Exposure to Wollastonite in a Mining and Milling Facility A Prospective Cohort Study
ublished Date: November 20, 2020
Exposure to Wollastonite in a Mining and Milling Facility: A Prospective Cohort Study
Matt Stewart*, and Andrew Ghio
P. O. Box 5150, Norwalk, CT 06856-5150, USA, Tel: 223-263-0119
*Corresponding author: Matt Stewart, P. O. Box 5150, Norwalk, CT 06856-5150, USA, Tel: 223-263-0119, E-mail: firstname.lastname@example.org
In March of 2016, the American Conference of Governmental Industrial Hygienists (ACGIH) adopted a new threshold limit value (TLV®) for “calcium silicate, naturally occurring as wollastonite” (Chemical Abstract Service number 13983?17?0). The adopted recommendation is a TLV®– time weighted average (TWA) of 1 mg/m3 inhalable particulate matter, containing no asbestos and <1% crystalline silica. The rationale for establishing a recommended exposure limit for the inhalable fraction of a given substance is prevention of an adverse health effect in the upper respiratory tract. The adverse health effects from overexposure to wollastonite considered relevant for establishing the 1 mg/m3 TLV®, included decrements in lung function and pneumoconiosis. The objective of this study was to characterize the size of wollastonite aerosol in a mining / milling facility and any associated impact on worker pulmonary function/lung structure. In this study, the size distribution of wollastonite particles in a mining and milling facility and potential respiratory consequences of their exposure were determined. Various size selective air sampling techniques were deployed, computer-controlled scanning electron micrographs (CCSEM) collected, and worker pulmonary function tests and chest radiographs analyzed. Size selective air sampling demonstrated that the majority (70 to 88% of the mass per unit volume of air collected) of the wollastonite particles in the mining and milling facility exceeded respirable size. Spirometry and chest X-rays demonstrated that workers in the mining and milling facility experienced no loss in pulmonary function and no radiographic evidence of pneumoconiosis since 2008.
KEYWORDS: Wollastonite; Calcium Silicate; Acicular; Toxicity; Health Surveillance; Epidemiology; Pulmonary Function Test; Respirable Particulate; Inhalable Particulate
Industrial hygiene has historically characterized and evaluated airborne particulates in terms of total and respirable dust levels. However, other size fractions are recognized to be of potential biological significance. Accordingly, additional size fractions began to be more fully characterized in the early 1990s including the “inhalable” and “thoracic” size fractions. Inhalable particulate matter (IPM) is that material which is inhaled through the nose and mouth having a median cut point for the aerodynamic diameter of inhalable particulates ranging from approximately 57 µm to 100 µm . Thoracic particulate matter, is that material which is inhaled through the nose and mouth penetrating beyond the larynxwith a median cut point of 10 µm (those particulates with an aerodynamic diameter at or less than 10 µm and reflects a maximum particle size of 25 µm) . Respirable particulate matter, is that material which is inhaled through the nose and mouth penetrating to the unciliated airways with a median cut point of 4 µm (those particulates with an aerodynamic diameter at or less than 4 µm and reflects a maximum particle size of 10 µm) .
The ACGIH Threshold Limit Value (TLV®) - Chemical Substance (CS) Committee is in the process of re-examining substances encountered in particulate form with the objective of defining: 1) the size fraction most closely associated with health effects of concern and 2) the mass concentration within that size fraction which should represent the TLV® .
Wollastonite is a “needle-like”, crystalline calcium metasilicate with the ideal molecular formula Ca3(Si3O9) (Figure 1). It exists as several triclinic and monoclinic polytypes possessing a Mohs hardness between 4 ½ and 5 . The typical aspect ratio of this silicate is reported to be 3:1 to 20:1 . In March of 2016, the ACGIH adopted a new TLV® for “calcium silicate, naturally occurring as wollastonite” (Chemical Abstract Service number 13983?17?0) (ACGIH, 2016). The adopted recommendation is a threshold limit value (TLV®)– time-weighted average (TWA) of 1 mg/m3 IPM containing no asbestos, and <1% crystalline silica. Prior to adoption of this recommended exposure limit, the TLV® of 10 mg/m3 for inhalable dust (particles not otherwise specified) and 3 mg/m3 for respirable dust (particles not otherwise specified) were applicable .
For wollastonite, the adverse health basis declared by the ACGIH for establishing the 1mg/m3 TLV® was respiratory injury and disease, including decrements in lung function and pneumoconiosis, as delineated by several epidemiologic studies. Hanke et al. reported lung function changes (significantly lower FEV1/FVC) at exposures less than or equal to 30 mg/m3- years "total dust", assuming a mean working lifetime of 40 years . That is, a cumulative lowest-observed-adverse-effect-level (LOAEL) exposure equivalent to an average exposure of 0.75 mg/m3 over a working lifetime. A corresponding no-observed-adverse-effect-level (NOAEL) of 0.5 mg/m3 "total dust" was calculated, assuming the NOAEL was two-thirds of the LOAEL. The corresponding inhalable particle NOAEL was obtained by multiplying the "total dust" level by a factor of 2, which was the mid-point of the range of values reported by Tsai and Morgan for converting "total dust" to "inhalable aerosol" levels among mining production workers . In describing their approach to deriving the NOAEL, Tsai and Morgan stated that while a respirable particulate size fraction may be more appropriate for the observed health effects, there are insufficient data to recommend a respirable level.
Limited information is available on the nature of worker exposure to the inhalable fraction of wollastonite in the mining sector and the associated impact of worker respiratory health. Workers at a wollastonite production facility in Willsboro, New York have been studied . The conclusion of this work was that lung function parameters forced expiratory volume in one second (FEV1) and the ratio of FEV1 to forced vital capacity (FVC) decreased as a function of the cumulative dust concentration among non-smokers. A similar study was carried out in 1983 with 46 Finnish workers with a longer duration of exposure . However, observed adverse health effects may have been attributable to smoking, and/or exposure to respirable crystalline silica, rendering conclusions unclear. A group of these Finnish workers (49 men) were further assessed in 1990 and while three workers demonstrated ILO profusion of 1/0, the applicability of these findings to the workers subject to that study are questionable because tremolite fibers were found in airborne particulate samples collected in the Finnish operations and many of those in the cohort had exposure to asbestos. Lung function in this Finnish cohort also did not correlate with exposure intensity and no evidence was found indicating that long-term exposure to wollastonite causes parenchymal fibrosis of the lungs . A number of researches have concluded that wollastonite is to be considered a low-toxicity substance [11-14]. Additional information on worker exposure and the associated impact on the respiratory system would benefit the effort to establish the most appropriate exposure limits for wollastonite.
In an effort to clarify the size distribution of wollastonite particulates in the atmosphere of a mining and milling facility and associated respiratory health status of workers, various size selective air sampling techniques were deployed, computer-controlled scanning electron micrographs (CCSEM) collected, and pulmonary function tests and chest radiographs prospectively analyzed beginning in 2008.
Bulk Material Characterization
Wollastonite has an ideal composition of 51.7% silicon dioxide and 48.3% calcium oxide but can also contain trace to minor amounts of aluminum, iron, magnesium, manganese, potassium, sodium, or strontium substituting for calcium. It is usually white but also may be gray, cream, brown, pale green, or red depending on the impurities and grain size . The typical particle size distribution for the primary bulk finished product being produced at the time of the collection of air samples was generated by means of a Micromeritics Sedi Graph 5100 using the sedimentation method with water.
Aerosolized Material Characterization
Computer controlled scanning electron microscopy (CCSEM) was applied to air samples collected on 0.4 µm pore size polycarbonate filters. Size-selective sampling trains were used to collect “total dust,” thoracic dust, and respirable dust. Due to the lack of availability of a 0.4 µm pore size polycarbonate filter for the inhalable fraction, CCSEM was not applied to this size fraction.
Total dust sampling and analysis was completed in accordance with NIOSH Method 0500 (particulates not otherwise regulated, total). The total dust sampling train consisted of a 0.4 µm pore size polycarbonate filter housed in a standard 37 mm filter holder (closed face). Air was drawn through the train by means of a GilAir BDX II Air Sampler sampling pump field calibrated to a flow rate in the range of 2.0 liters/minute. Thoracic dust analysis was completed in accordance with NIOSH Method 0500 (particulates not otherwise regulated, total). The thoracic dust sampling train consisted of a Mesa Labs BGI GK2.69 cyclone equipped with a 0.4 µm pore size polycarbonate filter housed in a standard 37 mm filter holder. Air was drawn through the train by means of a GilAir 3 Personal Air Sampler sampling pump field calibrated to a flow rate in the range of 1.6 liters/minute. Respirable dust sampling was completed in accordance with NIOSH Method 0600 (particulates not otherwise regulated, respirable). The respirable dust sampling train consisted of a Zefon Heavy duty 10mm Dorr?Oliver cyclone equipped with a 0.4 µm pore size polycarbonate filter housed in a standard 37 mm filter holder. Air was drawn through the train by means of a GilAir Personal Air Sampler air sampling pump field calibrated to a flow rate in the range of 1.7 liters/minute. All air sampling pumps were field calibrated immediately prior and following air sampling each day with a TSI 4100 Series, Model 4199 air flow calibrator to three significant figures.
CCSEM yielded quantitative information with respect to each of the three size fractions (total, thoracic and respirable) sampled. A portion of each of the polycarbonate filters was mounted on a carbon planchette and analyzed using an automated procedure. The SEM examined each field of view, seeking particles that were visible against the filter background. The SEM then scanned and identified particles to determine their average size, recorded a low magnification image and the elemental composition. The SEM scan of the field of view was continued until all particles in that field were analyzed. The SEM then selected a different random field and repeated the analysis. This continued until 2,500 particles had been identified and analyzed. Particle size distributions by average diameter and particle mass distribution by average diameter were generated from these scans.
In 2017, in addition to characterization of particle size by CCSEM, side-by-side comparisons of conventional gravimetric air sampling techniques were undertaken with the closed-cassette total dust sampling technique, the IOM inhalable particulate matter sampler, the Mesa Labs BGI GK2.69 thoracic particulate matter cyclone, and the Zefon Heavy duty 10 mm Dorr?Oliver respirable particulate matter cyclone (Zefon International, Inc., Ocala, FL). Each of these sampling trains was placed next to one and other to collect an air sample under identical conditions to compare the mass per unit volume of aerosol collected by each. Total, inhalable, thoracic, and respirable dust samples were taken twice in the mill area and once in the crusher area under identical conditions.
Exposure Monitoring Air Sampling Techniques
An industrial hygiene survey was conducted at the wollastonite processing facility. The survey included collection of full-shift area and personal breathing zone air samples for total, inhalable, and respirable dust at each location along the production process (at the mineral deposit during ore drilling & transport to the surface, at the ore crusher, at the ball mill, and at the finished product packaging line).
The facility under evaluation has a medical surveillance program, through which pulmonary function is assessed on a regular basis (generally annually). Spirometry was interpreted using a standardized approach. American Thoracic Society/European Respiratory Society (ATS/ERS) recommendations for spirometry were followed [16,17]. The confidence intervals used were those provided by the Intermountain Thoracic Society .
A total of 36 individuals have been employed at the facility at some point during the study period. Of those 36 individuals, 16 were exposed for a sufficient duration and also submitted for a pulmonary evaluation at the beginning and end of the study period. Table 1 provides a summary of the reason for exclusion of those not included in the study group. The initial and final pulmonary function test (PFT) for the 16 included individuals employed at this wollastonite production facility from 2008 through 2018/2019 were evaluated to determine the impact on pulmonary health. In addition, chest radiographs are collected and evaluated by a NIOSH certified B-reader in accordance with the 2000 edition of the ILO guidelines for interpretation of radiographs .
Occupational exposure limits (OELs), including ACGIH TLV®-TWA, MSHA PEL-TWA, and NIOSH REL-TWA were compared with the concentrations determined in this study for total, respirable, and inhalable wollastonite dust. The result are provided in tables 2, 3, and 4. To calculate the mean and standard deviation of the concentration measured, the value of the detection limit was used for those concentrations measured at or below the detection limit.
In this study, the typical mineralogical composition of the wollastonite ore studied, as determined by x-ray fluorescence (w/w), was 68-86% wollastonite (CaSiO3), 7-12% diopside (CaMgSi2O6), 2-6% calcite (CaCO3), 3-16% prehnite (Ca2Al2Si3O10(OH)2), 0-1% hedenbergite (CaFe2+Si2O6), and 0-1% quartz (SiO2).
Qualitative information on particulate morphology was collected using CCSEM. The typical particle size distribution for the primary bulk finished product being produced is shown in table 5. Expressed in terms of cumulative mass fraction versus diameter it can be seen that the bulk material had a mass median diameter of 10 µm (Figure 2). Particle shape varies depending on the intended use. Based on readily available technical data bulletins, these products have an aspect ratio ranging from < 4:1 to 19:1. Populations of particles have an average length ranging from 9.6 to 24.5 µm and an average diameter ranging from 2.5 to 6.7 µm.
As an illustration of the physical appearance of the aerosolized particulates, figure 3 shows a field image (400 µm across) collected during application of CCSEM to the 0.4 µm polycarbonate filters from a single area respirable air sample collected at the ball mill. The blocky to acicular shape is apparent in the field image. CCSEM also yielded quantitative information with respect to each of the three size fractions (total, thoracic and respirable) sampled. A CCSEM analysis was not undertaken on the inhalable size fraction due to the lack of availability of a suitable polycarbonate filter with the appropriate pore size. As indicated in tables 6 and 7, it is evident that regardless of whether the respirable or thoracic size-selective air sampling device is used, the distributions are virtually identical and skewed toward the lower end of the size range. Importantly, a comparison of the CCSEM scan of the filtrates generated from total particulate matter filter in relation to the filtrates from the respirable and thoracic size selective devices demonstrated that the total particulate matter filtrate had a very similar particle size distribution.
Results from the side-by-side conventional gravimetric air sampling effort using the closed-cassette total dust sampling technique, the IOM inhalable particulate matter sampler, the Mesa Labs BGI GK2. 69 thoracic particulate matter cyclone, and the Zefon Heavy duty 10mm Dorr?Oliver respirable particulate matter cyclone are provided in table 8. While a small sample set, this exercise provides a measure of the relative difference in mass concentration measured by each size-selective device. Notably, the measured atmospheric concentration of total dust in all three sampling events was consistently below the concentration of inhalable dust by 18% to 29%.
The average area and personal breathing zone air sampling results from monitoring efforts completed throughout the facility annually from 2010 through 2018 are provided in table 2 (total dust), table 3 (inhalable dust) and table 4 (respirable dust). In tables 2, 3, and 4, an overall average and standard deviation are presented for the entire time period. As explained below, these averages were used to develop exposure-years values (exposure intensities) for use in correlation against change in pulmonary function. These demonstrate:
- A total of 34 personal breathing zone samples of airborne respirable dust were collected between 2010 and 2018; 9 on the ball mill operator, 12 on the 50 pound bag packing crew, 7 on the mill crusher operator, and 6 on a miner. A total of 55 area samples of airborne respirable dust were collected between 2010 and 2018; 13in the ball mill area, 10at the 50 pound bag packing area, 8 in the mill crusher area, 9 in the general palletizing/packing area and 15 in the mine.
- A total of 11 personal breathing zone samples of airborne total dust were collected between 2010 and 2018; 2 on the ball mill operator, 6 on the 50 pound bag packing crew, two on the mill crusher operator, and one on a miner. A total of 64 area samples of airborne total dust were collected between 2010 and 2018; 17 in the mill area, 11 at the 50 pound bag packing area, seven in the mill crusher area, 19 in the general palletizing/packing area and 10 in the mine.
- Airborne inhalable dust concentration was measured only for the past three years (2016, 2017 and 2018). A total of 4 personal breathing zone samples of inhalable respirable dust were collected between 2016 and 2018; 0 on the ball mill operator, 4 on the 50 pound bag packing crew, 0 on the mill crusher operator, and 0 on a miner. A total of 12 area samples of airborne respirable dust were collected between 2016 and 2019; 4 in the mill area, 2 at the 50 pound bag packing area, 2 in the mill crusher area, 4 in the general palletizing / packing area and 0 in the mine.
With respect to the characterization of airborne respirable dust concentrations and airborne total dust concentrations, this represents a fairly robust dataset. However, because airborne concentrations of inhalable dust have been measured for only three years, this dataset is more limited.
Cumulative airborne dust exposures were developed by determining the number of years each of the workers served in a given job function and multiplying this value by the average airborne dust concentration calculated for that job function. Table 9, 10, and 11 summarize the results. If the average personal breathing zone airborne dust concentration was higher, it was used. If the average area airborne dust concentration was higher, it was used. This resulted in a worst case scenario for cumulative exposure years.
Table 12 and 13 summarizes the data associated with the medical surveillance of the 16 individuals employed at this wollastonite production facility from 2008 through 2018/2019, who remained employed in 2018/2019. Dust concentrations measured over the time period during which pulmonary function was evaluated (2008–2018) are presented in table 2 (total dust), table 3 (inhalable dust), and table 4 (respirable dust). Where multiple values were available, they were averaged.
All members of the study cohort were male. Three were current smokers three were ex-smokers with the remainder being lifetime non-smokers. The mean age (+/- standard deviation) was 41.3 (+/-16.5). The mean percentage predicted FEV1 was 111% (+/-15%) while mean percentage predicted FVC was 112% (+/-14%). The mean absolute ratio of FEV1/FVC was 81% (+/-5%). Spirometry from two individuals was consistent with mild obstruction while one was consistent with mild restriction. The remainder had normal values of spirometry.
The change in lung function parameters percentage predicted FEV1, percentage predicted FVC, and percentage predicted FEV1/FVC between 2008 and 2018/2019 was evaluated. In these 16 individuals, when averaged, a small reduction in percentage predicted FVC (-1% reduction) was observed. An improvement in percentage predicted FEV1 (6%) was observed. There was also an improvement in percentage predicted FEV1/FVC (9%). As indicated in table 13, the 95% confidence interval for the average of each of these parameters among this 16 member cohort supports the conclusion that there was no reduction in pulmonary function over the period observed. For change in FVC% and change in FEV1%, the confidence intervals included zero and for FEV1/FVC%, it was greater than zero. The change in FVC% and the change in FEV1% are both normally distributed. As such, a simple paired t test can be applied to determine if there has been a change in these parameters for this population of workers over time. In both cases, application of this simple paired t test indicates that the null hypothesis (there is no change) cannot be rejected. There is no apparent change in the population’s FVC% or the FEV1% from 2008 to 2019. The Change in FEV1/FVC% is not normally distributed. As such, the “sign” test is preferred for determining if there is a change in this parameter for this population of workers. Application of the “sign” test indicates that the null hypothesis (there is no change) must be rejected. There is an apparent change in the population’s FEV1/FVC% from 2008 to 2019 – it has increased. This increase is not indicative of pulmonary impairment. Obstructive lung disease often results in the reduction in the FEV1% (an obstruction of air escaping from the lungs), resulting in a reduction in the FEV1/FVC% ratio. Restrictive lung disease often results in an equal reduction in FEV1% and FVC% due to fibrosis, resulting in a relatively normal FEV1/FVC% .
In their description of the evaluation of small populations of workers’ PFT performance over time, D. Christiani and D. Wegman (2006) stated “…the average value of a group of tests has less variability than an individual test results. For example, individual measurements of FEV1 and FVC that vary between 80% and 120% of the population standards are still considered normal; a group of 10 – 20 actively working people, however, should have a mean result much closer to the standard values (100%)” . Similarly, Hankinson, et al., assert that if the average population difference is as little as 5% lower – that is, 95% of the predicted value – then an adverse health effect in that population should be seriously considered . As evidenced by the paired t-test, there has been no statistically significant reduction in FEV1% or FVC% among these wollastonite workers. Consistent with the assertions of Christiani & Wegman, and Hankinson, this supports a conclusion that there has been no impairment in pulmonary function. As described by Parker, the statistically significant increase in FEV1%/FVC% is not indicative of pulmonary impairment. See Appendix A for the details of this statistical analysis of the change in these parameters from 2008 to 2019.
Scatter plots were created whereby exposure intensity (exposure- years) for each of the particle size fractions (total, inhalable and respirable) was plotted against change in each of the three measures of pulmonary function (FVC%, FEV1% and FEV1/FVC%) to assess correlation. A total of nine scatter plots resulted – Figures 4 to 12. The scatter plots further support the conclusion that exposure intensity (measured as exposure – years) is not correlated with impairment (a reduction in pulmonary function) among the workers studied. When smokers are eliminated from the population, there is no change in the results of the scatter plots analyses.
Chest roentgenograms were interpreted by a B-Reader in 2019. In all 16 cases, the chest radiographs were found to demonstrate no profusion of small opacities and no pleural abnormalities. There was no evidence of pneumoconiosis on any chest X-ray.
Size and distribution characteristics of the particles at the wollastonite mining and milling operation facility by means of CCSEM and conventional gravimetry revealed:
- Measurement of respirable size fraction and thoracic size fraction yielded virtually identical average diameters in the range of 1 to 5 microns. CCSEM analysis of the total particulate sampling train filter (closed cassette) yielded a very similar particle size distribution (see Table 6).
- Characterization of particle mass distribution by CCSEM resulted in similar observations to those made for size distribution. The average diameter ranged from 2.5 to 5 microns (See Table 7).
- When used side-by-side, the conventional size selective air sampling and associated gravimetric analysis illustrated that from 70% to 88% of aerosol by mass is of the larger inhalable size fraction (see table 8).
The CCSEM characterization of the morphology and particle size distribution of aerosolized dust collected by the IOM inhalable dust sampler was not undertaken during this study due to the lack of availability of appropriate filer media. Considering the fact that between 70% and 88% (of the mass per unit volume of air) of the airborne particulates present in the atmosphere within this facility is likely greater than 10 µm in diameter, further study of its morphology and size distribution by means of CCSEM may be of value.
Regarding exposure of workers at a wollastonite mining and milling facility, this evaluation demonstrated that:
- The highest dust exposures were experienced by individuals filling 50 pound bags.
- Dust concentrations in excess of the ACGIH recommended TLV®-TWA were experienced by individuals filling 50 pound bags, operating the crusher, and operating the 50 pound bag palletizer.
- Regulatory limits in worker personal breathing zones were not exceeded during sampling.
Personal breathing zone airborne particulate concentrations have never been measured in excess of the 10 mg/m3 total dust MSHA permissible exposure limit (PEL). Though not used by MSHA to assess compliance, area airborne particulate matter concentrations exceeded the 10 mg/m3 total dust MSHA PEL in 2013 and 2011 at the mill crusher and at the ball mill in 2013 as well as at the 50 pound bagging line in 2011. While only three years of data are available, personal breathing zone and area inhalable wollastonite concentrations were in excess of the new 1 mg/m3 ACGIH TLV® - TWA at the packing and palletizing areas and the mill crusher. On average, respirable dust concentrations throughout the facility were below the 3 mg/m3 ACGIH TLV® - TWA. Respirable dust concentrations were highest in the vicinity of the mill crusher with an average concentration of 1.24 mg/m3, followed by the bag filling area with an average concentration of 1.17 mg/m3.
On average, total dust concentrations throughout the facility were below the 10 mg/m3 ACGIH TLV® - TWA for total dust. Total dust concentrations showed a similar trend to that seen with respirable dust. The highest concentrations were in the vicinity of the mill crusher with an average concentration of 6.84 mg/m3, followed by the bag filling area with an average concentration of 4.63 mg/m3. Respirable and total dust concentrations in the mining operation were considerably lower than those measured in other areas of the facility. Respirable dust concentrations were generally below detection in the mine and total dust concentrations were at no time in excess of 0.4 mg/m3. Inhalable dust concentrations have yet to be measured at the mine but are expected to be very low based on historic respirable and total dust concentrations.
Regarding air sampling, a number of studies have demonstrated that the IOM sampler often provides higher concentrations of particulates relative to the standard closed-face 37 mm total dust sampling cassette in side-by-side air sampling events [23,24]. It has been theorized that the degree of difference is a function of the actual particle size in the sampled atmosphere, with greater variability resulting from more coarse particles [25-28]. Discernment of this phenomenon is beyond the scope of this study but worthy of further exploration.
Medical surveillance included spirometry and chest x-rays. Despite the potential for exposure to inhalable wollastonite dust in excess of the new 1 mg/m3 ACGIH TLV® - TWA at this facility, the pulmonary function status of this population of workers can be considered to be normal. Analysis of the data for the 16 exposed workers who had taken part in the study in both 2008 and 2018/2019 revealed that none of the three lung function parameters FEV1%, FVC %, nor FEV1/FVC % indicate a degradation of lung function.
Importantly, the absence of an impact on the pulmonary health of this worker population is in contrast to that which was observed by Hanke W, et al. (1984)  despite the shorter duration (1976 through 1982) of the Hanke study. Hanke observed that the impact seen on the pulmonary health of that population of wollastonite workers may have been due to “significant smoking effects”. The absence of an impact on the pulmonary health of this worker population is also in contrast with that which was observed by Huuskonen MS, et al. (1983) . Though the study by Huuskonen was of a longer duration, including workers with as many as 30 years of exposure, the wollastonite mineral assemblage studied by Huuskonen was known to contain silica and the impact of smoking among the worker population was unable to be discerned.
Exposure to wollastonite in the operations reported on in this study, over the 10 year study period, has not in this population of 16 workers been shown to cause respiratory impairment. Chest X-rays were interpreted to reveal no evidence to support pneumoconiosis.
A recognized limitation is that the population of workers evaluated is relatively small (i.e. n = 16). In addition, ten years is a short period of time in an assessment of an impact of an exposure on pulmonary function (though other studies have been published of an even shorter duration, such and Hanke W, et. al. ). Also, the side-by-side comparison of the various size-selective air sampling trains would benefit from additional data points. In future years, to more fully characterize the airborne inhalable dust concentrations on this worker population, measurements should be taken on the mill operator, mill crusher operator, and miners.
Despite inhalable dust concentration being measured in excess of the new 1 mg/m3 ACGIH TLV® - TWA for wollastonite in a number of areas in a mining and milling facility, there has been no apparent adverse impact on worker pulmonary function and no evidence of lung damage upon review of chest roentgenograms.
Declaration of Interest
The lead author is an employee of the organization that owns the facility under study. The co-author is paid to provide routine evaluations of chest x-rays and pulmonary function tests as part of an on-going medical surveillance program.
 The only exceptions were for the mill manager and the technician, both of whom were salaried personnel. These individuals would be present in the general mill area, not exposed to job-specific personal breathing zone dust concentrations experienced by hourly personnel.
 Inhalable but too large to be collected by the thoracic size selective cyclone.
 The personal breathing zone respirable dust concentration of 30 mg/m3 measured in 2017 is likely in error due to contamination of the filter with entrained product. When omitted, the average for this operation is 0.9 mg/m3
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