Ann. occup. Hyg., Vol. 47, No. 3, pp. 235-240, 2003
© 2003 British Occupational Hygiene Society
Published by Oxford University Press
Dust Exposure During Small-scale Mining in Tanzania: A Pilot Study
1 Section for Occupational Medicine, Department of Public Health and Primary Health Care, University of Bergen, Ulriksdal 8c, N-5009 Bergen, Norway; 2 Department of Physiology, Muhimbili University College of Health Sciences, PO Box 65316, Dar es Salaam, Tanzania; 3 Work and Health, PO Box 11242, Arusha, Tanzania
Received 1 July 2002; in final form 2 September 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Small-scale mining in developing countries is generally labour-intensive and carried out with low levels of mechanization. In the Mererani area in the northern part of Tanzania, there are about 15 000 underground miners who are constantly subjected to a poor working environment. Gemstones are found at depths down to 500 m. The objectives of this pilot study were to monitor the exposure to dust during work processes, which are typical of small-scale mining in developing countries, and to make a rough estimation of whether there is a risk of chronic pulmonary diseases for the workers. Personal sampling of respirable dust (n = 15) and ‘total’ dust (n = 5) was carried out during three consecutive days in one mine, which had a total of 50 workers in two shifts. Sampling started immediately before the miners entered the shaft, and lasted until they reappeared at the mine entrance after 5–8 h. The median crystalline silica content and the combustible content of the respirable dust samples were 14.2 and 5.5%, respectively. When drilling, blasting and shovelling were carried out, the exposure measurements showed high median levels of respirable dust (15.5 mg/m3), respirable crystalline silica (2.4 mg/m3), respirable combustible dust (1.5 mg/m3) and ‘total’ dust (28.4 mg/m3). When only shovelling and loading of sacks took place, the median exposures to respirable dust and respirable crystalline silica were 4.3 and 1.1 mg/m3. This study shows that the exposure to respirable crystalline silica was high during underground small-scale mining. In the absence of personal protective equipment, the miners in the Mererani area are presumably at a high risk of developing chronic silicosis.
Keywords: crystalline silica; quartz; mining; developing country; Tanzania
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Small-scale mining in developing countries provides employment for an estimated 13 million people (ILO, 1999). These mines are labour-intensive, with low levels of mechanization, and working conditions are generally far removed from international labour standards. Underground miners are exposed to heat, noise and vibrations, as well as to dust and inorganic gases. There is usually no provision of collective or personal protective equipment.
In the United Republic of Tanzania, the mining industry has increased dramatically during the last three decades. Today, the number of employees in small-scale mining in this country is ~450–600 000, which is more than in any other African country (ILO, 1999). The Mererani mining area is situated 50 km from Arusha in northern Tanzania, and is the only source of the precious stone Tanzanite. The most famous gem variety is the purplish-blue Tanzanite, which was discovered in 1967. The Tanzanite gemstones are found in graphitic gneiss containing quartz and feldspar at depths down to 500 m.
Respirable particles are formed whenever silica-bearing rock is drilled, blasted or crushed. In large-scale mines in South Africa and China the exposure to crystalline silica is normally <0.5 mg/m3 (Beadle and Bradley, 1970; Dosemeci et al., 1995). However, there seems to be no published studies on dust exposure in small-scale mining in developing countries.
It is well established that moderate exposure (0.05–0.1 mg/m3) to crystalline silica for 20–45 yr is associated with chronic silicosis (Greaves, 2000). Silica is also considered a human carcinogen by the International Agency for Research on Cancer (IARC, 1997), and Greaves (2000) concludes that individuals who inhale silica dust in sufficient amounts to cause fibrosis are at increased risk of lung cancer. Furthermore, respirable coal dust might contribute to chronic bronchitis and decreased lung function (Soutar and Hurley, 1986).
The objectives of this pilot study were to monitor exposure to dust during work processes typical for small-scale underground mining in developing countries, and to roughly estimate whether there is a risk of developing pulmonary diseases for the workers.
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MATERIALS AND METHODS |
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Location
The management of the Small Miners Association approved our access to the Mererani mining area in Tanzania. About 300 privately owned underground mines in Mererani are members of this association, employing at least 15 000 miners. The criteria for the selection of one mine for the present study were: (i) the mine had to be a member of the Small Miners Association; (ii) it had to be possible to carry out personal dust sampling during the day shift on three consecutive working days; (iii) the work processes during the days of monitoring had to be representative for normal working days.
The selected mine had 50 workers in two shifts. The length of the day shift varied from 5 to 8 h, while the night shift lasted up to 18 h.
Job description
The workers entered the mines through vertical shafts with footsteps carved into the wall (Fig. 1). The unsupported tunnels were ~1.5 m high, and no machinery was used except compressors and pneumatic drills. The compressors were also used to supply air into the long, dead-end tunnels. Wet methods to suppress dust generation on drilling and blasting were not used. After blasting with explosives, the rock was shovelled using hand tools, and loaded into small sacks that were later carried to the surface. The workers did not use personal protective equipment, such as respirators, helmets, gloves or hearing protection, and their only light source was a torch attached to the miner’s head by a rubber band. The workers did not leave the mine during the working shift.
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Dust sampling
Personal dust sampling was carried out during three consecutive days in one single underground mine in August 2001. Only day shifts were selected due to practical problems with transport to the remote mining area after dark. On each of these 3 days, the mine boss selected seven workers to wear the sampling equipment. These miners were asked about their age and for how long they had been working in the mines.
Each day five respirable dust samples and two total dust samples were taken. On the first and second day of measurement, the workers reported to have drilled twice a day with pneumatic tools for ~1 h before blasting the rock with explosives. The rest of the day they shovelled the blasted rock. The third day was spent on shovelling the blasted rock and loading the transport sacks. The investigators did not enter the mines to verify this information.
Respirable dust and ‘total’ dust were collected on 37 mm cellulose acetate filters (pore size 0.8 µm). For sampling of respirable dust, the filters were placed in a 37 mm SKC Conductive Plastic Cyclone, and for sampling of ‘total’ dust in closed-faced, 37 mm Millipore cassettes. SKC Sidekick (Model 224-50) pumps with sampling flow rates of 2.2 and 2.0 l/min were used for respirable and ‘total’ dust, respectively.
Samplings were started immediately before the miners entered the shaft, and lasted until they reappeared at the mine entrance. The sampling time varied between 5 and 8 h, depending on how much time the workers spent in the mine. On day 2, one of the ‘total’ dust samples was rejected because the pump had stopped.
Analysis of dust samples
Respirable and ‘total’ dust were quantified by gravimetric analysis at X-lab in Bergen, Norway, using a Mettler AT 261 Delta Range, with an accuracy of 0.05 mg. After acid digestion (HNO3/H2O2) in a microwave oven, the ‘total’ dust samples (n = 5) were analysed for elements with inductively coupled plasma-atomic emission spectrometry (ICP-AES, ARL 3410+, Thermo ARL).
Nine of the respirable dust samples (three from each day of sampling) were analysed at SGAB Analytica Laboratory, Sweden for crystalline silica by X-ray diffraction on silver membrane filter using NIOSH method 7500. For the remaining six respirable dust samples (two from each day of sampling), the same laboratory indirectly determined the respirable combustible dust (RCD) by gravimetric analysis before and after heating to 550°C. At this temperature carbon oxidizes to form carbon dioxide. In this analysis it was assumed that the amount of organic carbon was negligible and that the RCD mainly consisted of carbon originating from the graphitic gneiss. Selection of respirable samples for analysis of RCD or for crystalline silica was done by systematically assigning the two samples with the lowest identification number on each sampling day to RCD determination and the remaining samples to analysis of crystalline silica.
Results from the exposure measurements are compared to threshold limit values (TLVs) which refer to the guidelines given by the American Conference of Governmental Industrial Hygienists (ACGIH, 2001).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The average age of the 21 miners who participated in the personal dust sampling study was 29.0 yr (range 18–38 yr). On average, they had worked in these mines for 4.9 yr (range 0.5–14 yr).
On days 1 and 2, when drilling and blasting were carried out, the exposure measurements showed high median levels of respirable dust (15.5 mg/m3), respirable crystalline silica (2.4 mg/m3) and ‘total’ dust (28.4 mg/m3) compared to the respective TLVs (Table 1). The highest value for crystalline silica was 3.4 mg/m3. The median exposure to RCD was also relatively high (1.5 mg/m3).
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On day 3, when only shovelling and loading of sacks took place, the median level of respirable dust (4.3 mg/m3), was significantly lower than on days 1 and 2 (P = 0.002; Mann–Whitney non-parametric test) (Table 1). Table 1 also shows higher levels of crystalline silica, RCD and total dust on days 1 and 2 than on day 3, but these differences were not tested due to the low number of samples.
When the data for the 3 days of sampling were pooled together, the overall median respirable dust level was 10.6 mg/m3, and the median level of respirable crystalline silica was 1.4 mg/m3 (Table 2). The median crystalline silica (n = 9) and RCD contents (n = 6) of the selected respirable dust samples were 14.2% (interquartile range 10.5–16.4%) and 5.5% (interquartile range 2.1–20.8%), respectively.
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The elements with the highest median content in the ‘total’ dust samples (n = 5) were calcium (7.4%, interquartile range 4.7–9.2%), aluminium (3.9%, interquartile range 2.7–4.7%) and iron (1.6% interquartile range 0.7–2.2%). The median exposure to calcium was 1.1 mg/m3, corresponding to 1.5 mg/m3 of calcium oxide, i.e. ~77% of the TLV (Table 2). The median exposures to the other elements were <1.0 mg/m3, and <10% of their respective TLVs (Table 2). The exposures to arsenic, silver, cadmium and lead were <1.0 µg/m3 (not shown in table).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The measurements showed very high dust levels during underground, small-scale mining, particularly on the two sampling days when drilling and blasting operations were carried out. On those days, the exposure to respirable crystalline silica was about 50 times higher than the TLV guidelines. The median content of crystalline silica in the respirable dust samples was 14.2%. As expected, the respirable dust levels were considerably lower when only shovelling of blasted rocks and loading of sacks took place, but the respirable crystalline silica concentration was still very high compared to the TLV.
Due to the lack of respiratory protective devices, the reported levels of dust should represent the actual personal exposures on the days of measurement, and the miners considered their activities on the days of measurements to be representative of normal working days. However, the results from the limited number of samples in this pilot study should only be taken as an indication of the average personal exposure levels of the miners.
Many factors probably contribute to the heavy dust exposures, such as drilling and blasting without any water for dust suppression, insufficient ventilation systems and blasting when the miners were evacuated to a relatively short distance away. Thus, the concentration of post-explosion dust was presumably still very high when the miners re-entered the blasting site.
Occupational exposure to crystalline silica is probably one of the most documented workplace exposures. Published studies are mainly restricted to investigations done in large and medium-sized mining companies where the working conditions have generally improved during recent decades. In South African gold mines in Witwatersrand, the level of free crystalline silica was ~50 mg/m3 in 1905, decreasing to ~2 mg/m3 by 1930 and to 0.2–0.5 mg/m3 by 1938 following improvements in ventilation and the use of water to control dust (Sluis-Cremer, 1986). In surveys taken during 1965–67, the levels of crystalline silica in these gold mines ranged from 0.05 to 0.84 mg/m3 (Beadle and Bradley, 1970). The content of free silica in the respirable dust from these gold mines was ~30% (Sluis-Cremer, 1986; Hnizdo and Sluis-Cremer, 1991). In 20 Chinese mines, the mean levels of respirable silica have decreased from 4.89 to 0.39 from 1950–59 to 1980–89 after preventive measures were introduced (Dosemeci et al., 1995). From 1988 to 1992, the mean respirable quartz level in underground mining in the USA was <0.09 mg/m3 (Watts and Parker, 1995). The quartz content of these samples varied from 3.4 to 13.3%.
The present study confirms the assumption that respirable dust and crystalline silica exposure during small-scale mining might be considerably higher than found in published studies from mining companies in South Africa, China and USA. The Mererani underground mining area for small miners covers only about 2–3 km2. Although the crystalline silica content of the dust presumably varies with the exact location and depth of the mine, the present results indicate that the approximate number of 15 000 underground small-scale miners in this area are highly exposed to crystalline silica. According to the International Labour Organization (ILO, 1999), the working conditions in the Mererani mines are typical of small-scale mining in many developing countries. Sakari and Muchiri (1997) concluded that the prevalence of silicosis in many African countries is likely to be high, and called for further surveys. However, we could not find any previous studies on crystalline silica exposure in small-scale mining in Africa, and the prevalence of silicosis in this type of mining has not been studied in developing countries.
The incidence of silicosis of severity 1/1 or greater show lifetime risks of 55–92%, assuming 45 yr of continuous exposure to silica at 0.10 mg/m3, which corresponds to a cumulative exposure of 4.5 mg/m3-yr (Hnizdo and Sluis-Cremer, 1993; Steenland and Brown, 1995; Kreiss and Zhen, 1996; Chen et al., 2001). In those studies, the mean time from the first exposure until the onset of silicosis was 18–41 yr. In our study, the mean number of years of work in the underground mines was 4.9 yr for the 21 workers. The median exposure to crystalline silica for the three sampling days was 1.4 mg/m3, resulting in a cumulative exposure of 6.9 mg/m3-yr. As no records were kept for information about the workers, the calculation of the mean years of exposure for a larger group of miners was not possible. However, our limited amount of information strongly indicates that the risk of acquiring silicosis is very high in this working environment. Furthermore, the presence of silicosis is also reported to increase the risk of developing pulmonary tuberculosis, which is associated with the HIV epidemic in this part of the world (Cowie, 1994). Results from the exposure measurements also indicate that concurrent exposure to respirable combustible dust, which probably consists of carbon-containing material from the graphitic gneiss, might aggravate the respiratory effects caused by crystalline silica. On the other hand, none of the elements identified on the total dust samples was present in concentrations indicating any health risk.
In summary, this pilot study shows that exposure to respirable crystalline silica was high during underground small-scale mining. Consequently, the 15 000 miners in this limited area are presumably at high risk of developing chronic silicosis. Improved ventilation and provision of respiratory protection devices would have reduced the health risk of the workers. However, the International Labour Organization concludes that the underlying poverty aspects of small-scale mining in developing countries makes sustainable technical improvements difficult, unless attention is being paid to the economic, social and labour conditions in the society (ILO, 1999).
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FOOTNOTES |
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* Author to whom correspondence should be addressed. Tel: +47-55-58-60-73; fax: +47-55-58-61-05; e-mail: magne.bratveit{at}isf.uib.no
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