Mercury vapor levels in exhaust air from dental vacuum systems may exceed occupational safety limits

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Objective:

This study was undertaken to determine mercury (Hg) vapor levels in the air exhausted from dental vacuum systems. Methodology. Hg vapor concentrations from the dental vacuum system exhaust ports of three dental clinics were measured utilizing the Jerome 431-XTM mercury vapor analyzer and the United States Occupational Safety and Health Administration’s (OSHA) method ID-140 in units of ngHg/m3. Air velocity measurements and temperatures were determined with a constant temperature thermal anemometer. Hg emissions per unit time were then calculated in ng Hg/min. Ambient Hg concentrations from a location approximately 1000 feet away from the closest clinic sampled in this study were measured with an Ohio Lumex Inc. RA-915+TM Hg vapor analyzer. 

Results:

Mean Hg vapor concentrations analyzed with the Jerome 431-XTM were: 46,526, 72,211, and 36,895 ng/m3 for clinic I (110 chairs), clinic II (30 chairs) and clinic III (2 chairs), respectively. Mean Hg vapor concentrations utilizing OSHA method ID-140 were 45,316, 73,737, and 35,421 ng/m3, respectively. Air flow values were: 11.6, 1.8, and 0.5 standard m3/min, respectively. Hg emission data utilizing air flow measurements were calculated to be 532,684, 131,353, and 18,079 ng/min, respectively, (P < 0.001). There was no statistical difference between the two methods used to measure Hg vapor concentrations. The mean Hg concentration in ambient air approximately 1000 feet from the nearest clinic sampled was 13.2 ng/m3. 

Discussion

Anthropogenic Hg emissions to the atmosphere total an estimated 158 tonnes/year in the United States with approximately 87% coming from combustion sources [1]. Roughly  80% of the total Hg emissions come from the following four sources: coal-fired utility boilers, municipal waste combustors, industrial coal-fired boilers, and medical waste incinerators [1]. Coal, on average, contains approximately 0.1 mg/kg  Hg and when it is burned, all this Hg is vaporized [14].

Atmospheric Hg deposition into lakes, rivers and streams leads to the production of organic Hg, some of which is incorporated into the food chain [1]. Microorganisms, especially sulfate-reducing bacteria in the sediment, transform inorganic Hg into organic forms [15,16]. Almost the entire Hg burden in fish tissue is organic Hg [1]. In 2002, 45 states had fish consumption advisories due to elevated Hg concentrations in fish tissue [17]. Regulators in the United States are currently proposing rules to stem the release of Hg fromcoal-fired power plants, a major source of Hg emissions to the environment. 

While total Hg load to the atmosphere from dental vacuum systems pales in comparison to the 51 tonnes of Hg/year released from coal-fired power plants, the concentration of Hg in waste air from dental vacuums is higher. In the combustion zone of coal-fired boilers, elemental Hg vapor concentrations range from 1 to 20g/m3 (1000–20,000 ng/m3) [14]. The grand mean for all clinics and both methods from dental vacuum systems in this study was 51,684 ng/m3, making the concentration of Hg in air from dental vacuums 2.6–51.7 times higher than Hg concentrations in the combustion zones of coal-burning power plants.

The mean Hg vapor concentration in ambient air at a site away from the nearest dental clinic sampled was 13.2 ng/m3. This value is comparable to published values for ambient Hg vapor concentrations in urban settings which range from 2 to 20 ng/m3 [9,12,13]. 

Two previous studies have surveyed Hg release from dental vacuum systems.

The earlier effort [6] found mean Hg vapor concentrations of 0.092mg/m3 (92,000 ng/m3) with a range from 0.010 to 0.237mg/m3 (10,000–237,000 ng/m3). Authors sampled eight dental office vacuum systems with the Jerome 431-XTM instrument, but made no attempt to measure air flow from vacuum ports.

A second study [7] measured Hg vapor concentrations from an in-office dental aspirator (vacuum system). Hg vapor concentrations of 250g/m3 (250,000 ng/m3) were seen in the breathing zone of the dentist and Hg vapor concentrations up to 1400g/m3 (1,400,000 ng/m3) were seen from the aspirator exhaust port. The dental aspirator was discharging exhaust air directly into the operatory in excess of human exposure limits.

While modern dental treatment facilities are designed with ventilation systems that can rapidly exchange air, high concentrations of Hg vapor may be a cause for concern, especially if vacuum systems vent directly into treatment rooms. 

exposure_limits

Table 6 lists human exposure limits for Hg vapor from three different agencies.

The OSHA permissible exposure limit for inorganic Hg vapor is 0.1mg/m3 (100,000 ng/m3) as an 8h time weighted average [8]. In this study, five measurements reached or exceeded this permissible exposure limit.

The National Institute for Occupational Safety and Health (NIOSH) has a recommended exposure limit [8] for Hg vapor of 0.5mg/m3 (50,000 ng/m3). Mean Hg concentration in dental vacuum exhaust air from clinic II was found to be 72,974 ng/m3, which is almost 1.5 times higher than the NIOSH recommended exposure limit.

The American Conference of Governmental Industrial Hygienists (ACGIH) has a threshold limit value of 0.025mg/m3 (25,000 ng/m3) [8]. The ACGIH limit is half the NIOSH limit and based on concern that chronic lowdose exposure to Hg vapor may impact human health.  

An unanticipated finding of this study was the similarity in values between the two methods used to determine Hg vapor concentrations. The Jerome 431-XTM analyzer takes measurements at a single point in time while OSHA method ID-140 samples continuously (over 3–7h in the case of this study). The reason for the comparable values is not understood and is the subject of ongoing research.

When comparing Hg vapor concentrations from the various clinics, clinic II had a 1.6 times higher Hg concentration than clinic I, but with fewer dental chairs. This anomaly might be due to differences in the vacuum systems. Clinic I uses a much larger system to generate vacuum (two large turbines in addition to four rotary vane vacuum pumps) and the volume of air discharged from clinic I (110 chairs) is 6.4 times larger than from clinic II (30 chairs): 11.6 versus 1.8 standardm3/min, respectively.

To control for the confounding factor of different airflow rates in the three clinics, the concentration data was converted into ng Hg/min (by multiplying Hg concentration in ng/m3 by air flow in m3/min). With this adjustment, the larger clinic (clinic I) had the highest load value of 532,684 ng Hg/min compared to 131,353 ng Hg/min for clinic II. Clinic III with only two chairs showed the lowest Hg concentration, the lowest air flow and the lowest Hg load/min (18,079 ng Hg/min).

Hg load in ng/min appears to correlate directly with the number of dental chairs. Plotting the number of dental chairs against mean ng Hg/min (from the three clinics) yields a linear correlation (Fig. 1) with an R-squared value of 0.9983. The R-squared value between amalgam surfaces placed and ng Hg/min was 0.9740. Analysis using surfaces of amalgam placed provides only a partial accounting of Hg load since data for the number of amalgam surfaces removed is not available.

Conclusion:

The two methods used to measure Hg vapor yielded similar estimates of Hg concentrations. Hg vapor release to the atmosphere fromdental vacuums can be substantial and occasionally exceed permissible exposure limits. Additional studies are indicated to confirm these findings and assess the following issues:

  • Wet vacuum systems usewater to cool and seal the vacuum pumps and these systems exhaust air into plumbing lines either via ‘P’ traps or through floor drains. Since many small dental clinics use wet vacuum systems, sampling should be expanded to include clinics using these systems.
  • A number of Hg sorbent materials are available in the market and should be tested to determine their capacity to reduce Hg vapor emissions from dental vacuum systems. 
  • Speciation studies to determine the chemical form of Hg in the exhaust air should be undertaken. Elemental Hg vapor (Hg0) resides in the atmosphere for up to a year or more, while reactive gaseous mercury (Hg2+) has the shortest atmospheric half-life as well as the quickest dry-deposition velocity of the mercury species [18]. Reactive gaseous mercury is thought to be the species most easily transformed into methyl mercury.

Mercury vapor levels in exhaust air from dental vacuum systems

Dental Materials 23 ( 2007 ) 527–532

Mark E. Stone, Mark E. Cohen, Brad A. Debban

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