Potential airborne asbestos exposures in dentistry: a comprehensive review and risk assessment

Figures & data

Figure 1. Per the “lost-wax” casting method, a wax model is first hand-crafted, mounted onto a base by a sprue wire, and surrounded by a metal ring (a). A piece of asbestos casting ring liner is cut, wetted for approximately one minute, and formed around the inside of the metal ring (b). Investment powder is then mixed with water to the desired consistency, poured around the wax model inside of the casting ring (with the ring liner in place), vibrated to ensure complete filling of the ring, and then allowed to harden for approximately 30 min to one hour (c and d). After the investment material hardens, the wax is subsequently burned out of the investment material in a heat treatment process lasting from 45 min to two hours, and reaching temperatures of approximately 250–900 °C (approximately 500–1650 °F), depending upon the composition of the investment material (e). Molten metal, typically gold, is then used to fill the negative space left by the burned out (or “lost”) wax in order to form the desired dental fixture (f). The invested casting is cooled for approximately six minutes at room temperature prior to being quenched in a cold-water bath, allowing for further setting of the investment material; the investment material and casting ring liner are subsequently cleaned from the final product with an abrasive sandblaster or a hand-finishing tool (g and h).

Figure 2. Periodontal dressing powder (a) is mixed with eugenol (b), rolled into a “sausage shape” (c), and applied to a patient’s gums to assist with the healing process by protecting the wound from physical trauma and providing stability at the surgical site (d).

Table 1. Summary of asbestos bulk analysis data for casting ring liner.

Study	Content (%)	Fiber type	Method of identification
Cutright et al. (1980)	–	Chrysotile	SEM with EDS
Sichletidis et al. (2009)	–	Chrysotile	XRD, SEM
Millette (2013)	80–90	Chrysotile	PLM
	0.02	Tremolite/actinolite†	TEM
	80–90	Chrysotile	PLM
	0.002	Tremolite/actinolite†	TEM
	60–80	Chrysotile	PLM
	0.0002	Tremolite/actinolite†	TEM
Millette et al. (2019)	55	Chrysotile	PLM
	0.006	Tremolite/actinolite†	TEM
	50	Chrysotile	PLM
	0.0005	Tremolite/actinolite†	TEM
	55–60	Chrysotile	PLM
	0.002	Tremolite/actinolite†	TEM
	55–70	Chrysotile	PLM
	0.1	Tremolite/actinolite†	TEM
	80	Chrysotile	PLM
	0.003	Tremolite/actinolite†	TEM
	85–95	Chrysotile	PLM
	0.003	Tremolite/actinolite†	TEM
	50–60	Chrysotile	PLM
	0.04	Tremolite/actinolite†	TEM
	60–80	Chrysotile	PLM
	0.01	Tremolite/actinolite†	TEM
	40–60	Chrysotile	PLM
	0.009	Tremolite/actinolite†	TEM
	50–70	Chrysotile	PLM
	0.006	Tremolite/actinolite†	TEM

†Mineral habit was not specified.

SEM: scanning electron microscopy; EDS: energy-dispersive X-ray microanalysis; XRD: X-ray powder diffraction; PLM: polarized light microscopy; TEM: transmission electron microscopy; –: not reported.

Table 2. Summary of asbestos bulk analysis data for investment material.

Study	Content (%)	Fiber type	Method of identification
Compton and Millette (2012)	ND	–	PLM

ND: not detected; PLM: polarized light microscopy; – : not reported.

Table 3. Summary of asbestos bulk analysis data for periodontal dressing powder.

Study	Content (%)	Fiber type	Method of identification
Guglani and Allen (1965)	6†	–	–
Legan et al. (1973)	7.5	–	–
Otterson and Arra (1974)	7.1	–	–
	12.8	–	–
Bakdash and Frydman (1976)	3	–	–
	7.1	–	–
	14.29	–	–
Fulton et al. (1989)	7.1	Chrysotile	SEM with EDS

†The authors noted that this PDP contained “6 parts asbestos” (Guglani and Allen 1965, p. 19).

SEM: scanning electron microscopy; EDS: energy-dispersive X-ray microanalysis; –: not reported.

Table 4. Summary of glovebox fiber release data for casting ring liner.

Study	Task	Sample duration (min)	Concentration (NIOSH 7400 PCM) (f/cc) – (Adjusted†)	Reported chrysotile asbestos content (NIOSH 7402 TEM) (%)	Reported asbestos concentration (AHERA TEM) (structures >0.5 µm/cc)
Millette et al. (2019)	"Handling dental tape"	10	1.2‡	94	18.3
	"Tearing tape, wet press"	10	3.1‡	100	55.8
	"Tearing … tape"	37	0.47	99	5.6
	"Tearing … tape"	37	0.33	95	6.3

†PCM concentrations were reportedly adjusted with NIOSH 7402 TEM % asbestos results. ‡Millette et al. (2019) reported air concentrations as a range; however, since the samples were collected during the same sampling event and for the purpose of this assessment, we therefore averaged the two reported airborne fiber concentrations.

NIOSH: National Institute of Occupational Safety and Health; PCM: phase contrast microscopy; TEM: transmission electron microscopy; AHERA: Asbestos Hazard Emergency Response Act.

Table 5. Summary of exposure study data for casting ring liner – all tasks.

Study	Task	Sample duration (min)	N	Sample type	Concentration (f/cc)	Method of detection	Reported chrysotile asbestos content (NIOSH 7402 TEM) (%)	Reported asbestos concentration (TEM) (structures ≥5 µm/cc)	Reported 8-h TWA (f/cc)	Calculated 8-h TWA (f/cc)
Brune and Beltesbrekke (1981)	Dismantling	–	1	Personal	21	Light microscopy	–	–	–	NA
	Dismantling	–	1	Personal	27	Light microscopy	–	–	–	NA
Tulachka et al. (1987)	Placement	60	1	Personal	<0.01	SEM	–	–	–	NA
	Placement	480	1	Area	<0.01	SEM	–	–	–	NA
	Dismantling	480	1	Area	<0.01	SEM	–	–	–	NA
Davis (1995)	Tearing, placement	30	1	Personal	0.092	PCM	–	0.200	0.006	–
	Tearing, placement	30	1	Personal	0.061	PCM	–	0.025	0.004	–
	Tearing, placement	30	1	Personal	0.065	PCM	–	<0.025	0.004	–
	Tearing, placement	90	1	Area	0.005	PCM	–	–	0.001	–
	Tearing, placement	90	1	Area	0.005	PCM	–	–	0.001	–
Sichletidis et al. (2009)	Tearing, placement	240	1	Personal	0.008	PCM	–	–	–	0.004000
Jung (2011)	Tearing, placement	420	14	Personal	0.0162†	PCM	–	–	–	0.014175
	Tearing, placement	420	2	Personal	0.0251†	PCM	–	–	–	0.021963*
	Dismantling	420	14	Personal	0.0046†	PCM	–	–	–	NA
	Dismantling	420	2	Personal	0.0067†	PCM	–	–	–	NA
Millette et al. (2019)	Tearing, placement	3	1	Personal‡^	3.5	PCM	100	–	–	0.021563
	Tearing, placement	3	1	Area^	0.7	PCM	100	–	–	0.004375**
	Tearing, placement	4	1	Personal‡^^	1.6	PCM	100	–	–	0.013250
	Tearing, placement	4	1	Area^^	<0.07	PCM	–	–	–	0.000292#
	Tearing, placement	4	1	Personal‡^^^	1.3#	PCM	96	–	–	0.010833
	Tearing, placement	4	1	Area^^^	<1.35	PCM	–	–	–	0.005625#
	Tearing, placement	5	1	Personal‡^^^^	1.1	PCM	>90	–	–	0.010938
	Tearing, placement	5	1	Area^^^^	0.1	PCM	–	–	–	0.000729
	Tearing, placement	5	1	Personal^^^^^	2.0	PCM	>90	70 (str ≥0.5 µm/cc)	–	0.020833
	Tearing, placement	5	1	Area^^^^^	–	–	–	–	–	–
	Tearing, placement	6	1	Personal‡^^^^^^	0.69	PCM	92-93	–	–	0.008625
	Tearing, placement	6	1	Area^^^^^^	0.14	PCM	93.3	–	–	0.001750

†Geometric mean. ‡Millette et al. (2019) reported air concentrations as a range; however, based on the respective underlying unpublished reports, these air samples do not appear to be independent samples, but rather samples that were collected simultaneously on both lapels of the individual performing the given task. For the purpose of this assessment, we therefore averaged the two reported airborne fiber concentrations whenever Millette et al. (2019) presented them as a range. Some of these ranges are indeed averaged in the results of the underlying unpublished reports. In addition, the air concentration values as reported by Millette et al. (2019) were TEM-adjusted, resulting in PCM-equivalent air concentration values. *Upper-bound, personal 8-h TWA (= [0.0251 f/cc × 420 min]/480 min). **Upper-bound, area (bystander) 8-h TWA (= [0.7 f/cc × 3 min]/480 min). ^As reported in the respective underlying unpublished report (Compton and Millette 2010b). ^^As reported in the respective underlying unpublished report (Compton and Millette 2011c). ^^^As reported in the respective underlying unpublished report (Compton and Millette 2010d). ^^^^As reported in the respective underlying unpublished report (Millette 2005c). ^^^^^As reported in the respective underlying unpublished report (Millette 2005d). It was noted in this report that there was a “filter cassette problem” with one of the personal samples, and so it was not included in the results. There was also a reported “sampling pump problem” for the area sample, and so this sample was also not included in the results. ^^^^^^As reported in the respective underlying unpublished report (Compton and Millette 2011a). #Calculated assuming 1/2 of the limit of detection.

NIOSH: National Institute of Occupational Safety and Health; TEM: transmission electron microscopy; TWA: time-weighted average; SEM: scanning electron microscopy; PCM: phase contrast microscopy; NA: not applicable; –: not reported

Table 6. Summary of exposure study data† for periodontal dressing powder – all tasks.

Study	Task	Sample duration (min)	Sample type	Concentration (f/cc)‡	Method of detection
Dyer (1967)	Opening bottle with shaking	–	Personal	–	–
	Opening bottle without shaking	–	Personal	–	–
	After mixing	5	Personal	–^	–
Otterson and Arra (1974)	During mixing	5	Personal	<0.297	PCM
	During mixing	17	Personal	<0.074	PCM
	During mixing	17	Personal	<0.074	PCM
	During mixing	17	Personal	<0.074	PCM
	During mixing	18	Personal	<0.070	PCM
	During mixing	20	Personal	<0.0063	PCM
	During mixing	28	Personal	<0.0044	PCM
	During mixing	4	Area	<0.396	PCM
	During mixing	23	Area	<0.0054	PCM
	During mixing	23	Area	<0.054	PCM

†Data is presented as reported in the underlying study. ‡Entire dataset was censored and, therefore, 8-h TWA values were not calculated. ^Dyer (1967) reported that the personal sample taken after the commencement of mixing PDP “did not show any asbestos fibres” (Dyer 1967, p. 507).

TWA: time-weighted average; ND: not detected; PCM: phase contrast microscopy; NA: not applicable; –: not reported.

Table 7. Summary of mesothelioma case reports among asbestos-containing dental product users.

Study	N	Disease	Sex	Age at diagnosis (years)	Time period of exposure	Location	Job title	Product used	Additional asbestos exposures?
Reid et al. (1991)	1	Pleural mesothelioma	Male	60	–	U.K.	Dentist	–	No
	2	Pleural mesothelioma	–	–	–	–	Dental technician	–	–
Ogus and Bourne (2007); Fry (2008, 2009)	3	Mesothelioma (type unspecified)	Male	64	1962–1966	U.K.	Dental student, house surgeon, and senior house officer	–	Yes – "tunnels" of hospital
Markowitz and Moline (2017)	4	Pleural mesothelioma; calcified pleural plaques	Male	70	Beginning in 1970 for an unspecified duration	U.S.	Dentist (student)	CRL	Yes – application and sanding of joint compound during home renovation from 1976–1977
	5	Pleural mesothelioma	Male	55	1971–1975	U.S.	Dentist (student)	CRL	No
	6	Pleural mesothelioma; pleural thickening	Male	55	1969–1973 for two months	U.S.	Dentist (student)	CRL	No
	7	Mesothelioma of the pleura, diaphragm, pericardium, and lung parenchyma; benign pleural plaque	Male	59	1964–1969	U.S.	Dentist (student)	CRL	No
	8	Pleural mesothelioma; pleural thickening	Male	69	1967–1973	U.S.	Dental technician (student)	CRL	Yes – use of asbestos-containing gloves in lab; use of asbestos sheets in high school chemistry class; father worked at a shipyard for five years in the 1950s
Mensi et al. (2017)	9	Pleural mesothelioma	Male	72	1953–1957	Italy	Dental technician	CRL	Yes – use of asbestos-containing gloves
	10	Pleural mesothelioma; pleural thickening	Female	61	1946–1991	Italy	Wife and daughter of dental technicians	CRL	Yes – held a slab of asbestos cement while her husband drilled into it in the 1980s
	11	Pleural mesothelioma; pleural thickening; peritoneal carcinosis	Male	79	1952–1956	Italy	Dental technician	CRL	Yes – some domestic electrical work
	12	Pleural mesothelioma; hilar fibrosis; pleural thickening	Male	70	1960–1999	Italy	Dental technician	CRL	No

CRL: casting ring liner; –: not reported

Table 8. Cumulative exposure estimates for various dental professionals.

Product by task	Dental professional	Duration (years)‡	Cumulative exposure (f/cc-years)
			Personal*	Area**
Casting ring liner†
Tearing, placement	Dental assistant	5.4	0.12	0.024
	Dental hygienist	10.6	0.23	0.046
	Dental laboratory technician	12.9	0.28	0.056
	Dentist	15.1	0.33	0.066

†Casting ring liner was the only dental product evaluated for which measurable exposure data were reported. ‡Based on mean employment tenure as reported by Maguire (1993). *Calculated using the upper-bound 8-h TWA of 0.022 f/cc. **Calculated using the upper-bound 8-h TWA of 0.0044 f/cc.

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