%)
Never	10 (0.7%)	3 (0.7%)

For the dental group, professional and workplace factors did not have any significant effect on urinary mercury levels. The focusing factors were the number of spills staff were exposed to, whether they used bulk or precapsulated amalgam, duration of work in dental facilities, duration of work with amalgam, average number of working days per week with amalgam, and the number of amalgam fillings and removals carried out or assisted per week. Comparison on the good and bad practices based on compliance with the Code of Practice for Dental Mercury Hygiene (MDC, 2002) carried out by dental health care workers is shown in Table 5. It was found that most dental staff followed good clinical practice in their routine work. The urinary mercury levels were found to be significantly different for dental workers using squeeze cloths in their practice (Mann–Whitney test, p = 0.03).

Table 5. Comparative work practices of dental health care workers.

Work practice	Following good practice guidelines, n (%) and median (IQR) of urinary mercury level (µg/L)	Not following good practice guidelines, n (%) and median (IQR) of urinary mercury level (µg/L)	p Value (Mann–Whitney test)
Method of amalgam residue disposal, n = 1384	1212 (86.2%)	194 (13.8%)	p = 0.748
	2.35 (2.63)	2.44 (2.65)
Squeeze cloth, n = 1409	1534 (82%)	337 (18%)	p = 0.03*
	(do not use squeeze cloths)	(use squeeze cloths)
	2.25 (2.59)	2.65 (3.11)
Use of gloves, n = 1868	1301 (69.6%)	567 (30.4%)	p = 0.546
	2.26 (2.34)	2.36 (2.63)
Face mask, n = 1868	1159 (62.04)	709 (38.0)	p = 0.849
	2.34 (2.65)	2.3 (2.67)
Mercury spill, n = 1868	1789 (95.8)	79 (4.69)	p = 0.358
	2.31 (3.27)	2.93 (2.63)

Further analysis of multiple logistic regression was conducted to determine the risk factors associated with the level of urinary mercury. All the possible risk factors such as dental and nondental health care workers, gender, smoking behavior, taking alternative treatment, the usage of cosmetic products that might contain mercury, and seafood consumption were analyzed together using stepwise multiple logistic regression. After controlling for other factors, the results showed that only the usage of cosmetic products that might contain mercury was found to be significantly associated with the urinary mercury levels (OR = 15.237; CI: 3.612–64.276).

Discussion

The population can be exposed to mercury from many sources. Exposure occurs through natural occurrences in the environment, such as volcanic activity, movement of streams, and biological processes. Sources could be anthropogenic, such as from industrial processes, mining, and combustion of fossil fuel and incineration of medical and chemical wastes (World Health Organization [WHO], 2003). Some fungicides and bactericides may also contain mercury, as the mercury compounds are highly toxic to fungi and other microorganisms (Apostoli et al., 2002; Maliszewska et al., 1985). Humans are also exposed to mercury from their own dental amalgam fillings and from consumption of seafood (Barregard et al., 2006). Dental amalgam continuously releases mercury, which results in exposure in those individuals possessing fillings composed of this material (U.S. Food and Drug Administration [FDA], 2009). Blood pressure monitors (sphygmomanometers) and thermometers may contain mercury and become a source of mercury exposure if they break or leak. It was reported that in a study of 1107 individuals from 15 countries, participants who had no known occupational, medicinal, or other exposures to mercury had the following urinary mercury levels: 78% <0.5 μg/L; 86% <5 μg/L; 89% <10 μg/L; 94% <15 μg/L; 95% <20 μg/L (Goldwater, 1977). Richardson et al. (2011) updated the link between dental amalgam and levels of Hg in various bodily fluids and tissues, and improved the quantification of Hg exposure from dental amalgam on a population basis from the post-2000 U.S. population (from the National Health and Nutrition Examination Survey [NHANES]). Studies have showed that seafood consumption was associated with higher urinary mercury levels (Apostoli et al., 2002; Barregard et al., 2006; Freije and Awadh, 2011; Levy et al., 2004).

All health care workers should be warned of the potential toxicity of mercury. Nondental health care workers would also be needed to be aware of how to handle and disposal of mercury, in case they are exposed to spills of mercury from broken sphygmomanometers and thermometers.

Mercury hygiene is an important practice, as without proper mercury handling, dental personnel may suffer from varying degrees of mercury intoxication, including fatality. There are a number of studies which demonstrated significantly higher levels of urinary mercury in those who worked with dental amalgam compared to those who did not (Decharat et al., 2014; Hörsted-Bindslev, 2004; Jones, 1981). However, if dental health care workers practice good mercury hygiene, it is possible to reduce mercury body burdens to a level that does not differ significantly from those who are not occupationally exposed to mercury (Hörsted-Bindslev, 2004).

The mean and prevalence rates of high urinary mercury levels of the dental and nondental health care workers in Malaysia were found to be low and did not differ significantly between groups. It was also found that among the dental health care workers, occupational (i.e., professional and workplace) factors were not associated with urinary mercury levels. This possibly reflects good mercury hygiene among the Malaysian dental health care workers.

“The Code of Practice for Dental Mercury Hygiene,” promoted by the Oral Health Division in the Ministry of Health of Malaysia, plays a role in creating a safer environment for dental health care workers by inculcating good work habits (MDC, 2002). The majority of dental health care workers in our study appeared to be following good practice in their daily work, that is, in their disposal of amalgam residue, use of gloves and face masks, and refraining from use of squeeze cloths. A small fraction of dental health care workers however, still required to improve their work practice to follow the recommended guidelines (Table 5). A better awareness of mercury hygiene and toxicity among those who work with amalgam is important to keep the risk of mercury exposure low among the dental profession. Our results found that squeezing the excess mercury into the cotton cloths could increase the risk for mercury exposure. This study found that the proportion of respondents (18.0%) who squeeze excess mercury into the cotton cloths is lower than that (32.2%) reported by Iwaola et al. (2015).

Our results found that urine mercury level were significantly different between respondents who eat seafood more than five times per week compared to those who eat it less frequently or not at all (Mann–Whitney test, p = 0.003). A study by Levy et al. (2004) observed that a higher level of urine Hg was reported with children who visited dentists for amalgam fillings than with the children without amalgam fillings. This level was significantly higher in those who consumed a lot of fish. From the results of a study conducted in California, it was found that 89% of the respondents had Hg levels exceeding the reference dose (RfD) and that the respondents consumed 30 types of fish (Hightower and Moore, 2003).

Our finding from multiple logistic regression demonstrated that the usage of cosmetic products which might contain mercury was significantly associated with the urinary mercury levels (OR = 15.237; CI: 3.612–64.276). Cosmetic products may affect the urine mercury level of the users. A study by McRill et al. (2000) noticed that high urine mercury levels among respondents indicated that the use of a cosmetic cream constituted a significant exposure. Sixty-six among the 89 respondents showed an elevated level of urine mercury. However, this finding warrants further investigation.

Conclusion

From the investigation, it was found that mean urinary mercury levels of dental and nondental health care workers were low. Exposure to dental amalgam is not associated with high mercury exposure. However, the usage of cosmetic products containing mercury and high seafood consumption may lead to the increase of exposure to mercury.

Funding

This study was funded by the Malaysian Ministry of Health with a project code: MRG-2004-2. This study is also supported by the FRGS/2/2014/SG03/UKM/03/2. Many thanks to the Director General of the Ministry of Health, Director of the Institute for Medical Research, Director of the Oral Health Division of the Ministry of Health, Department of Occupational Safety and Health, our collaborators, research team members, state project coordinators, and individuals who contributed to this study.

Additional information

Funding

This study was funded by the Malaysian Ministry of Health with a project code: MRG-2004-2. This study is also supported by the FRGS/2/2014/SG03/UKM/03/2. Many thanks to the Director General of the Ministry of Health, Director of the Institute for Medical Research, Director of the Oral Health Division of the Ministry of Health, Department of Occupational Safety and Health, our collaborators, research team members, state project coordinators, and individuals who contributed to this study.

Notes on contributors

M. Sahani

M. Sahani is a Senior Medical Lecturer and Chairperson, School of Diagnostic Science and Applied Health at the Faculty of Health Sciences, Universiti Kebangsaan Malaysia, Jalan Raja Muda Abdul Aziz, Kuala Lumpur, Malaysia.

N.S. Sulaiman

N.S. Sulaiman works as a Medical Researcher at the Institute for Medical Research, Jalan Pahang, Kuala Lumpur, Malaysia.

B.S. Tan

B.S. Tan works as an Associate Professor at the School of Dentistry, Institute of Research, Development and Innovation, International Medical University, Kuala Lumpur, Malaysia.

N.A. Yahya

N.A. Yahya works as a Medical Researcher at the the Institute for Medical Research, Jalan Pahang, Kuala Lumpur, Malaysia.

Z.F. Anual

Z.F. Anual works as a researcher at the Faculty of Health Sciences, Universiti Kebangsaan Malaysia, Jalan Raja Muda Abdul Aziz, Kuala Lumpur, Malaysia.

W.R. Wan Mahiyuddin

W.R. Wan Mahiyuddin works as a Researcher at the Institute for Medical Research, Jalan Pahang, Kuala Lumpur, Malaysia.

M.F. Khan

M.F. Khan is a Senior lecturer and Research Fellow at the Centre for Tropical Climate Change System, Institute of Climate Change, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia.

K.A. Muttalib

K.A. Muttalib recently retired. Formerly worked as a Deputy Director, at the Oral Health Division, Ministry of Health, Putrajaya, Malaysia.