Exposure to mercury by dental health workers is associated with amalgam restorations in dental practices. Dental amalgam is a mixture of metals, consisting of liquid mercury and a powdered alloy composed of silver, tin, and copper. Approximately 50% of dental amalgam is elemental mercury by weight. During the amalgam preparation and tooth restoration process, the mercury vapor is emitted into the air [
Early symptoms can be unspecific and present as tiredness, loss of appetite, irritability, anxiety, agitation, and depression. Later symptoms can develop as memory loss, difficult sleep patterns, and personality changes [
The main objectives of this study were to determine airborne mercury levels in dental clinics and the mercury concentrations in urine samples of dental health workers. In addition, we aimed assessing any associations between mercury levels in urine and airborne mercury concentrations in dental clinic, as well as with descriptive factors, such as demographics, job positions, working environments, and behavioral hygiene.
The study population consisted of dental health workers (16 dentists, 70 dental hygienists, and 38 dental assistants) who worked at 17 community hospitals in Nakhon Si Thammarat Province in the South of Thailand between May and September 2013.
124 exposed subjects were recruited. 30 exposed subjects were male and 94 exposed subjects were female. Control subjects, matched to exposed subjects by gender, were recruited from the workers who worked at the same community hospitals but had not had occupational contact with mercury. The inclusion criteria of the exposed group were dental health workers aged between 20 and 60 years who had experienced and contacted mercury on their daily routine work, for at least one year. They agreed to participate in the study and provided written informed consent.
The 248 subjects (124 exposed and 124 unexposed) were interviewed using structured questionnaire interviews. Spot urine samples (30 mL) were collected that extended from the time the subjects went to bed through the first urination of the morning. The urine samples were kept in polypropylene sampling vessels and stored at −20°C prior to analysis.
In the questionnaire interviews, detailed descriptive information was collected, including personal descriptive characteristics, dietary habit, occupational life styles, working positions, working environment, and personal hygiene. Direct observations were also made and recorded to confirm the questionnaire interviews. At the end of shifts, the subjects were also interviewed.
Area air samples were taken at 3 areas within the dental healthcare office including the area around the base of the chair, the area around the storage area for waste amalgam, and the work surface where the preparation of amalgam usually taken place. Personal air samples were collected in the exposed subject’s breathing zone. All samples were collected, with personal air samplers, for analysis of mercury concentrations by solid sorbent tube (hopcalite in single section, 200 mg, SKC Inc., PA, USA) (Gilian, Gilair-5RP Air Sampler). Before use, each air sampler was calibrated to obtain a flow rate of 0.2 L/min for a recommended sampling time of 8-hour time-weighted average. Samplers were attached to the pumps with flexible tubing and air was collected. Samples were capped and packed securely for shipment. The concentrations of mercury vapor were analyzed using cold-vapor atomic absorption spectrophotometer (CVAAS) (NOISH method 6009) [
Creatinine in urine was measured using a test kit based on the Jaffé reaction. (Merckotest number 3385; Merc, Darmstadt, Germany).
Quantitatively transferred the hopcalite sorbent and the front glass wool plug from each sampler tube into a 100 mL volumetric flask. 2.5 mL of concentrated HNO3 and 2.5 mL of concentrated HCl were added, mixed, and allowed standing for a further 1 hour or until the black sorbent was dissolved. The solution’s color changed to dark brown, which was carefully diluted to 50 mL with deionized water. This process was maintained until the blue-green color was sustained; then a further 2.5% w/w HNO3 and 2.5% HCl were added and mixed. Mercury in air samples was analyzed by CETAC M6000A cold-vapor atomic absorption spectrometer (CVAAS) mercury analyzer. This method of mercury in air samples (hopcalite in single section, 200 mg) determination was modified from NIOSH 6009 [
CETAC M6000A cold-vapor atomic absorption spectrometer (CVAAS) was used for cold vapor analysis. Instrumental parameters were a slit width of 0.5 rim, wavelength of 253.7 nm, photo multiplier voltage of 4 mA, no background correction, and a delay time before reading of 55 or 70 sec. The delay time was reduced to 55 sec to reduce digest volume used in analysis. The limit of quantization (LOQ) corresponded to 0.01 absorbance units, which was produced by solutions containing 1.0
Two milliliters of each urine sample was mixed with 0.1 mL of 35% w/w nitric acid, 0.2 mL of 50% w/w sulfuric acid, and 0.5 mL of 5% w/v potassium permanganate; then microwave digestion was carried out at an elevated temperature for 15 minutes. The sample solution was allowed standing at room temperature. If the solution’s color changed from purple to brown, then a further 0.5 mL of permanganate solution was added, mixed, and allowed standing for a further 8 hours. This process of adding successive aliquots of permanganate solution and allowing the reaction to proceed was maintained until the purple color was sustained. With increasing masses of dissolved organic materials, increasing volumes of permanganate solution are required. After the permanganate reaction was completed, 0.4 mL of 2.5% (w/v) potassium persulfate was added and mixed; then it was placed in an incubator at 95°C for at least 2 hours before cooling down to room temperature. Next, 0.5 mL of 5% (w/v) hydroxylamine hydrochloride and 1 mL of 10% SnCl2 solution were added with an accessory dispenser. The total volume was made up to 10.0 mL with reagent water and mixed well prior to determination. This method of urinary mercury determination was modified from Ham, 1997 [
Urine mercury was analyzed by CETAC M6000A cold-vapor atomic absorption spectrometer (CVAAS) mercury analyzer. Field blank samples and laboratory blank samples were used in all of the analyses as a quality control. Determination of urinary mercury levels was calibrated by preparing a series of standard additions containing 0, 10, 20, and 40
Descriptive statistics (means and SD) were used to characterize the difference between exposed and unexposed groups including demographic characteristics, mercury vapor levels, and urinary mercury concentrations, frequencies, and percentages.
The data were tested for the normality using a Kolmogorov-Smirnov test. The data were normally distributed. An independent
248 subjects participated in this study. Most of the subjects (51.6%) were aged between 30 and 40 years. The group of exposed subjects consisted of 30 smokers (24.2%) and 94 nonsmoking exposed subjects (80.6%), while the group of unexposed subjects consisted of 78 smokers (62.9%) and 46 nonsmoking unexposed subjects (37.1%).
More unexposed subjects drank alcohol (62.9%) than did the exposed subjects (19.4%). In this study, 57.3% of exposed subjects consumed fish and seafood ≥3 times/month and 42.7% of them consumed fish and seafood ≤3 times/month, while 68.5% of unexposed subjects consumed fish and seafood ≥3 times/month and 31.5% of them consumed fish and seafood 43 times/month (Table
Descriptive characteristics of the exposed and unexposed subjects (
Parameters | Exposed |
Unexposed |
---|---|---|
Sex | ||
Male | 30 (24.2) | 30 (24.2) |
Female | 94 (75.8) | 94 (75.8) |
Age (yrs) | ||
20–30 | 40 (32.3) | 20 (16.1) |
>30–40 | 66 (53.2) | 62 (50.0) |
>40–50 | 16 (12.9) | 42 (33.9) |
>50 | 2 (1.6) | 0 |
Cigarette smoking | ||
No | 94 (75.8) | 46 (37.1) |
Yes | 30 (24.2) | 78 (62.9) |
Alcohol drinking | ||
No | 100 (80.6) | 46 (37.1) |
Yes | 24 (19.4) | 78 (62.9) |
Dietary habit (fish or seafood consumption) | ||
≤3 times/month | 53 (42.7) | 39 (31.5) |
≥3 times/month | 71 (57.3) | 85 (68.5) |
The accuracy of airborne mercury analysis was checked by running 3 samples of Standard Reference Material (SRM). The limit of detection (LOD) was 0.5
Environmental mercury vapor samplings and percentage of mercury airborne levels exceeded (% OELs).
Personal and area samplings |
| |||||
---|---|---|---|---|---|---|
|
Mean | Median | Min | Max | Number of mercury airborne levels exceeded | |
Chairs | 24 | 9.42 | 5.70 | 0.20 | 31.10 | 2 (8.3) |
Amalgam storages | 17 | 19.28 | 18.00 | 10.00 | 29.00 | 6 (35.3) |
Preparation areas | 17 | 8.88 | 10.50 | 0.70 | 20.50 | 2 (11.8) |
Total areas samplings |
|
|
|
|
|
|
Personal air samplings | 124 | 15.60 | 12.20 | 2.00 | 38.00 | 22/124 (17.7) |
The mean urinary mercury levels of the exposed and unexposed subjects were significantly different (
Urinary mercury levels of exposed and unexposed subjects.
Metal | Exposed |
Unexposed |
|
---|---|---|---|
Mercury ( |
|||
Mean | 8.24 | 2.00 | <0.0 |
Standard deviation | 1.89 | 0.11 | |
Range | 2.00–22.84 | 1.00–10.00 |
There was no significant difference in urinary mercury levels among job positions (
Descriptive characteristics of urinary mercury levels, PPEs used, and personal hygiene, behaviors, and dietary habit.
Parameter | Number of mercury exposed dental health workers | Urinary mercury mean |
SD |
|
---|---|---|---|---|
Position | ||||
Dentists | 16 | 5.37 | 1.29 | 0.182 |
Dental hygienists | 70 | 8.75 | 1.95 | |
Dental assistants | 38 | 8.66 | 1.16 | |
Duration of work (yrs) | ||||
≤5 | 10 | 3.15 | 0.02 | 0.0 |
>5 | 114 | 8.47 | 1.09 | |
PPEs uses | ||||
Mask | ||||
Yes | 110 | 7.19 | 0.89 | <0. |
No | 14 | 16.84 | 1.28 | |
Glove | ||||
Yes | 30 | 6.91 | 1.25 | <0. |
No | 94 | 12.59 | 1.32 | |
Safety glasses | ||||
Yes | 10 | 7.64 | 1.68 | 0.223 |
No | 114 | 5.30 | 0.87 | |
Ate snacks/drank water during work | ||||
Sometimes | 88 | 7.80 | 1.76 | 0.252 |
Always | 36 | 9.46 | 1.17 | |
Wash hands before lunch | ||||
Sometimes | 62 | 9.58 | 0.97 | 0.0 |
Always | 62 | 6.98 | 1.53 | |
Wash hands before dinner | ||||
Sometimes | 56 | 9.26 | 1.51 | 0.166 |
Always | 68 | 7.48 | 1.25 | |
Clean cloths | ||||
Everyday | 24 | 7.64 | 0.68 | 0.134 |
2-3 days | 16 | 5.30 | 0.87 | |
Week or more | 84 | 9.04 | 0.08 | |
Dietary habit | ||||
≤3 times/month | 53 | 5.23 | 1.68 | 0.0 |
≥3 times/month | 71 | 9.85 | 0.88 |
To predict the urinary mercury levels of dental health workers, a multiple linear regression model was constructed (Table
Multiple linear regression of dietary habit, occupational life style, PPEs used, and personal hygiene behaviors on urinary mercury levels in dental health personnel.
Parameters | Regression coefficient | SE |
|
---|---|---|---|
Position (dentists, dental hygienist, and dental assistants) | 0.0005 | 0.0002 | 0.082 |
Duration of work (more than 5 yrs versus less than 5 yrs) | 0.0024 | 0.0010 | 0. |
Mask using (yes versus no) | −0.0477 | 0.0118 | <0.0 |
Glove using (yes versus no) | −0.0259 | 0.0193 | <0.0 |
Snack eating/water drinking at work (always versus sometimes) | 0.1470 | 0.0294 | 0.054 |
Hand washing before lunch (always versus sometimes) | −0.0483 | 0.0114 | <0.0 |
Hand washing after work (always versus sometimes) | −0.0479 | 0.0159 | 0.0 |
Dietary habit (fish and seafood consumption; ≤3 times/month versus ≥3 times/month) | 0.0026 | 0.0015 | 0.0 |
There were significant correlations between levels of mercury in urine and the sampling areas at the mercury storage areas (
The correlation plot of airborne mercury levels versus dental health workers’ mercury levels.
17.3% of the area air samplings (10/58 samples) exceeded the OELs in dental clinics. The highest mercury concentrations were found at the base of the dental chairs, amalgam storages, and preparation areas where an amalgamator was used. This study had similar findings as the previous study by Langworth, 1997 [
In this study, mercury exposure concentrations were determined using a long exposure period to mercury and inorganic mercury method. This study also showed that the urinary mercury levels in dental health workers were higher than in unexposed subjects. The results were similar to the previous study by Zimmer et al., 2002 [
In this study the urinary mercury levels in dental health workers were
Saengsirinavin and Pringsulaka, 1988 [
Dental health workers who consumed fish and seafood ≥3 times/month had significantly higher urinary mercury levels than those who consumed fish and seafood ≤3 times/month, similar to the study conducted by Zolfaghari et al., 2007 [
For the duration of work, workers who had worked ≥5 years had significantly higher urinary mercury levels than those who had worked <5 years. This may be due to a lack of appropriate PPE use and environmental area prevention, leading to higher accumulations in their bodies [
Personal hygiene and behavioral risk factors were also associated with urinary mercury levels (Table
These poor protective practices meant that dental health workers were likely to carry mercury contamination elsewhere, potentially exposing their homes and families. Paraoccupational or take-home exposure among workers’ families may cause mercury poisoning among family members [
There were significant correlations between urinary mercury levels and environmental mercury levels. Nixon et al. [
In addition, aerosols and exhaust air from dental vacuum systems will be inhaled despite wearing face masks, which may provide little, if any, barrier to mercury vapors entering the lungs and being absorbed. However, several previous studies have indicated that good personal hygiene was an essential factor in minimizing exposure to mercury vapor [
This study demonstrated that urinary mercury levels were associated with airborne mercury levels and hygiene behaviors of dental health workers. This study showed that improving dental health workers hygiene habits can reduce urinary mercury levels. This study recommends conducting education and training about personal hygiene to minimize occupational mercury vapor exposure. In addition, engineering controls are also recommended to reduce mercury vapor exposure.
Further study increasing the sample size of participants would also be beneficial for a better understanding of this health risk.
This study was approved by the ethical committee of Thaksin University Review Board. All of participants received a clear explanation of the purpose of this study and agreed to participate using signed consent forms.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank dental health workers at the community hospitals in Nakhon Si Thammarat Province, Thailand, for allowing them to collect urinary mercury and airborne samples. The authors would also like to thank the staff of the Central Equipment Unit, Faculty of Medicine Technology, Mahidol University, for their assistance in sample analysis. This study was supported by grant from the Faculty of Health and Sports Science, Thaksin University.