Quartz Exposure in Agriculture: Literature Review and South African Survey

ABSTRACT

Objectives: To review the published literature on respirable quartz exposure and associated disease in agricultural related settings systematically and to describe personal respirable dust and quartz measurements collected on a sandy soil farm in the Free State province of South Africa.

Methods: The published studies on exposure to respirable silica and quartz in agriculture and related settings (to June 2009) were searched systematically through ‘PubMed’ and critiqued. A farm in the sandy soil region of the Free State province of South Africa producing typical crops for the region was identified and 138 respirable dust and respirable quartz measurements were collected from July 2006–August 2008 during major farming operations.

Results: In total, 17 studies were identified: 11 investigated respirable quartz exposure on farms and 6 quartz-related diseases in agricultural settings. They provided convincing evidence of a respirable quartz risk on sandy soil farms but scant evidence of associated disease. Respirable quartz measurements from the South African farm ranged from not detectable to 626 μg m−3 and confirmed the quartz risk as some concentrations exceeded generally accepted occupational exposure limits in all jobs evaluated, even though the majority of respirable dust concentrations were well below a commonly used occupational exposure limit of 2 mg m−3. Twelve of 138 respirable dust measurements (9%) and 18 of 138 respirable quartz measurements (13%) exceeded commonly used occupational exposure limits of 2 mg m−3 and 100 μg m−3, respectively. The highest time weighted average respirable quartz concentration of 626 μg m−3 was during wheat planting activities. Fifty-seven percent of the respirable quartz measurements exceeded the American Conference of Governmental Industrial Hygienists (ACGIH) Threshold Limit Value (TLV) of 25 μg m−3. Quartz percentages of the respirable dust ranged from 0.3 to 94.4% with an overall median of 13.4%.

Conclusion: Despite its ubiquity, little is known about quartz exposure in the agricultural industry; but this study demonstrates significant potential for overexposure in some settings. Further research is required to quantify quartz exposure and identify settings and tasks that place farmers and farmworkers at risk of quartz-associated diseases so that controls can be implemented.

METHODS

Study population and design: South African farm

Personal respirable dust and quartz measurements were collected over 17 days between July 2006 and August 2008 during major farming operations on a sandy soil farm in the eastern Free State province of South Africa. The sandy soil region was identified by a pedologist using geological maps and the actual farm was selected by virtue of ‘convenience sampling’ i.e. access to the farm was granted. This region is ∼2 million hectares in size and the farm selected was situated centrally in the sandy soil area. Agriculture in this region is dominated by grain and livestock with a median rainfall of 450–500 mm per annum (Statistics-South-Africa, 2002). The farm under study is representative of this region in terms of farming activities and commodities and was ∼850 hectares in size. Crops and livestock produced on the farm included maize, wheat, sunflower, water melon, pumpkin, potatoes, and cattle. Major tasks undertaken on the farm are shown in Table 2. Mechanical harvesting of crops was done in modern closed-cabbed harvesters and preliminary measurements in the harvesters indicated quartz exposures below the limit of detection (LOD). Therefore, for the purpose of this study, farmworkers operating cabbed harvesters were not included. However, personal occupational exposures in these harvesters will be addressed in a subsequent study. Twenty people worked on the farm during the time of the study, and major tasks undertaken over the annual farming cycle were identified by the farmer. All workers completing the tasks on the sampling days agreed to participate. Tasks were selected to be representative of those performed by farmworkers throughout the study period. No respiratory protective equipment was used by the farmworkers. Furthermore, all tractor drivers measured during the study used open-cabbed tractors to perform the tasks on the farm. No screening for the presence of respiratory disease in the worker population studied was done.

Soil grain size analysis

A soil sample was collected on the farm under study and analyzed at the School of Geosciences of the University of the Witwatersrand for grain size analysis. A sample of 150 g was soaked overnight in water with <1 g of sodium hexametaphosphate added to aid clay deflocculation. Samples were wet sieved through a 63-μ sieve in order to separate sand from silt and clay. The clay–silt suspension was poured into a cylindrical flask filled with water. After ∼54 min, the top 5 cm of suspension (containing only clay-sized particles) was extracted using a pipette. Cylinders were then topped up with water and the process repeated until the amount of clay left in the solution was negligible. The analysis showed that the soil was 86% sand, 10.7% silt, and 3.6% clay.

Dust sampling

The farm was visited six times (which summated to 17 days in total) and during the time of the exposure assessment, 138 PBZ measurements were collected for the purposes of evaluating respirable dust and quartz exposure of the farmworkers. Respirable dust was collected using the HSE MDHS 14/3 method (HSE, 2000); wherein, PBZ samples were collected by means of a Higgins-Dewell cyclone on pre-weighed 25-mm polyvinyl chloride filters using personal sampling pumps (Gillian® Gil Air) connected to sampling trains calibrated on-site to a flow rate of 2.2 l min−1 in order to determine the respirable fraction. Farmwork seldom involves 8-h shifts, but management at the farm agreed to allow sampling for 8 h a day only and therefore, the duration of the PBZ measurements was ∼8 h in length (mean 495 min). Calibration was verified after each measurement. After sampling, cyclones were removed and filter cassettes were closed and capped for transport. The filters were equilibrated in an environmentally controlled weighing area for at least 2 h and weighed before and after sampling on a 5-decimal (g) microbalance. A respirable dust concentration using the difference in filter weight, after adjustment for blanks, and sampling volume was calculated for each sample.

Quartz anaylsis

After post-weighing, filters were sealed in petri slides and transported to the National Institute for Occupational Health (NIOH) in South Africa for quartz analysis using X-ray diffraction (XRD) as specified in the HSE MDHS 101 method (HSE, 2005). The results were expressed in microgram per cubic meter and the LOD reported by the NIOH was 22 μg.

Data analysis

Respirable dust and quartz concentrations were evaluated against reference values of 2 mg m−3 for respirable dust and 100 μg m−3 (South African Occupational Exposure Limit), 50 μg m−3 [National Institute for Occupational Safety and Health Recommended Exposure Limit (REL)], and 25 μg m−3 (American Conference of Governmental Industrial Hygienists Threshold Limit Value) for respirable quartz. Forty-nine of 138 measurements of respirable quartz were found to be below the analytical LOD of 22-μg quartz; these were assigned a value of LOD/2‾√ for calculation of average exposure (Nieuwenhuijsen, 1997). Both the respirable dust and respirable quartz measurements could be best described with a lognormal distribution. Geometric means and geometric standard deviations were used to present the average and distribution of concentrations. The measurements are presented by date of visit and major tasks undertaken on the farm (Tables 3 and 4). Quartz percentages in the respirable fraction of the dust were calculated by dividing the respirable quartz concentration by the respirable dust concentration.

Ethics

Written informed consent was obtained from all participants in the study that was approved by the University of the Witwatersrand Human Research Ethics Committee (clearance number M070252).

Quality assurance

A quality control system was in place for both the gravimetric and XRD analytical analyses. For the gravimetric analysis, an approved inspection authority registered with the South African Department of Labour and a qualified occupational hygienist registered with the Southern African Institute for Occupational Hygiene monitored all aspects of the procedure from filter preparation, calibration of the pumps to filter weighing. Three blank filters were kept and treated in the same manner as the filters used during actual sampling for each day of measurement and analysis for all blank filters showed concentrations below the limit of detection. Quartz was measured in an experienced laboratory; where, both the gravimetric and analytical laboratories were accredited by the South African National Accreditation System.

RESULTS

One hundred and thirty-eight personal respirable dust and quartz samples were collected from July 2006–August 2008 during typical farming activities from a single farm identified to have sandy soils. Measured respirable dust and quartz concentrations by agriculture activity are presented in Tables 3 and 4. The overall geometric mean time weighted average (TWA)-respirable dust and quartz concentrations were 0.32 mg m−3 and 35.35 μg m−3, respectively, with the GSDs being 3.24 and 2.28, respectively. Respirable dust concentrations were relatively low: only 9% of the samples exceeded 2 mg m−3 with the highest concentration measured during wheat-planting operations (6.49 mg m−3). Nevertheless, Table 4 shows overexposure to respirable quartz in all farming tasks evaluated. Thirteen percent of the measurements exceeded the South African Department of Labor (DoL) OEL of 100 μg m−3, 31% exceeded the National Institute for Occupational Safety and Health REL of 50 μg m−3 and 57% of the measurements exceeded the widely used reference value of the American Conference of Governmental Industrial Hygienists (ACGIH) TLV–TWA of 25 μg m−3, with a maximum of 626 μg m−3 measured during wheat-planting operations. Quartz percentages of the respirable dust ranged from 0.3 to 94.4% with a median of 13.4% (Table 5).
