We hypothesized that the action of drilling amalgam with a dental high-speed hand piece, even when using protocols and all feasible engineering controls to minimize mercury vapor, would still generate an aerosol of particles that would be heated sufficiently to produce increased mercury vapor. We designed the study to answer the following questions:
What concentration of mercury vapor can be reached from particulate generated from the removal of dental amalgam restorations using a high-speed drill?
How long can the particulate volatilize mercury vapor?
Is the peak vapor generated associated with the mass of the mercury in the particulate?
Does the amount of amalgam removed in each sample affect the peak Hg vapor?
Does the amount of amalgam removed in each sample affect the mass of mercury in particulate collected?
Amalgam fillings are a widely used dental restorative material and have been utilized since the nineteenth century. Use of the material is declining in the developed nations but is increasing in the developing countries. It has been estimated that global use of mercury for dental amalgam in 2015 was 226–322 tones . The longevity of an amalgam filling has a very large range but is expected to provide service on average for 10 years. After the amalgam’s service period has ended, removal is required which is generally achieved using a high-speed drill. There are other indications where dental amalgam removal is required and these are listed later in the background. Dental amalgam consists of approximately 50% mercury and 50% base metals. The toxicity of mercury is well established.
Dental workers, including dentists, dental assistants, dental hygienists, dental students, dental instructional staff, and dental laboratory and sterilization technicians are all at risk of mercury exposure if they work with dental amalgam [2,3,4,5,6,7,8,9,10,11,12,13,14,15]. Dental associations and dental schools have specific policies on the use of amalgam because of its mercury content. Additionally, Safety Data Sheets or SDS (formerly Material Safety Data Sheets or MSDS) from manufacturers of amalgam outline the risk of mercury exposure when using amalgam.
Dental workers have higher levels of mercury [3, 5, 10, 12, 16,17,18] as measured in blood, urine, stool, nails, hair and organs. Dental workers also have a higher prevalence of health issues consistent with chronic mercury exposure than controls. These health problems include adverse neurological conditions [2, 3, 6,7,8,9, 17, 19], while there are also some possible indications that exposure to elemental mercury may also affect reproduction [20,21,22].
Mercury levels either in the tissue of the dental worker or in the dental working environment have been found to be lower than established safety levels. It has, however, also been argued that adverse health effects can occur at such levels and after many years . In a Swedish study conducted to assess mercury exposure and health effects in dental personnel, the researchers found that the mean mercury levels in the personal space of 44 dental workers were well below established safety thresholds. The urinary mercury levels of this group were not elevated when compared to a control group. Despite the unremarkable levels of mercury measured in air and urine, central nervous symptoms of the dental group were significantly higher than the controls . This suggests that the current safety limits may be too lenient.
Although many jurisdictions have set biological exposure indexes, it was concluded within a review from 2012 that “it has not been possible to set a level for mercury in blood or urine below which mercury related symptoms will not occur .” Others claim that there is an unreliability in the current methods to measure various tissue samples to determine mercury exposure . Further, WHO has stated: “Recent studies suggest that mercury may have no threshold below which some adverse effects do not occur.” 
It is certain that there are susceptible subsets of the population that are more likely to be affected by chronic mercury exposure. In a recent study of the effects of mercury in a cohort of children, genetic polymorphisms were identified that made the participants more susceptible to mercury , and additional research has explored this pertinent genetic component . Specifically, the role of genetic profiles in dental workers’ reactions to mercury has been examined [7,8,9].
The World Dental Federation (FDI) recommends avoiding direct skin contact with mercury or freshly mixed dental amalgam and avoidance of mercury vapour sources including during the removal of dental amalgam 
It has been claimed that the respirable particulate matter represents the largest share of daily Hg-exposure for the practicing dentist . By use of standard exposure assessment methods it was found that a dentist who removes four amalgam fillings per day will inhale 38 mg of mercury derived from amalgam particulate, by far exceeding any level considered safe. When respirable amalgam particles are deposited in the lungs, they reach body temperature that enhances vaporization over days and thus, also subsequent absorption.
It is important to understand that although particulate is the exposure source, it is the vapor that comes off the particulate that is of interest because in this form, it is very easily taken into the body by the lungs and the skin. The amount of Hg vapor from amalgam increases with stimulus [15, 30, 31]. These stimuli generally cause an increase in temperature which increases the vapor pressure. The dental high-speed drill can spin up to speeds of 350,000 rpm , and therefore, can generate friction and increase the heat of the material being removed.
As long as installation of amalgam continues (and for years after it ends), there will be a need to remove amalgam from teeth. There are several circumstances that require the removal of dental amalgam from teeth using a high-speed dental drill. These include, but are not limited to, the following scenarios: sectioning of a tooth to facilitate dental extraction, failed seal of an existing amalgam restoration, recurrent decay under a filling, fracture of a tooth with an amalgam filling, adjustment of an incorrect bite, preparation for a fixed or removable prosthesis, root canal access opening, reshaping of an existing amalgam, removal of an amalgam that has an open inter-proximal contact, removal to prevent galvanism with another intra-oral metal, removal for health reasons, removal to reduce exposure of mercury, treatment of periodontal disease, and removal due to mercury sensitivities.
The guidelines for assessing mercury presence in the work place is outlined by OSHA under Method Numbers ID-140  for vapor and ID-145  for particulate. The complete processes suggested in these documents are complicated and beyond the scope of this paper; however, there are some specific items in the methods that are pertinent to this study. The assessment of mercury in the workplace is accomplished by using three different techniques. The first is the use of a passive or active mercury vapor sampling device for atmospheric mercury vapor levels. The second is the use of a vacuum with a filter cassette to collect mercury containing particulate in the air, and the third method is the use of wipes to collect mercury containing particulate on surfaces.
There are several drawbacks to these methods with respect to assessing mercury exposure from amalgam particulate generated by drilling. Vapor samplers do not assess particulate or localized mercury vapor generated from particulate if the sensors are in a location that does not have access to the particulate. If particulate in air is assessed using the vacuum filter cassettes, according to procedure described in OSHA ID-145 section 5.4, the cassettes are to be sealed after taking the sample and sent to the lab to determine the mass of the mercury in the particulate. The cassette does not allow access to the filter in a way that assessment of mercury vapor from this particulate can be made. The standardized wiping technique for determining particulate on surfaces as outlined by OSHA ID-145 states that a wet gauze is used to wipe a 10 cm × 10 cm square. The most concerning surfaces that require assessing for particulate presence are the potential skin areas that may be exposed to the amalgam particulate. This is particularly true because skin is a known route of absorption for mercury. The shape of the hands, arms, face, chest and other parts of the dental worker’s and dental patient’s anatomy that may be exposed to amalgam particulate during dental operations do not lend themselves to classical surface wiping. When considering all of these drawbacks with respect to standard occupational assessment protocol for mercury exposure in dentistry, it is evident that these processes do not quantify the extent of mercury exposure.
There is very little information on the levels of mercury vapor that can be emitted from fresh amalgam particulate generated from the dental high-speed drill. There are two limitations that may have prevented this effort. The first is that the respirable particulate is inhaled and not available for measurement of vapor because it is inaccessible to measuring devices. Second, as mentioned previously, current OSHA standards to collect particulate actually prevent the volatilization measurements because the concern of particulate assessment is to determine the mass of the mercury in the particulate, not the vapor emitting from the particulate.
There are others who have aimed to measure exposure by amalgam removal, but have failed to quantify mercury vapour from the particle matter generated [15, 35,36,37,38].
While these and other studies have examined the role of the dental drill in generating mercury releases [15, 35,36,37,38,39,40], it appears as though there have been no detailed attempts in the scientific literature to quantify the level of mercury that may vaporize from the surface of freshly ground dental amalgam particulate. By doing so, perhaps an under-estimated occupational mercury exposure in dentistry can be identified. This is the aim of this study.
Mercury vapor can be absorbed via inhalation and skin . It is these two routes of absorption of mercury that prompted the design of this study. Assessing the concentration of localized mercury vapor that the skin or the lungs would endure after particulate exposure required measuring the vapor derived from particulate at as close a range as reasonably possible. In this methodology, we aim to illustrate the potential mercury vapour exposure from particulate that comes into contact with these two organ systems.