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Research overview

Mercury exposure among dental personnel

The Norwegian Knowledge Centre for the Health Services has conducted a systematic review on available research on mercury exposure and adverse health outcomes in dental health care workers.


  • Year: 2011
  • By: Norwegian Knowledge Centre for the Health Services
  • Authors Hammerstrøm KT, Holte HH, Dalsbø TK, Vist GE, Steiro A, Lidal IB, Gundersen M, Reinar LM, Jamtvedt G.
  • ISSN (digital): 1890-1298
  • ISBN (digital): 978-82-8121-395-1

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Key message

Mercury exposure in dental personnel is primarily connected to the use of amalgam restorations in dental care practice. In Norway, the use of mercury in dental restorations has been prohibited since 2008. Interim permissions that allowed the use of amalgam on special indications ended 31.12.2010.

The Norwegian Knowledge Centre for the Health Services has conducted a systematic review on available research on mercury exposure and adverse health outcomes in dental health care workers. We searched systematically in databases and contacted experts in Norway to help identify scientific research publications. From 981 identified references, we included 134 papers.

Key messages:

  • Dental personnel in Norway were exposed to mercury to variable degrees in the 1960s, 1970s and 1980s. The highest exposure is found in the 1960s. 
  • Mercury concentrations in urine and in other body tissues were generally higher in dental personnel than in unexposed control groups, both in Nordic and international studies.  
  • Dental care assistants in general had higher concentrations of mercury in urine compared to dentists.  
  • Six studies that reported 64 health outcomes for dental care assistants compared to unexposed control groups showed 20 outcomes in favour of the control group and three outcomes in favour of dental care assistants.
  • Nine studies that reported 62 health outcomes for dentists compared to unexposed control groups showed 13 outcomes in favour of control groups and 13 in favour of dentists.
  • Some studies lacking control groups, but of high methodological quality, showed statistically significant associations between high levels of mercury in urine and less favourable results on outcomes such as reduced attention, memory and coordination. The same associations are indicated in studies comparing dental assistants to unexposed populations.

Summary

Background
Mercury exposure in dental personnel is connected to the use of amalgam restorations in dental care practice. Preparation of amalgam restorations in dental offices can emit mercury vapour and mercury particles in the air. In Norway, the use of mercury in dental restorations has been prohibited since 2008. From 1995-2002, there was a 95% decline in the use of amalgam restorations in primal teeth and an 85% decline in permanent teeth in children and adolescents. Interim permissions that allowed the use of amalgam on special indications ended 31.12.2010.

Chronic exposure to mercury might give adverse health outcomes. Early symptoms might be unspecific, and include tiredness, loss of appetite, irritability, anxiety, agitation and depression. Later symptoms might include memory loss, difficult sleeping patterns and personality change. Furthermore, chronic mercury poisoning might lead to tremor, sight disturbance and polyneuropathy.

The Norwegian Knowledge Centre for the Health Services was asked by the Norwegian Directorate of Health to conduct a systematic review on mercury exposure and effects on health in dental health care workers.

Method
On December 12, 2010, we searched the following databases systematically: Ovid Medline, Ovid EMBASE, Ovid PsycINFO, The Cochrane Library, Web of Science and TOXLINE. Additionally, we contacted experts to help identify research papers and references to relevant studies.

The inclusion criteria were: studies describing to what extent dental health care personnel had been exposed to mercury, as well as studies measuring health outcomes in dental personnel that had been exposed to mercury or comparative studies on health outcomes in exposed dental personnel and controls not exposed to mercury, o r follow-up studies on health outcomes in dental personnel exposed to mercury. We included studies that measured mercury in body tissues and/or in the dental care setting or studied that estimated such exposure. We included the following study designs: systematic reviews, controlled trials, case-control studies, cohort studies, surveys, cross sectional studies, registry data, patient series, interrupted time series and case studies.

Two researchers independently screened titles and abstracts according to the inclusion criteria. After exclusion, two researchers independently assessed the methodological quality of the studies, with the use of checklists available from the Knowledge Centre website. We also held a meeting in January, seeking feedback from Norwegian research communities and other stakeholders on the subject.

Results
We identified 1445 titles from the database searches, and received 594 publications from the research communities; after duplicate removal the number of unique references was 981. Of these, we reviewed 380 articles in full text of which 134 papers were included.

We identified 83 studies on mercury exposure in dental health care personnel. 

Norwegian studies do not differ from the other Nordic countries or from other international studies. It seems that dental care assistants have been more exposed to mercury than dentists. Most studies reported that among the exposed there are individuals with a much higher level of mercury in tissues (urine, hair, nails) compared to the average for the whole group. Several authors state that four to five percent of the respondents had high levels (including some ”extreme” measurements) of mercury.

One study showed that dentists had a mean of 240 nmol/l mercury in urine in the 1960s versus 22 nmol/l in the 1990s. Corresponding levels for dental care assistants in were 160 nmol/l and 21 nmol/l respectively. This study also showed that on an individual basis, there were measurements above 200 nmol/l mercury in urine in the 1960s relatively often. 200 nmol/l mercury in urine is the biological threshold limit value for mercury determined by work environment authorities in Norway. There were no documented measurements above this level in the 1990s.

We included 48 papers from 35 studies that measured health outcomes after mercury exposure. 

Fifteen cross sectional studies reported health outcomes for dental health care workers compared to unexposed control groups. Six studies that reported 64 health outcomes for dental care assistants compared to unexposed control groups showed 20 outcomes in favour of the control groups and three outcomes in favor of dental care assistants. Nine studies that reported 62 health outcomes for dentists compared to unexposed control groups showed 13 outcomes in favour of control groups and 13 in favour of dentists.

Results from studies that investigated the associations between dose of exposure and health outcomes, showed associations between higher levels of mercury in urine and less favourable results on some outcomes. Statistically significant associations between mercury in the urine and outcomes such as reduced attention, memory, coordination and headache for both male dentists and female assistants were found. There were also statistically significant associations between mercury in the urine and depression and skin disease for assistants. Verbal intelligence and reaction time were not associated with the level of mercury in urine.  

Discussion
We have reviewed and summarised available research on exposure of mercury and health outcomes in dental health care workers in a systematic review. One of the limits of this review might be the limitation to dental health care workers. However, the inclusion of other mercury exposed workers/employees would not have helped us to answer to what extent dental health workers have been exposed to mercury.   

Statistically significant findings do not necessarily translate into clinically important findings. The question of whether or not significant differences are clinically important will vary from test to test, and has to be judged from case to case.

Conclusion
Dental health personnel in Norway have been exposed to mercury to varying degrees. The highest exposure is shown in the 1960s. Concentrations of mercury in urine and other tissues were higher for dental personnel than that found in unexposed populations. Dental health care assistants in general had higher concentrations of mercury in urine than dentists. Many studies reported individuals with extreme values. In the 1990s and 2000s no values exceeding 200 nmol/l of mercury in urine amongst dental health care workers are measured.

15 cross sectional studies reported health outcomes for dental health care workers compared to unexposed control groups. Six studies that reported 64 health outcomes for dental care assistants compared to unexposed control groups showed 20 outcomes in favour of the control groups and three outcomes in favor of dental care assistants. Nine studies that reported 62 health outcomes for dentists compared to unexposed control groups showed 13 outcomes in favour of control groups and 13 in favour of dentists. Some studies lacking control groups, but of high methodological quality, showed statistically significant associations between high levels of mercury in urine and less favourable results on outcomes such as reduced attention, memory and coordination. The same associations are indicated in studies comparing dental assistants to unexposed populations.

This is a publication from the Norwegian Knowledge Centre for the Health Services. The Knowledge Centre became part of the Norwegian Institute of Public Health 01/01/2016.