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Exposure to respirable crystalline silica in South African farm workers

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2009 J. Phys.: Conf. Ser. 151 012005 (http://iopscience.iop.org/1742-6596/151/1/012005) View the table of contents for this issue, or go to the journal homepage for more

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Inhaled Particles X, (23–25 September 2008, Manchester) Journal of Physics: Conference Series 151 (2009) 012005

IOP Publishing doi:10.1088/1742-6596/151/1/012005

Exposure to respirable crystalline silica in south african farm workers Andrew Swanepoel1,2, David Rees1,2, Kevin Renton2, Hans Kromhout3 1

University of the Witwatersrand, School of Public Health, Johannesburg, South Africa. 2

National Institute for Occupational Health, Johannesburg, South Africa.

3

Environmental Epidemiology Division, Institute for Risk Assessment Sciences, University of Utrecht. [email protected] Abstract Although listed in some publications as an activity associated with silica (quartz) exposure, agriculture is not widely recognized as an industry with a potential for silica associated diseases. Because so many people work in agriculture; and because silica exposure and silicosis are associated with serious diseases such as tuberculosis (TB), particular in those immunological compromised by the Human immunodeficiency virus (HIV), silica exposure in agriculture is potentially very important. But in South Africa (SA) very little is known about silica exposure in this industry. The objectives of this project are: (a) to measure inhalable and respirable dust and its quartz content on two typical sandy soil farms in the Free State province of SA for all major tasks done on the farms; and (b) to characterise the mineralogy soil type of these farms. Two typical farms in the sandy soil region of the Free State province were studied. The potential health effects faced by these farm workers from exposure to respirable crystalline silica are discussed.

1. Introduction To date, there have been no studies published in South African on personal quartz exposure (commonly known as respirable crystalline silica) and agriculture and therefore it is unknown to what extent quartz exposure is a risk in the SA agriculture industry. South Africa has a large agricultural sector and is a net exporter of farming products such as maize, wheat and sunflower. Out of a population of 47.9 million people, the agricultural industry contributes around 8.8% (1.164 million people) of formal employment (13.234 million people) in South Africa. Although listed in some publications as an activity associated with silica (quartz) exposure, agriculture is not widely recognized as an industry with a potential for silica associated diseases. Several good international published studies on silica exposure in agriculture indicated variable exposure levels between various operations. Studies have reported that farming activities in sandy soils can produce quartz levels well above generally accepted occupational exposure limits (OELs) and that sandy soils are commonplace in drier regions of many countries. Because so many people work in agriculture; and because silica exposure and silicosis are associated with serious diseases such as tuberculosis (TB), particularly in those immunological

c 2009 IOP Publishing Ltd

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compromised by the Human immunodeficiency virus (HIV), silica exposure in agriculture is potentially very important. 2. Objectives The objectives of this study are: (a) to measure respirable dust and its quartz content on three typical sandy soil and sandy loam soil farms in the Free State and North West provinces of central South Africa for all major tasks done on the farms; and (b) to characterise the mineralogy soil type of these farms. 3. Methods Two typical farms in the sandy soil region of the Free State province (Farm 1 and 2) and a typical farm in the sandy-loam region of the North West province (Farm 3) were identified by a pedologist. The actual farms were selected by convenience sampling because access to the farms was available. Personal breathing zone samples were measured in all workers on the farms involved in the following operations: maize planting and harvesting, watermelon and pumpkin handling, sunflower planting and ground preparation. Respirable dust was measured using the standard widely used Health and Safety Executive (HSE) laboratory Method for the Determination of Hazardous Substances (MDHS 14/3 – General methods for sampling and gravimetric analysis of respirable and inhalable dust) and a Higgens-Dewell cyclone was used operating at a flow rate of 2.2 ℓ/min. Respirable quartz was measured and analysed using Xray diffraction (XRD) at the National Institute for Occupational Health (NIOH) as specified in the HSE analytical method MDHS 101 (Crystalline silica in respirable airborne dust – Direct-on-filter analysis by infrared spectroscopy and X-ray diffraction). A hundred and forty nine breathing zone measurements of approximately eight hours in length (mean 467 min) were collected over a complete cycle of a year on the three identified farms. Data was analysed using the S-Plus 6.2 statistical software package. Written informed consent was obtained from all participants in the study and the study was approved by the University of the Witwatersrand Human Research Ethics Committee. Representative soil samples were collected at a depth of 3– 4 centimeters (cm) for analysis by XRD to describe the mineralogy of the soils. 4. Results A hundred and forty nine (n=149) breathing zone measurements of approximately eight hours in length (mean 467 min) were collected over a complete cycle of a year on the three identified farms. Values below the detection limit were assigned half the detection limit for statistical analysis. Table 1 . Respirable quartz exposures during farming operations (mg/ m3 ) on three central South African farms. Farms n Mean Median Min Max Inter % % % ±SD Quartile 0.1a 0.05b 0.025c Range mg/m3 mg/m3 mg/m3 Farm 1

13

Farm 2

77

Farm 3

59

0.0.1 ±0.05 0.06 ±0.12 0.04 ±0.01 0.05 ±0.01

0.03

0.020

0.630

1.872

30.7%

30.8%

53.9%

0.03

0.021

0.314

0.029

14.3%

26%

50.7%

0.02

0.021

0.166

0.010

6.8%

15.3%

37.3%

22.1%

45.6%

Total: 149 0.02 0.020 0.630 0.020 12.8% Farm 1,2,3 a 0.1 mg/m3 is the South African occupational exposure limit (OEL) for quartz b 0.05 mg/m3 is the NIOSH recommended exposure limit (REL) c 0.025 mg/m3 is the ACGIH Threshold Limit Value (TLV)

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The overall mean Time Weighted Average (TWA) respirable quartz concentration was 0.05 mg/m3 with a maximum of 0.63 mg/m3 measured on farm 1 during sunflower planting operations of the study (Table 1). Thirteen percent of the measurements exceeded the South African Department of Labor (DoL) OEL of 0.1 mg/m3, 22% exceeded the National Institute for Occupational Health and Safety (NIOSH) recommended exposure limit (REL) of 0.05 mg/m3 and 46% of the measurements exceeded the widely used reference value of the American Conference of Governmental Industrial Hygienists (ACGIH) of 0.025 mg/m3 (TLV-TWA). The overall mean TWA respirable dust concentration was 0.57 mg/m3 with a maximum of 6.06 mg/m3 measured on farm 1 during sunflower planting operations of the study (Table 2). Only 7% of the measurements exceeded the South African Department of Labor (DoL) OEL of 2 mg/m3. Table 2 Respirable dust exposures during farming operations (mg/ m3 ) on three central South African farms Farms n Mean Median Min Max Inter Quartile %  2a ±SD Range mg/m3 Farm 1

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1.472 0.101 0.017 6.062 1.872 23.1% ±0.47 Farm 2 77 0.6106 0.246 0.009 5.689 0.363 7.8% ±0.12 Farm 3 59 0.3111 0.170 0.019 2.803 0.309 1.7% ±0.58 Total: 149 0.57 0.227 0.009 6.062 0.473 6.7% Farm 1,2,3 ±0.01 a 2 mg/m3 is the South African occupational exposure limit (OEL) for respirable dust.

5. Discussion Many studies indicated that quartz exposure in agriculture might be a risk to farm workers, but generally no systematic data are available on the magnitude of the risk. Although exposure to quartz during agricultural activities is variable, exposure can extend throughout the growing season. The duration of specific activities varies due to many factors, and lifetime exposure to quartz may be considerable. A number of possible explanations exist for the paucity of cases of silicosis. Exposure assessment in agriculture is difficult because of the varied and cyclic nature of the farmers work and the diverse locations of the farms. Exposures may vary with local farming practice and specific tasks performed on the farm, commodities grown or raised, geography, environmental factors (i.e. temperatures, season changes, relative humidity, rainfall, wind speeds, wind directions) and other factors such as soil characteristics (i.e. soil temperatures, mineralogy of soil, soil moisture) and tractor characteristics (tractor speed and enclosed cabins). Variations in soil components (e.g., silicates, iron oxides) can affect the pathogenicity of silica. Silica potency is reduced by other soil components such as iron oxides and clay silicates. In addition, silica in soils is likely to be aged, with most farming activities producing little freshly fractured material. Fresh-fractured silica dust is known to be more pathogenic by contact with the air (e.g., most silica in soil), however little is known about the potential for fresh-fractured silica in the soil that might be caused by contact with metal during activities such as plowing and discing. Only a small portion of the soil is in contact with the machinery, while the majority of dust is from the disturbed soil. Thus, it seems likely that fresh fractured dust could not comprise more than a small fraction of repirable soil dust. Under-diagnosis is also a possible explanation. Cases of silicosis have been reported, however, and silica exposure in farming deserves further research.

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Notably, these widely used “safe” limits for quartz of 0.05–0.1 mg/m3 have been shown not to be protective. The American Conference of Governmental Industrial Hygienists (ACGIH) has recommended a threshold limit value (TLV) for quartz of 0.025 mg/m3. 6. Conclusion These data suggest that quartz exposure is a possible risk in the farming industry. If it is, this is a serious public health concern in South Africa given the country’s high TB rates and the number of people involved in agriculture. Therefore it needs to be further investigated Keywords: silica, quartz exposure, respirable, agriculture, farm workers.

TWA resp dust 8

1.0

TWA resp quartz

OEL (2 mg/m³)

0.8

OEL (0.1 mg/m³) NIOSH (0.05 mg/m³) ACGIH (0.025 mg/m³)

6

n=13

*

*

*

*

*

*

*

*

*

*

*

4

mg/m³ 0.4

mg/m³

0.6

*

n=59

n=77 * *

* ** ** ** ** ** ** ** ** ** ***

*

* * ** * ** ** ** ** **

Farm 1

Farm 2

2

* *

Total

* **

n=59 * * * ** * ** ** 0

0.2

n=13 n=77

n=149

0.0

n=149

Farm 3

* * *

* *

** ** **

* *

*** ** *** **** ***** **

* * *

Total

*

** * **

* **

*** *** ** ***** **

*** * ** ** ** ** **

Farm 1

Farm 2

Farm 3

TWA resp quartz = the time weighted average concentration of respirable quartz. TWA resp dust = the time weighted average concentration of respirable dust. Figure 1.

Respirable quartz and respirable dust concentrations (mg/m3) during farming operations (mg/ m3 ) on three central South African farms (n=149).

4