Occupational exposure to crystalline silica at Alberta work sites.

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Occupational Exposure to Crystalline Silica at Alberta Work Sites a

b

Diane Radnoff , Maria S. Todor & Jeremy Beach

c

a

Jobs, Skills, Training and Labour, Safe, Fair and Healthy Workplaces Edmonton, Alberta, Canada b

School of Public Health, University of Alberta, Edmonton, Alberta, Canada

c

Division of Preventive Medicine, University of Alberta, Edmonton, Alberta, Canada Accepted author version posted online: 30 Jan 2014.

To cite this article: Diane Radnoff, Maria S. Todor & Jeremy Beach (2014) Occupational Exposure to Crystalline Silica at Alberta Work Sites, Journal of Occupational and Environmental Hygiene, 11:9, 557-570, DOI: 10.1080/15459624.2014.887205 To link to this article: http://dx.doi.org/10.1080/15459624.2014.887205

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Journal of Occupational and Environmental Hygiene, 11: 557–570 ISSN: 1545-9624 print / 1545-9632 online c 2014 Crown copyright Copyright DOI: 10.1080/15459624.2014.887205

Occupational Exposure to Crystalline Silica at Alberta Work Sites Diane Radnoff,1Maria S. Todor,2 and Jeremy Beach3 1

Jobs, Skills, Training and Labour, Safe, Fair and Healthy Workplaces Edmonton, Alberta, Canada School of Public Health, University of Alberta, Edmonton, Alberta, Canada 3 Division of Preventive Medicine, University of Alberta, Edmonton, Alberta, Canada

Downloaded by [George Mason University] at 00:20 08 July 2014

2

Although crystalline silica has been recognized as a health hazard for many years, it is still encountered in many work environments. Numerous studies have revealed an association between exposure to respirable crystalline silica and the development of silicosis and other lung diseases including lung cancer. Alberta Jobs, Skills, Training and Labour conducted a project to evaluate exposure to crystalline silica at a total of 40 work sites across 13 industries. Total airborne respirable dust and respirable crystalline silica concentrations were quite variable, but there was a potential to exceed the Alberta Occupational Exposure Limit (OEL) of 0.025 mg/m3 for respirable crystalline silica at many of the work sites evaluated. The industries with the highest potentials for overexposure occurred in sand and mineral processing (GM 0.090 mg/m3), followed by new commercial building construction (GM 0.055 mg/m3), aggregate mining and crushing (GM 0.048 mg/m3), abrasive blasting (GM 0.027 mg/m3), and demolition (GM 0.027 mg/m3). For worker occupations, geometric mean exposure ranged from 0.105 mg/m3 (brick layer/mason/concrete cutting) to 0.008 mg/m3 (dispatcher/shipping, administration). Potential for GM exposure exceeding the OEL was identified in a number of occupations where it was not expected, such as electricians, carpenters and painters. These exposures were generally related to the specific task the worker was doing, or arose from incidental exposure from other activities at the work site. The results indicate that where there is a potential for activities producing airborne respirable crystalline silica, it is critical that the employer include all worker occupations at the work site in their hazard assessment. There appears to be a relationship between airborne total respirable dust concentration and total respirable dust concentrations, but further study is require to fully characterize this relationship. If this relationship holds true, it may provide a useful hazard assessment tool for employers by which the potential for exposure to airborne respirable silica at the work site can be more easily estimated. Keywords Crystalline silica, respirable dust, exposure

Address correspondence to: Diane Radnoff, 10808-99 Avenue, Edmonton, Alberta, Canada, T5K0G5; e-mail: diane.radnoff@ gov.ab.ca.

INTRODUCTION

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ilica or silicon dioxide is one of the most widespread and abundant minerals in the Earth’s crust. While it occurs in both crystalline and amorphous forms, it is the crystalline forms which are of most concern when considering health effects. The most common type of naturally occurring crystalline silica is quartz. Because quartz silica is ubiquitous, there is a high potential for occupational exposure to airborne respirable silica for workers in many industries and occupations.(1) Where rock, sand, soil, or silica-containing products are mechanically broken down, moved, handled, or disturbed, airborne respirable silica dust may be generated and occupational exposure may occur. Respirable crystalline silica dust particles (particulate with a 50% cut-point of 4.0 μm aerodynamic diameter) can be inhaled and deposited in the lungs, leading to a number of pulmonary conditions including silicosis.(2) Freshly broken or ground respirable silica appears to cause greater pulmonary reactions than aged silica. Silicosis may be acute, accelerated, or chronic. Chronic silicosis, the most common form of disease, usually occurs after low to moderate levels of exposure to silica for more than 10 years. All forms of the disease are incurable and symptoms are often difficult to manage. The disease may be progressive even if exposure to respirable crystalline silica ceases, and can lead to serious loss of lung function and even death.(3,4) Exposure to crystalline silica may also increase the risk of developing tuberculosis, chronic renal insufficiency, and autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, and sarcoidosis.(3–9) In 1997 the International Agency for Research on Cancer (IARC) classified crystalline silica from occupational sources as a Group 1 carcinogen, the evidence of increased risk being clearest for those with silicosis.(10) Since there is not a clear dose threshold for silicosis, any reduction of exposure to crystalline silica should reduce the potential risk of silicosis and associated diseases.(11) In 1974 the National Institute of Occupational safety and Health (NIOSH) recommended a time-weighted average

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(TWA) OEL to crystalline silica of 0.05 mg/m3.(12) In 2006, the American Conference of Governmental Industrial Hygienists (ACGIH) adopted a threshold limit value (TLV) for respirable crystalline silica of 0.025 mg/m3 as an 8-hr TWA.(13) These exposure values create a reference point for defining workplace exposure standards in many jurisdictions across the world. In the United States, OSHA recently proposed adopting the NIOSH REL as a new exposure standard for crystalline silica.(14) In 2009, Alberta adopted 0.025 mg/m3 as the 8-hr OEL for crystalline silica. For work shifts longer than 8 hr, the Alberta Occupational Health and Safety (OHS) legislation requires the OEL to be adjusted to compensate for the longer period of exposure. Using the formula in the legislation, the OEL for a 10-hr work shift would be reduced by 30% to 0.0175mg/m3 and for a 12-hr work shift by 50% to 0.0125 mg/m3.(15) According to CAREX (CARcinogen EXposure) Canada, the number of workers potentially exposed to crystalline silica in Canada is estimated to be 350,000. In Alberta alone, there may be almost 106,000 workers potentially exposed to crystalline silica. However, the information used by CAREX to estimate populations potentially exposed to silica is primarily based on historical occupational exposure data, published scientific literature, and census information rather than current crystalline silica exposure data.(16) Estimating the actual numbers of workers at risk of exposure and the extent of exposure to crystalline silica in Alberta is challenging since current exposure surveillance data are not widely available. Following the adoption of the OEL for crystalline silica, the OHS Policy and Program Development Branch of Alberta Jobs, Skills, Training and Labour initiated a project to address issues around crystalline silica exposure in Alberta workplaces. One of the key objectives of the project was to evaluate occupational exposure to crystalline silica in Alberta workplaces and to identify worker occupations and activities in Alberta for which exposure to crystalline silica is a potential hazard. This article presents the data collected from Alberta workplaces, identifies the industries and occupations with the highest potential exposure to respirable crystalline silica, and explores the relationship between the total airborne respirable dust concentrations, percentage of quartz in airborne respirable dust, and airborne respirable quartz concentrations.


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METHODOLOGY Selection of Work Sites Measurements of airborne respirable crystalline silica were collected between 2009 and 2013 at 40 work sites across 13 industry groups in Alberta, as listed in Table I. Measurements of total respirable dust were also collected at two-thirds of the work sites for most industry groups except asphalt plants, cement plants, and the limestone quarry. Participation in the exposure assessments was on a voluntary basis. The work sites were identified by directly contacting the companies in the target industries and via contact through industry associations. The selection aimed to ensure that a wide variety of work 558

operations were included. For each work site that participated, there was an attempt to ensure that all worker groups at the work site on that day were represented in the occupational sampling. Sampling and Analytical Methods The crystalline silica and respirable dust samples were collected and analyzed according to NIOSH Methods 600 and 7500. For the samples collected in 2010 and 2011, the sample collection method was slightly modified by using a different sampler and increasing the volume of air drawn through the filter from 2.5 L/min to 2.75 L/min. This modification was done to evaluate the ability to measure airborne crystalline silica concentrations for comparison to the Alberta OEL, even when the OEL had been adjusted lower to account for work shifts longer than 8 hr. However, it was observed that a sufficient sample volume could be collected to ensure the required sample concentration detection level using a lower flow rate, so this modification was not used for samples collected in 2012. The air samples were collected for a full work shift in the workers’ breathing zone using personal sampling pumps preand post-calibrated to draw a known amount of air through a 37 mm 5 μm PVC filter cassette attached to an SKC aluminum cyclone (SKC, Inc., Eighty Four, Pa.) (2.5 L/min samples) or an SKC GS-3 cyclone (SKC, Inc., Eighty Four, Pa.) (2.75 L/min samples). Both cyclones are designed to meet the ACGIH and International Organization for Standardization/European Standardization Committee respirable size-selection curve and have a 50% cut-point of 4.0 μm aerodynamic diameter at these flowrates.(17,18) About 16% of the samples consisted of duplicates. All samples were analyzed by an American Industrial Hygiene Association (AIHA)-accredited laboratory (Galson Laboratories or Bureau Veritas). For quartz analysis, the laboratory quantitative detection limit was 0.010 mg. R

Data Analysis Descriptive statistics were used to characterize respirable quartz and total respirable particulate concentrations in terms of geometric mean (GM), geometric standard deviation (GSD), 95% confidence interval (CI) of GM, median, minimum, maximum, and percentage of samples greater than Alberta OEL and NIOSH REL. In cases where the sample size (N) was less than six, GM, GSD, confidence intervals, and median were not calculated.(19) Exposure data were checked for normality using the Shapiro-Wilk test and Q-Q plots. The results showed that data were positively skewed in all industry groups except the limestone quarry and aggregate mining and crushing, as well as for two-thirds of the worker occupations. Logtransformation did not result in the data becoming normally distributed. Outliers were examined but there was no evidence these results were erroneous and so they were retained in the analyses. Linear regression analysis was used to examine the relationship between airborne respirable quartz and total respirable particulate concentrations. Statistical analyses were conducted with SPSS statistical software (version 21; IBM SPSS).


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TABLE I. Industry Groups Included in Exposure Assessments Industry Group

Number of Work Sites Evaluated

Abrasive Blasting

5

Aggregate Mining and Crushing

3

Asphalt Plant

2

Cement Plant Demolition Earth Moving/Road Building Foundry Limestone Quarry

3 1 3 4 1

Manufacturing, Misc. Mining New Construction

2 5 4

Oil and Gas

5

Sand and Mineral Processing

2

To facilitate comparison of respirable quartz exposures for each industry group in relation to the Alberta OEL, exposure severities were calculated by dividing the TWA exposure by the OEL. Exposure severity measures were used by Occupational Safety and Health Administration’s (OSHA) Integrated Management Information System (IMIS), and they allow for the comparisons of possible health effects in industries with similar silica exposure levels.(20) RESULTS Descriptive Statistics Overall, 343 samples were collected from 287 workers and analyzed for respirable crystalline silica (quartz and cristobalite). Of these, 11 samples were collected from blasters in abrasive blasting operations with the sampling equipment attached to the worker’s collar but positioned under the cape of the protective helmets used by the blasters. This was done because the blasters frequently removed their helmets throughout the work shift, even while conducting abrasive blasting. Since the results for these samples may not accurately represent worker exposure compared to the other samples collected (samples may have been shielded by the helmets or the airflow from the helmet may have interfered with the

Description Four of five firms conducted abrasive blasting as part of coating operations (pipe, tanks, and so on) involving blasting of bare metal in preparation for coating application. One firm conducted blasting of concrete driveways (surface finishing). Three of the five firms used silica substitutes. Extraction of sand and gravel, sorting of products and gravel crushing Manufacture of asphalt products; aggregate is mixed with oil, recycled asphalt, and other ingredients. Manufacture of cement and cement products (e.g., cement pipe) Cleanup of demolition waste Road grading and repair, new road construction Casting of a variety of non-ferrous metal products Extraction of limestone; includes blasting and drilling to break up stone and material handling Manufacture of decorative stone products and insulation Coal mining (above and underground) and oil sands extraction New construction of commercial buildings (vertical construction), including the various tasks (carpentry, concrete work, electrical work, and so on) involved to complete the building Variety of operations including bulk plants at which products used in drilling operations are mixed (3), drilling/well servicing (1), and hydraulic fracturing (1) Grading and bagging of sand and mineral products

pump flowrate), they were excluded from further analysis in this work. In addition to quartz, cristobalite was only detected in three samples above the detection limit. These samples were collected at a decorative stone product manufacturing operation. Since cristobalite is not naturally present in large quantities in Alberta, it is suspected that the source was the marble stone being processed on the day of the assessment. The cristobalite component of these three samples was not included in the data analysis. For 55 samples (16.6%), the measured amount of quartz was below the laboratory quantitative detection limit of 0.010 mg. For this analysis, where the quartz values were reported below the limit of detection (LOD), they were replaced by a value equal to the LOD divided by the square root of two.(21) The distribution of samples recorded below the LOD is shown in Table II. In addition, 205 of the samples collected were analyzed for total airborne respirable dust. Of these, 182 samples had a total airborne respirable dust concentration above the LOD so a percentage of crystalline silica in total respirable dust could be calculated. Only these samples paired with the corresponding respirable airborne quartz concentration were included in data analyses comparing total airborne respirable dust and respirable silica. In cases where the


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TABLE II. Descriptive Statistics for Airborne Respirable Quartz Exposure by Industry


Industry Sand and Mineral Processing New Construction Aggregate Mining and Crushing Abrasive Blasting Demolition Oil and Gas Foundry Manufacturing, Misc. Mining Asphalt Plant Earth Moving/ Road Building Cement Plant Limestone Quarry

N

Min. (mg/m3)

Max (mg/m3)

GM (mg/m3)

95% CI of GM

% Above NIOSH REL

Below LOD N

%

16

0.024

1.7

0.090

0.055–0.148

2.51

94

81

0

0

44 22

0.013 0.004

1.0 0.19

0.055 0.048

0.040–0.075 0.029–0.080

2.79 3.13

77 82

50 59

1 2

2 9

26 10 28 44 23

0.007 0.017 0.002 0.002 0.002

0.12 0.065 8.6 0.48 3.5

0.027 0.027 0.024 0.020 0.020

0.019–0.037 2.22 0.019–0.037 1.56 0.010–0.058 10.17 0.013–0.031 4.18 0.008–0.049 7.84

57 40 43 41 44

30 20 21 25 26

0 0 6 6 5

0 0 21 14 22

50 13 24

0.004 0.004 0.004

0.21 0.074 0.068

0.017 0.016 0.013

0.013–0.023 0.008–0.030 0.009–0.018

2.84 2.92 2.16

40 39 25

16 31 4

18 4 2

36 31 8

26 6

0.003 0.005

0.061 0.016

0.010 0.007

0.007–0.014 0.004–0.012

2.19 1.60

15 —

4 —

9 2

35 33

percentage of crystalline silica in total respirable dust was reported as a “” value, the numerical value was used.

Exposure to Airborne Respirable Silica by Industry The respirable airborne quartz exposure profile for the 13 industry groups is summarized in Table II. Mean and 95% confidence intervals of the log-transformed (base 10) respirable quartz exposure levels are compared against the log-transformed Alberta 8-hr OEL (−1.6) and NIOSH REL (−1.3) in Figure 1. The data show that exposure to quartz silica was widespread for many of the industries evaluated, with considerable heterogeneity within industry groups. The industries with the largest variation in the estimated GM and 95% CI of GM were oil and gas, manufacturing, and foundries, as shown in Table II. For oil and gas, the GM varied from 0.002 mg/m3 for drilling/servicing to 0.021 mg/m3 for bulk plants to 0.086 mg/m3 for hydraulic fracturing. For manufacturing, the GM varied from 0.004 mg/m3 for an insulation product fabricator to 0.085 mg/m3 for a decorative stone product manufacturer. The GM exposure was above the Alberta 8-hr OEL at 21 work sites; more than 60% of these were in five industry groups; sand and mineral processing, new construction, aggregate mining and crushing, abrasive blasting, and demolition. For sand and mineral processing, new construction, and aggregate mining and crushing, the lower end of the 95% CI was greater than the 8-hr OEL, indicating a very high potential for overexposure in these industry groups in particular. For 560

GSD

% Above Alberta OEL

sand and mineral processing and new construction, the GM exposures were also higher than the NIOSH REL. To compare and assess the magnitude of occupational exposure, exposure severities were calculated. The means, medians, interquartile range (IQR), minimum and maximum of the exposure severities are presented in Table III. In every case, the mean exceeded the median, indicating that the severity data were positively skewed. This indicates that the mean is affected by the outliers (extreme values) which are dragging it towards higher values.(22) The IQR value indicates the large variability of respirable quartz exposure. The median measure of severity was greater than one in the sand and mineral processing (3.56), new construction (2.02), aggregate mining and crushing (2.54), and abrasive blasting (1.24); however all industry groups, apart from the limestone quarry, had maximum levels above one indicating that there were some samples above the OEL in that industry group. Five industry groups had a maximum exposure severity more than 10 times the OEL. The maximum exposure severity was 344 times the OEL for oil and gas due to the very high exposures in hydraulic fracturing operations, 140 times OEL in the decorative stone manufacturing operation, 68 times the OEL in sand and mineral processing, 40 times the OEL in new construction, and 19.2 times the OEL in foundries. Exposure Profiles by Occupation Table IV presents the respirable quartz exposure profiles for the 19 worker occupation groups encountered during the exposure assessments; worker activities are summarized in


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FIGURE 1. Mean and 95% confidence intervals of log-transformed respirable airborne quartz exposure by industry compared with the log-transformed Alberta OEL and NIOSH REL (color figure available online).

Table V. The sample size for 4 of the 19 (21%) of the occupation groups was less than 6 samples, geometric means were not calculated in these cases. The GM exposure was above Alberta 8-hr OEL for a third of the occupation groups, with the highest GM found

for bricklayers and concrete cutting/coring/finishing (the two worker occupations were grouped together due to the similarity of work activities), equipment operators in underground mining, painters, laborers in non-mining operations, and plant operators. Of these five occupation categories, bricklayers

TABLE III. Exposure SeverityA by Industry Group Industry

N

Mean

Median

IQR

Min.

Max.

Abrasive Blasting Aggregate Mining and Crushing Asphalt Plant Cement Plant Demolition Earth Moving/Road Building Foundry Limestone Quarry Manufacturing Mining New Construction Oil and Gas Sand and Mineral Processing

26 22 13 26 10 24 44 6 23 50 44 28 16

1.41 2.98 1.04 0.55 1.17 0.69 2.17 0.31 11.35 1.23 4.49 28.39 7.33

1.24 2.54 0.40 0.32 0.90 0.48 0.78 0.28 0.48 0.60 2.02 0.76 3.56

1.58 3.30 1.87 0.51 0.58 0.67 1.76 0.23 2.02 1.18 2.18 1.69 2.43

0.30 0.16 0.17 0.14 0.68 0.14 0.90 0.18 0.80 0.16 0.52 0.08 0.96

4.80 7.60 2.96 2.44 2.60 2.72 19.20 0.64 140.00 8.40 40.00 344.00 68.00

ASeverities

were calculated by dividing the TWA exposure by the OEL.


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TABLE IV.

Descriptive Statistics for Respirable Quartz Exposure by Occupation

Occupation

N

Min (mg/m3)

Bricklayer & Concrete Coring/Cutting/ Finishing Equipment Operator (MiningUnderground) Painter Laborer (Non-Mining) Plant Operator Carpenter Equipment Operator (Non-Mining) Mechanic/Technician Maintenance Supervisor/Foreman Truck Driver Laborer (Mining, Above Ground) Equipment Operator (Mining, Above Ground) Professional Dispatcher/Shipping, Admin Electrician Lab Analyst/Technician Laborer (Mining, Underground) Welder

16

0.017

10

Max (mg/m3)

GM (mg/m3)

95%CI of GM

GSD

% Above Alberta OEL

% Above NIOSH REL

1.0

0.105

0.061–0.179

2.73

94

88

0.013

0.21

0.038

0.019–0.075

2.62

70

30

6 103

0.008 0.002

0.12 3.5

0.036 0.032

0.013–0.096 0.025–0.042

2.58 3.83

67 61

33 36

18 11 65

0.004 0.013 0.002

1.7 0.041 8.6

0.032 0.023 0.019

0.015–0.066 0.018–0.031 0.012–0.029

4.38 1.53 5.61

50 45 38

33 — 22

10 6 26 16 6

0.005 0.006 0.002 0.004 0.005

0.068 0.053 0.28 0.11 0.028

0.019 0.016 0.016 0.013 0.011

0.009–0.040 0.005–0.048 0.010–0.027 0.008–0.021 0.006–0.021

2.85 2.92 3.50 2.52 1.78

50 50 31 25 17

30 17 19 6 —

9

0.005

0.12

0.009

0.004–0.019

2.71

11

11

7 7

0.003 0.003

0.046 0.084

0.009 0.008

0.004–0.022 0.003–0.025

2.53 3.17

14 14

— 14

5 5

0.015 0.011

0.064 0.074

— —

— —

— —

80 60

60 40

3

0.013

0.036

—

—

—

33

—

3

0.002

0.13

—

—

—

33

33

and concrete cutting/coring/finishing workers were exposed to GM airborne respirable quartz concentrations higher than the NIOSH REL. The GSDs for the occupation groups ranged from 1.53 (carpenter) to 5.61 (equipment operator, non-mining) showing great heterogeneity of variances and highly variable exposure data. Figure 2 shows the mean and 95% confidence intervals of the log-transformed respirable airborne quartz exposure by occupation compared to the log-transformed (base 10) Alberta OEL and NIOSH REL. The laborers and equipment operators were the largest occupation groups evaluated. Their exposure data were further analyzed by industry to try to identify where exposure was most likely for these occupational groups. Figure 3 shows the mean and 95% confidence intervals of the logtransformed respirable airborne quartz levels for laborers by industry compared against the log-transformed Alberta 562

OEL and the NIOSH REL. GM exposures over the Alberta 8-hr OEL were found in sand and mineral processing (GM 0.93 mg/m3, 95% CI 0.63–0.136), aggregate mining and crushing (GM 0.89 mg/m3, 95% CI 0.059–0.131), new construction (GM 0.060 mg/m3, 95% CI 0.03–0.12), manufacturing (GM 0.042 mg/m3, 95% CI 0.009–0.094), foundry (GM 0.029 mg/m3, 95% CI 0.016–0.052), abrasive blasting (GM 0.028 mg/m3, 95% CI, 0.019–0.040), and demolition (GM 0.026 mg/m3, 95% CI, 0.019–0.036). Figure 4 shows the mean and 95% confidence interval of the log-transformed respirable airborne quartz concentrations for equipment operators by industry compared against the log-transformed Alberta OEL and NIOSH REL. Oil and gas (GM 0.055 mg/m3, 95% CI 0.009–0.320) and underground mining (GM 0.037 mg/m3, 95% CI 0.018–0.074) were the industry groups with GM exposure above the Alberta OEL for this group of workers. Within oil and gas, the highest


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TABLE V.

Summary of Worker Activities Number of Workers


Occupation Bricklayer & Concrete Coring/Cutting/Finishing

13

Equipment Operator (Mining-Underground)

10

Painter

6

Laborer (Non-Mining)

81

Plant Operator

16

Carpenter

8

Equipment Operator (Non-Mining)

51

Mechanic/Technician

9

Maintenance

5

Supervisor/Foreman

23

Truck Driver

15

Laborer (Mining, Above Ground)

5

Work Activities Concrete cutting, sawing, jack hammering, drilling, patching, grinding, sanding, smoothing; mixing of cement, and cleanup activities. Stone rubbing (manually rubbing a finishing stone over damp concrete to smooth the surface). Occupation category includes cement masons who grind, sand concrete, mix grout, and conduct cleanup activities. Brick layers cut cinder blocks using power tools, drill into concrete, conduct floor leveling, mix mortar, conduct cleanup as well as install bricks. Operate underground mining equipment including continuous miners, shovel cars, and roof bolters. Includes some equipment cleaning and routine maintenance. Painters in manufacturing operations (coating operations that involve abrasive blasting for surface preparation and foundries). Work activities include application of coatings with a brush or spray, cleaning the surface with compressed air, surface patching, and grinding/sanding and cleanup. Material handling, cleanup (usually by dry sweeping and compressed air), painting, patching paint and concrete (which may include grinding activities), forklift operation, equipment maintenance activities, and assisting with a wide variety of other tasks. Occupation category includes helpers and shop hands involved in abrasive blasting operations, workers involved in demolition (debris cleanup), workers in oil and gas operations, foundry workers (filling and emptying molds, grinding cut-offs), and flaggers in road construction activities. Operate the plant in a control room. May also conduct inspections of plant equipment. Framing (may require drilling into concrete walls to install the wood frame), build concrete forms from wood or styrofoam (drilling and chipping of concrete done as part of this activity), stone rubbing (manually rubbing a finishing stone over damp concrete to smooth the surface), build moulds in foundries, cleanup activities, sanding of drywall and concrete, caulking and install fireboard. Operating mobile equipment (e.g., backhoes, scraper, packers, drills, blenders in fracking operations) or operate a specific piece of plant or process equipment (e.g., blender, saws and cutting equipment, engraver) for the majority of the work shift. May also conduct maintenance activities on the equipment and equipment cleaning. Vehicle and equipment maintenance activities. Includes dry and wet cleaning of equipment. Variety of maintenance activities on process equipment, mobile equipment, equipment installation and inspection. Includes shop cleanup and equipment cleaning. Supervisory activities; may include office work as well as supervision throughout the work site (which may require driving around outdoor work sites). Also assist with other work tasks. Drive a truck for the majority of the work shift transporting materials on and off site; may also assist with loading and unloading. Similar activities to non-mining laborers, except in a mining operation. Includes oilers (responsible for inspection and routine maintenance of conveyor systems). (Continued on next page)


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TABLE V.

Summary of Worker Activities(Continued)


Occupation

Number of Workers

Equipment Operator (Mining, Above Ground)

9

Professional

6

Dispatcher/Shipping, Admin. Electrician Lab Analyst/Technician Laborer (Mining, Underground) Welder

7 4 5 3

Blaster

9

2

Work Activities Operation of a variety of heavy equipment at above ground mining operations (coal mining and oil sands extraction) including dozers, drills, shovels. May also conduct maintenance activities on the equipment and equipment cleaning. Health and safety professionals, surveyor; mainly office work but may include walking or driving around work site and collecting measurements. Coordinating activities and administration; usually within an office or control room. Install electrical conduit (includes drilling into concrete) and wiring. Collect samples, conduct quality control analysis on site. Similar activities to other laborers, except in an underground mining operation. Welding activities, some surface preparation (grinding, sanding), cleanup (generally manual sweeping). Conduct abrasive blasting, material handling, prep work, quality control, cleanup.

GM exposures were amongst the equipment operators at the hydraulic fracturing work site. Respirable Quartz Content in Total Respirable Dust Table VI shows the average percentage of quartz contained in airborne respirable dust, GM concentration of respirable quartz, and GM respirable dust concentration for the 182 sample pairs for which the total respirable dust was above the LOD. Within oil and gas, the percentage of quartz in total respirable dust was about 23% for all samples, but it was higher in hydraulic fracturing operations (45.8%) compared to the other oil and gas operations (9.9%). Overall, the average percentage of quartz contained in respirable dust was about 12% (excluding hydraulic fracturing), ranging from 2.6% (demolition) to 21.7% (sand and mineral processing). Although the respirable dust exposures (not adjusted for quartz content) were generally below the Alberta 8-hr OEL (3 mg/m3), the corresponding GM respirable quartz exposure measurements were greater than the Alberta 8-hr OEL for all industry groups except earth moving/road building and oil and gas operations (excluding hydraulic fracturing). The GSDs for GM total respirable dust concentrations ranged from 1.74 (aggregate mining and crushing) to 10.89 mg/m3 (hydraulic fracturing), showing a great heterogeneity of variances and highly variable exposures. Analysis indicated the airborne respirable quartz and total airborne respirable dust were associated (Spearman’s rho correlation coefficient = 0.78, p ≤ 0.001). In Figure 5, the plot of the log-transformed airborne respirable quartz and total airborne respirable dust concentrations are shown. Linear regression analysis also indicated a highly significant relationship between total airborne respirable dust and airborne 564

respirable quartz concentrations: Ln(Cquartz ) = −2.526 + 0.809 × Ln(CTRD ) Where Cquartz = respirable airborne quartz concentration in mg/m3 CTRD = total respirable airborne dust concentration in mg/m3 Figure 6 shows the GM airborne respirable quartz concentration and percentage of quartz in respirable dust by industry. While the figure suggests that where the percentage of quartz in total respirable dust was high there also tended to be a high respirable quartz exposure, this was not always the case. Analyses showed a fairly weak correlation between the two (Spearman’s rho correlation coefficient = 0.25, p = 0.001). The data suggested that the OEL for respirable quartz was often (but not always) exceeded when the percentage of quartz in total respirable dust was greater than 20% (e.g., hydraulic fracturing and sand and mineral processing). However, the GM respirable quartz concentration also sometimes exceeded the OEL when there was less than 10% quartz in total respirable dust such as in demolition and new construction. In fact, at the abrasive blasting work sites, where non-silica abrasives were used (containing less than 1% crystalline silica), the Alberta OEL was also exceeded. DISCUSSION wide variety of work sites, industries, and worker occupations were evaluated, confirming that crystalline silica exposure is a common potential hazard shared by many industries and occupations in Alberta. Workers potentially overexposed to airborne respirable quartz were identified at most of work sites evaluated, even where the GM exposure

A


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FIGURE 2. Mean and 95% confidence intervals of log-transformed respirable airborne quartz exposure by occupation compared with the log-transformed Alberta OEL and NIOSH REL (color figure available online).

at the work site was less than the 8-hr OEL. The potential for overexposure was particularly high in sand and mineral processing, new construction, aggregate mining and crushing, abrasive blasting, and demolition. A high potential for exposure to crystalline silica in sand and mineral processing, construction, abrasive blasting, and demolition activities is consistent with results reported in previous studies.(23,24,25) Within the manufacturing group, a high exposure severity was found at the decorative stone product manufacturing operation. These exposure results are consistent with those reported by Phillips et al. (2013) who found GMs of exposure ranging between 0.02 and 1.2 mg/m3 during fabrication of stone countertops; even higher exposures were reported during the dry-cutting associated with these operations.(26) Hydraulic fracturing was distinctly different from other oil and gas work sites and had order of magnitude higher exposures. A contributor to the higher exposures may be how the products containing silica are used at the work sites. For hydraulic fracturing, the sand was shipped dry and mixed at the work site just prior to pumping down the well while for the drillingservicing operation the drilling mud was mixed at the bulk plant and then shipped wet to the work site for use. Respirable quartz 12-hr TWAs ranging from 0.006 mg/m3 to 2.75 mg/m3 have previously been reported during hydraulic fracturing (overall GM 0.122 mg/m3) which is consistent with the mean

exposure found in the Alberta data (GM 0.086 mg/m3), even though the range of exposures was broader for the Alberta data (0.003 mg/m3 to 8.6 mg/m3).(27) The data collected for the mining operations appeared to be consistent with results found by others even though the operations evaluated in Alberta were coal and oil sands extraction. Data on respirable quartz exposure collected from various underground mining operations (South Dakota gold mining, tin mining in China, and South Africa gold mining) found mean airborne respirable quartz levels ranging between 0.03 to 0.1 mg/m3.(28,29) Weeks and Rose (2006) evaluated exposure data collected from 1998 to 2002 by the U.S. Mine Safety and Health Administration for metal and non-metal mines and found an overall mean exposure to crystalline silica of 0.027 mg/m3. About 27% of the samples evaluated in their review exceeded the NIOSH REL.(30) The exposure data for the limestone quarry were surprising as none of the samples was over the OEL and the GM exposure was quite low (0.007 mg/m3). Some work done in the 1990s to evaluate exposure to crystalline silica in similar operations found GM exposure values for limestone quarry and crushing operations to be 0.06 mg/m3. Of nine limestone operations evaluated, all had some exposures above the NIOSH REL of 0.05 mg/m3 and about 20% of the samples for limestone operations overall were over the NIOSH REL.(31)


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FIGURE 3. Mean and 95% confidence intervals of log-transformed respirable airborne quartz exposure for laborers by industry (color figure available online).

The operation evaluated in this study did have a number of engineering controls in place at the time of the assessment (wet drilling methods, ventilation systems in the crusher, control rooms, and vehicles) and the rock being processed may have been somewhat wet on the day of the assessment due to a previous snowfall. Occupations with the highest potential for exposure to airborne respirable quartz were bricklayer and concrete cutting/coring/finishing workers, equipment operators in underground mining, painters, laborers in non-mining operations and plant operators. Most of the electricians and half of the carpenters were also potentially exposed to silica over the OEL. The first two occupational categories are cited by other studies as being potentially overexposed to crystalline silica. For example, Beaudry et al. conducted a literature review on exposure to crystalline silica in construction and found that cement mason/concrete finishers and bricklayer/stone masons had GM exposure concentrations of 0.28 mg/m3 and 0.17 mg/m3, respectively.(32) Researchers have also identified a high risk for the development of silicosis in highway repair activities that involve jackhammering (mean exposure 0.276 mg/m3), concrete cutting and cleanup (mean 0.152 to 1.070 mg/m3), milling and cleanup of asphalt (0.007 to 0.041 mg/m3), and drilling dowels (0.107 mg/m3).(33)

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However, there were a number of worker occupations in the Alberta data where overexposure to crystalline silica was not expected (electrician, painter, and carpenter) for which the GM quartz exposure could approach or exceed the OEL. These are not normally occupations cited in the literature at risk for exposure to silica, although Rappaport et al. identified a high potential for exposure to airborne crystalline silica for painters, albeit when the painters were conducting abrasive blasting activities rather than painting which was the case in this study.(23) The exposures were likely due to specific tasks that the worker was doing at the time of the assessment (for example, the electricians were drilling through concrete to install electrical cables, the carpenters were installing framing into concrete walls, painters were using compressed air to clean surfaces of dust) or due to incidental exposure from other activities at the work site (painters at the foundry and abrasive blasting work sites). This finding suggests that consideration for potential exposure to crystalline silica should be given to all workers at a work site where there are activities or operations present that may create the potential for airborne respirable crystalline silica. Careful attention should be given to the actual activities the workers will perform, not just their occupation, when conducting the hazard assessment.


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FIGURE 4. Mean and 95% confidence intervals of log-transformed respirable airborne quartz exposure for equipment operators by industry (color figure available online).

GM TWA respirable dust exposures were below Alberta 8-hr OEL for respirable dust in all industry groups evaluated. However, some very high individual exposures occurred at the stone manufacturing, foundries, and at the hydraulic fracturing work sites, where peak levels of airborne respirable dust concentrations could exceed three to five times OEL. The results from the data analysis also indicate that airborne respirable quartz concentration may be related to the total airborne respirable dust concentrations. Other researchers have indicated that there can be a relationship between respirable and total dust. Historical work by Siltanen et al. and Verma found a good correlation between airborne respirable dust and total dust concentrations as well as a correlation between airborne respirable silica and total dust.(34,35) Ehrlich et al. also showed that there is a dependency of respirable crystalline silica concentration to total airborne dust concentration generated from silica sand processing operations (coefficient R2 = 0.88), although in this work a different method was used to collect samples (impactor methods).(36) If the relationship between airborne respirable silica and total respirable dust concentrations holds true, the formula could be used to calculate an estimate of airborne respirable silica concentrations where the total airborne respirable dust concentration is known. This could be a valuable hazard assessment tool for employers as total respirable dust concentrations can be measured using

direct-reading instruments. This relationship should be tested using other exposure data to evaluate its accuracy. In contrast, the percentage of respirable quartz in the total airborne respirable dust did not appear to be well correlated to the concentration of airborne respirable quartz. These results are consistent with those from a previous study which investigated the relationship between airborne respirable and total dust concentrations and the percentage of crystalline silica in total dust in a nonferrous foundry.(37) The Alberta OEL for respirable quartz could be exceeded even when the percentage of quartz in the total airborne respirable dust was low, and consequently employers should not assume the OEL for silica will not be exceeded where airborne dust does not contain a high proportion of silica. Limitations Exposure data were collected across a wide variety of industries and worker occupations in Alberta; however the list was not all-inclusive. Since participation in the exposure assessments was on voluntary basis, samples collected do not represent a random sample of exposure levels and the full range of industrial operations with potential exposure to airborne respirable crystalline silica. In some cases, industries with high potential for crystalline silica exposure elected to not participate in the study (e.g., ceramics manufacturing,


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TABLE VI.

Exposure Profiles for Respirable Quartz and Total Respirable Dust by Industry


Respirable Quartz (mg/m3)

Respirable Dust (mg/m3)

Industry

N

Average% Quartz

GM

GSD

GM

GSD

Abrasive Blasting Aggregate Mining and Crushing Demolition Earth Moving/Road Building Foundry Manufacturing Mining New Construction Oil and Gas-Bulk Plants Oil and Gas-Fracturing Sand and Mineral Processing

31 10

11.3 17.2

0.038 0.059

2.67 2.13

0.466 0.370

2.99 1.74

10 21

2.6 12.4

0.026 0.014

1.56 2.22

1.065 0.120

1.85 2.09

40 18 9 8 8 11 16

8.2 13.9 18.5 7.9 9.9 45.8 21.7

0.025 0.034 0.035 0.050 0.019 0.115 0.090

3.85 7.52 2.22 4.12 1.75 15.94 2.51

0.415 0.290 0.226 0.683 0.215 0.296 0.456

5.40 6.01 3.19 4.46 2.67 10.89 1.82

construction materials manufacturing, residential construction, and gravestone engraving). Exposure data collection was also limited due to logistical and scheduling issues (demolition activities, limestone quarry, oil and gas operations). As a result, the data collected may not be representative of the industry group overall. Sample size for 4 of the 19 (21%) occupation groups were less than 6 samples, limiting the ability of the study results to be generalized to the full occupational category. Moreover, some

FIGURE 5.

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of the occupational data were likely task-specific and may not represent average exposures for the entire occupation. Of some concern was that for some worker groups, particularly welders, the sample size was limited as many of the results had to be discarded because the samples were overloaded. Sampling was also inevitably limited by the work activities underway at the time of the sampling. Information on controls present at the work sites was collected during the assessments; however the control mea-

Plot of airborne respirable quartz concentration to corresponding total respirable dust concentration (color figure available online).


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FIGURE 6. Airborne respirable quartz concentration compared to respirable quartz percentage in total respirable dust by industry (color figure available online).

sures used at work sites were not quantitatively evaluated. A number of workers were using respiratory protective equipment (RPE) during the exposure monitoring and the impact of this on exposures was not incorporated in the analyses. Of the samples analyzed for crystalline silica, 16.6% were below the laboratory quantitative detection limit of 0.010 mg. Because of the uncertainty in the exact exposure concentrations below this quantitative detection limit, there may be a positive or negative bias in the exposure results that may limit the usefulness and generalizability of the data. CONCLUSION

H

azard recognition is the first step in protecting workers from exposure to hazardous substances. In this study data on airborne respirable crystalline exposure were collected for a variety of industries and occupations in Alberta. The highest potentials for exposure over the Alberta OEL occurred in sand and mineral processing, followed by new construction, aggregate mining and crushing, abrasive blasting, and demolition. Airborne exposures to crystalline silica above the Alberta OEL were found at almost all of the work sites assessed. In some

cases, workers in occupations where exposure to crystalline silica is not commonly considered a hazard were potentially overexposed. Since this was likely due to a combination of the task the worker was performing and incidental exposure from other workers’ activities at the work site, it is important that the employer consider all workers when there is a potential for respirable crystalline silica to become airborne from activities at the work site. While there appears to be a relationship between the airborne concentrations of respirable quartz and total respirable dust, there did not appear to be a correlation between the airborne respirable quartz concentration and the percentage of quartz in the total respirable dust. The potential for a relationship between airborne concentrations of quartz and total respirable dust should be explored further to confirm its validity as this could be a potential hazard assessment tool to help estimate airborne respirable silica concentrations.

ACKNOWLEDGMENTS

T

he authors would like to acknowledge the support of the Government of Alberta which provided funding for this


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work and Alberta employers who supported and participated in the exposure assessments.


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