Document J3ojnpdGNqZx2dxoGjq2yKQjr

Inaccuracy of area sampling for measuring the dust exposure of mining machine operators in coal mines Introduction This study examines the accu FJ. KISSELL AND H.K, SACKS can be made to run continuously over long periods, rather than for racy of area sampling for measuring the dust exposure of mining ma chine operators at coal mine work F.N. Kissell and H.K. Sacks are bath seniat scientists with the Pittsburgh Research laboratory. National Institute for Occupational Safety and Health. Pittsburgh, PA. Nonmeeting paper number DO- just an eight-hour shift. Area sam pling also relieves workers from the nuisance of having to wear a sam ing faces. Area sampling refers to general air monitoring for measur 331. Manuscript submitted for review October 2000 and accepted for publication May 2001. Discussion of this peer-reviewed and pling device for the entire work shift.Thus, depending on the level of ing employee exposures, typically at some fixed location near the worker. approved paper is invited and must be submitted to SME Publica tions Dept. prior to May 31, 2002. inaccuracy introduced, it is possible to imagine circumstances where The work was prompted by the area sampling provides better re development of a prospective new sults overall. type of dust sampling instrument called the machine- Inaccuracy resulting from use of the MMCRDM has mounted continuous respirable dust monitor (MMC- two sources. One is the MMCRDM itself (Kissell and RDM). However, the MMCRDM is more than a new Thimons, 2001).' The other is the potentially more sig sampling instrument. It also changes the location where nificant source of inaccuracy associated with the switch samples are collected. The current method of working- to area sampling. This study deals only with the inaccu face compliance sampling for coal mine dust uses "per racy resulting from the switch to area sampling. For this sonal sampling" equipment worn by workers. The purpose, the authors compared breathing zone samples MMCRDM, housed in a 73-kg (160-lb) box. must be to area samples using "personal samplers" in both loca mounted in a fixed location.Thus.it can only be used for tions. The effort had two parts:The first part was a litera area sampling. ture survey that extracted data from previous area Modern industrial hygiene practice has been to sampling studies. The second part was the authors own avoid area sampling and to sample airborne contami study on area sampling at one continuous miner face and nants using "personal sampling" equipment worn by at one longwall face. workers. It is well known that personal samples provide more accurate results than area samples when the con Literature survey on area sampling taminant source is nearby. Near contaminant sources, the The authors found 12 studies that are relevant to dilution air and the contaminants are not evenly mixed. area sampling of dust in coal mine working faces: Therefore, exposure measurements must be taken from Listak et al. (1999) conducted the most recent study the worker's breathing zone to be accurate (Leidel et al.. on area sampling, comparing fixed-location area samples 1977). However, area sampling equipment can have more functionality because of the relaxed size and weight restrictions on the instrumentation. For example, an area-sampling instrument can be more sensitive and 'The results of MMCRDM measurements and the performance of the MMCRDM itself are being reported elsewhere. Abstract This study examines the accuracy of area sampling for measuring the dust exposure of mining machine op erators in coal mines. The specific objective of this re search was to find locations where an area sampler might work better than earlier studies have indicated. The results show thatfixed-location area sampling can not accurately predict the dust exposure of a machine operator, even when the bestfixed location is sought, the fixed location is quite close to the operator and the bias due to the dust concentration gradient is corrected. In dustrial hygienists have known for many years that area sampling is unsuitable for measuring air contaminant exposures in the workplace. Near contaminant sources, the dilution air and the contaminants are not evenly mixed. Therefore, when workers are near contaminant sources, exposure measurements must be takenfrom the worker's breathing zone to be accurate. MINING ENGINEERING FEBRUARY 2002 33 f----------- ,L5. 111 mm in m 1rmi i ii i t i ft 11 1 rx____ L4 . FIGURE 1 Miner-bolter and loader showing operator and fixed-location sites. Key (El) Operator site L3 Fixed location site to operator breathing zone samples. Data at deep cut trations at three locations (headgate, support #50 and sections in five mines where remote operators controlled tailgate) along the face. the mining machines were gathered. Listak et al. con Kissell and Jankowski (1993) summarized several cluded that there was little predictive capability between studies conducted by Foster-Miller to reduce continuous the two locations. If the fixed-point dust level was 1.5 mg/ miner dust. During this work, Foster-Miller measured m3, then the 95% confidence level predicted operator dust levels at the boom hinge point and in the operator breathing zone exposure could vary from zero to 2.6 mg/ cab. The mean concentration ratio (hinge point: operator m3. cab) was 4.15, and the standard deviation was 1.85. In a study of nine longwalls, Sun et al. (1997) found Babbitt et al. (1990) obtained dust concentration no relationship between the dust concentration at the profiles around a longwall shearer during mining. The shearer operator and the concentration at the tailgate. profiles showed a strong gradient across the shearer. The However, Sun et al. also described another study con machine had a well-designed shearer-clearer system that ducted at one longwall in Australia. The Australian study functioned to hold the dust cloud against the face as it found a correlation between the shearer operator dust moved downwind from the shearer. When shield movers concentration and the average of the observed concen- worked within 15 m (50 ft) of the return side of the shearer, their dust levels were the TABLE 1 same or less than those measured at the shearer. Results of tests conducted by Divers et al. (1982). Kelly et al. (1990) measured tracer gas gradients at a full-scale model longwall shearer. Methane Respirable dust gas was released at the drums and concentration, mg/m3 the concentration at various loca- Shift Cab operator Remote Ratio cabzremote tions around the shearer was mea sured. One location was at the 1 4.34 0.14 31 headgate-side operator position and 2 3.33 3 1.95 0.09 0.08 37 another was the zone downwind of 24 the headgate-side drum on the face side of the shearer. Both the gradi- ent and the variability in the gradi TABLE 2 ent were high. The average ratio (face concentration/headgate opera Results of the five published studies that provide enough tor concentration) in 32 tests was 66, information to calculate a mean concentration ratio. with a standard deviation of 56. In addition, Kelly et al. obtained a dust Published study No. of mines Listak et at., 1999 5 Kissell and Jankowski, 1993 '"5 " Kelly et at. 1990 Lab test Divers et ai. 1982 1 Kost and Saltsman, 1977 ."8" ' Mean ratio F/O 3.07 4.15 66 30.7 3.53 RSD 0.59 0.45 0.85 0.21 0.81 profile map of the headgate area during mining. The dust level varied from less than 0.5 mg/m3 to more than 1.5 mg/m3 across the entry. Jayaraman et al. (1987) studied methods to reduce the dust level of continuous miner operators who use radio remote control to operate ma- 34 FEBRUARY 2002 MINING ENGINEERING FIGURE 2 Longwall op erator and fixed-location sites. chines equipped with dust scrubbers. It was found that, while these scrubbers remove a large portion of the dust, correct positioning of the operator can realize additional gains. When the machine operator positioned himself directly in front of the blowing curtain, his dust exposure level was 0.2 mg/m23. 4When the operator moved away from the curtain and positioned himself next to the miner, his dust exposure level was 3.1 mg/m3. Peng and Chiang (1986) took dust measurements near the shearer during longwall mining. Depending on the cut direction and the operator position measured, the concentration over the shearer ranged from 18% to 71 % above the operator concentration. Peng and Chiang also obtained other data about the dust gradient along the face. In the study, the dust concentration in the walkway 9 m (30 ft) downwind of the shearer varied from being similar to 12 times higher when compared to the walk way at the shearer. Changes in the shearer water sprays caused this large variation. Grayson and Peng (1984) conducted a regression analysis on the dust data from a longwall panel to predict the concentration at specific locations. It was found that by using "location on face" as a single independent vari able (Cl), dust level Cm in mg/m3 could be predicted as Cm = -0.2835 + 0.8106 VI, where the correlation coeffi cient r was equal to 0.8054, and r2 equaled 0.65. The fit ted model explained 65% of the total variation. However, to improve the predictive capability, it was necessary to incorporate other variables such as condi tion of roof, method of cutting, cutting time and air quan tity. In a study of six mines, Jankowski and Organiscak (1983) found that dust concentrations in the walkway 9 m (30 ft) on the return side of the shearer varied from being similar to three times higher when compared to the walkway at the shearer. Jankowski and Babbitt (1986) got similar results in a laboratory study using tracer gas, finding that the gas concentration downwind of the shearer varied from being similar to ten times higher than the walkway concentration at the shearer. Divers et al. (1982) conducted a three-shift dust study in a mine that used remote control to guide the miner. The mine also used a push-pull ventilation system with both blowing and exhaust curtains in the working place. The remote control operator positioned himself 3 to 6 m (10 to 20 ft) behind the miner. Respirable dust samples were taken at the cab and at the remote control operator location. The results are shown in Table 1. Rankin and Rodgers (1980) conducted dust concen tration surveys at locations away from the working face in intake air. Dust concentration correlations between area sampling locations and personal sampling of the section boss and shuttle-car operator were found. How- 2 The concentration ratio is analogous to the bias as described by Kennedy et al. (1995). Bias is the relative discrepancy between the mean of a distribution of measurements and the true concentration. In this paper, the conclusions are based on the assumption that all bias will be corrected. Fail ure to correct bias leads to greater errors than described here. 3 In dust sampling, the failure to correct area sampling bias invites "stra tegic sampling"-- that is, placement of dust samplers to achieve readings con sistently higher or lower than the true exposure of the worker. 4 It was assumed that the data cited in the literature survey are normally distributed. In the studies at the Federal #2 and Baker mines, Battelle con cluded that the data from these mines was normally distributed. TABLE 3 Federal #2 Mine results for the fixed-location sites and the personal sample in the cab. Concentration ratio between: Shown as Fixed loc. cab/pers. sample cab Fixed loc. right bolter/pers. sample cab Fixed loc. left bolter/pers. sample cab Fixed loc. right loader/pers. sample cab Fixed loc. left loader/pers. sample cab LI /El L2/E1 L3/E1 L4/E1 L5/E1 Mean ratio F/O 0.865 0.726 1.22 0.488 0.429 RSD 0.27 0.27 0.46 0.406 0.438 MINING ENGINEERING FEBRUARY 2002 35 TABLE 4 Federal #2 Mine results for the fixed-location sites and the remote operator location. Concentration ratio between: Fixed loc. cab/pers. sample remote op. Fixed loc. right bolter/pers. sample remote op. Fixed loc. left bolter/pers. sample remote op. Fixed loc. right loader/pers. sample remote op. Fixed loc. left loader/pers. sample remote op. Shown as L1/E5 L2/E5 L3/E5 L4/E5 L5/E5 Mean ratio F/O 2.49 2.07 3.32 1.33 1.17 RSD 0.285 0.299 0.304 0.243 0.325 ever, Rankin and Rogers cautioned not to expect good correlations near working-face dust sources, because shift-to-shift coefficient of variation values there varied from 25% to 60%. Kost and Saltsman (1977) measured dust concentra tions near continuous miner operators at eight sections in six different mines. It was concluded that the dust pro files showed a zone of high concentration near the face, separated from a zone of clean air. The boundary be tween these two zones was near the operator, which made the location of the sampler critical. Kost and Saltsman saw a large variation in the dust gradient from mine to mine, concluding that no single correction factor applies for all mines. Kost and Saltsman also saw a large variation in the concentration gradient from shift to shift in the same section, concluding that even a single-section correction factor is impractical. Establishing an accuracy criterion The issue at hand is whether a measurement made at a "fixed location" (an area sample) will effectively show how much dust a worker, typically the operator of a min ing machine, is breathing. Because the dust source is nearby, the average concentration at the two locations will obviously be different. In other words, there will be a gradient that can be represented by a "concentration ratio." If the variability in this ratio is small, then a fixedlocation measurement can be used to show worker expo sure when the concentration ratio is applied as a correction factor.21 For the literature survey results to be useful, one must convert those results to a common denominator and then compare them to well-established measures of accuracy. Fortunately, five of the studies gave enough in formation to enable the authors to calculate a concentra tion ratio between the fixed location and the mining machine operator for each shift.These concentration ra- 5 Assuming that all bias is corrected. 6 2s is the value corresponding to 95.4% of the measurements. 7A more complete statistical analysis of the area sampling studies at the Federal #2 and Baker mines was undertaken by Battelle Inc. under contract to NIOSH. Copies of this Battelle report are available from the authors. 8 In this mine, the air on the longwall face moved from the tailgate to the headgate, and so the headgate-end shearer operator was the designated sampling position. tios were then averaged over all shifts in the test, and the standard deviation calculated. The relative standard de viation (RSD) was calculated from the standard devia tion and the mean concentration ratio. Then, from the RSD in this concentration ratio, the authors could deter mine whether the accuracy criteria had been met. Two accuracy criteria. 25% and 50%, were used. These are analogous to the NIOSH instrumentation ac curacy criterion (Kennedy et al,, 1995) and the European Community standard for "screening measurements" (CEN, 1994). For normally distributed data4, 95% of the measurements fall within the range 1.96s, where s is the standard deviation. For example, assuming that the mean is 100, for the criterion of 25%, then 1.96s = 25 or s = 12.7. Because the mean is 100. the standard deviation di vided by the mean (called the relative standard deviation or RSD) is 0.127. Thus, the 25% accuracy criterion is met at RSD = 0.127 or less3. For the criterion6 of 50%, 2s = 50 or s = 25. Hence, the RSD is 0.25, and the 50% criterion is met at RSD - 0.25 or less. Results from the literature survey The five published studies that provided enough in formation to calculate a mean concentration ratio be tween the fixed location and the mining machine operator (F/O) and to calculate a relative standard de viation for the concentration ratios are shown in Table 2. These values for RSD fail to meet the 25% criterion, for which the RSD is 0.127. and, with only one exception (Divers et al.), they also fail to meet the 50% criterion, for which the RSD is 0.25. The average shift-weighted RSD for all the mines studied (omitting the lab test) was 0.58. Comprehensive study on area sampling in two mines The results from the literature survey showed that area sampling resulted in poor accuracy -- that is, high RSD values. Therefore, between January and March 1999. the authors conducted a comprehensive area sam pling study in two mines to determine if they could find locations where area sampling might yield better results. For this two-mine study, the authors used the per sonal samplers normally employed for coal mine compli ance sampling -- that is. a 10-mm Dorr-Oliver cyclone, a 36 FEBRUARY 2002 MINING ENGINEERING 37-mm filter and a Mine Safety Ap TABLE 5 pliances Co. Elf-Escort flow-con trolled pump operated at 2 L/min. Baker Mine results. Pump calibration was checked be fore each sampling shift using a Gillibrator calibrator and a filter load. The fixed location sample (the surrogate for the MMCRDM) was a package of three samplers, with the average concentration of the three designated as the fixed-location measurement. For the personal samples, one sampler was used with the cyclone attached to the lapel of the worker in the conventional man ner. MSE1A pre- and postweighed the filters to a precision of 11 pg in its automated sample-weighing fa cility (Kogut et al., 1997). One filter blank was established for each shift Concentration ratio between: Fixed loc.shield #42/pers.sample HGSO Fixed loc.shield #63/pers.sample HGSO Fixed loc.shield #84/pers.sample HGSO Fixed loc.shearer headgate end/pers. sample HGSO Fixed loc.shearer tailgate end/pers. sample HGSO Fixed !oc.#1 stage loader/pers.sample HGSO Fixed loc.#2 stage loader/pers.sample HGSO Shown as Mean RSD ratio F/O L42/ET 0.798 0.380 L63/E1 0.713 0.450 L84/E1 0.490 0.302 L1/E1 1.138 0.503 L2/E1 1.015 0.591 L3/E1 1.564 0.423 L4/E1 1.574 0.378 of samples, and filter post-weights were corrected for weight changes in the blank. TABLE 6 The data were analyzed using Kost and Saltsman study results. the same "concentration ratio" ap proach employed in the literature survey. The authors adopted7 this approach to allow a direct compari son to the literature survey results. Mine C C Fixed location (F) right rear post left rear post No. of shifts 12 12 Mean ratio (F/O) 0.666 0.806 RSD 0.39 0.33 Federal #2 Mine. The authors conducted the Federal #2 study on a miner-bolter section. Dust samples were collected for 11 shifts at five D E E F rear cab left rear post right rear post rear cab 10 8 8 8 0.690 0.934 1.07 0.956 0.38 0.60 0.58 0.28 worker exposure sites and five pos sible fixed-location sites for the MMCRDM. Concentration ratios were calculated be lation between the personal sample on the headgate-side tween each of the five fixed-location sites, and the two shearer operator and the fixed location on the headgate most prominent operator personal-exposure sites as side of the shearer (Ll/El), with an RSD value of 0.503. shown in Fig. 1. These operator exposure sites were in The results from the Federal #2 and Baker mines the miner cab and at the location where the operator were unfavorable, with an overall average RSD of 0.37. would stand if the machine were being operated re motely. Table 3 shows the results for the fixed-location sites Subset of data from samplers located within 760 mm (30 in.) of worker and the personal sample in the cab. Table 4 shows the The literature survey and the two-mine study results for the fixed-location sites and the personal yielded unfavorable results. Therefore, the authors re sample at the remote operator location. All of the RSD analyzed all of the data in the belief that samplers that values fail to meet the +25% criterion, and all but one were closer to each other might provide better correla fail the 50% criterion. tion. The authors focused on a subset of two cases in which the fixed location was within 760 mm (30 in.) of Baker Mine.The authors conducted the Baker Mine the machine operator. The literature survey and the two- study on a longwall section. Eleven shifts of dust samples mine study each provided one case. were collected at six worker exposure sites and at seven possible fixed-location sites for the MMCRDM. Concen Kost and Saltsman study Kost and Saltsman (1977) tration ratios were calculated between the most promi conducted a dust gradient study in six mines to assess the nent worker personal-exposure site, the headgate-end impact of moving the sampler away from the continuous shearer operator8 (HGSO) and each of the seven fixed- miner operator. The dust concentration measured at the location sites, as shown in Fig. 2. operator's lapel and the dust concentration measured These fixed-location sites were at shield #42. shield elsewhere on the mining machine were compared. In #63, shield #84. on the shearer at the headgate end. on four of the mines, Kost and Saltzman placed samplers on the shearer at the tailgate end and at two places on the the rear post of the canopy that covered the operator stage loader. Results are shown in Table 5. All of the cab. These were the samplers closest to the operator, and RSD values fail to meet both the 25% and the 50% they were never more than 610 mm (24 in.) from the criteria. The most surprising finding is the lack of corre sampler on the operator's lapel. TABLE 7 Federal #2 study results. Concentration ratio between: Fixed loc.cab/pers. sample cab Fixed loc.right bolter/pers. sample right bolter Fixed loc.left bolter/pers. ' sample left bolter Shown as LI /El L2/E2 L3/E3 Mean ratio (F/O) 0.865 0.859 1.00 RSD 0.27 0.32 0.39 Distance 18 in. 30 in. 30 in. Table 6 gives the mean concentration ratio for each mine and the std.dev./mean or relative standard devia tion (RSD). Every value fails both the 25% and the 50% criteria. This is a surprising result considering that the samplers were within a few feet of each other. Federal #2 study. In the Federal #2 study, three data pairs represented cases where the operator sampler and the fixed-location samplers were very close to each other. These were as follows: the fixed-location measure ment at the miner cab vs. a personal sample on the con tinuous miner operator who sits in the cab (Ll/El): the fixed location at the right bolter vs. the personal sample at the right bolter (L2/E2); and the fixed location at the left bolter vs. the personal sample at the left bolter (L3/ E3). The mean ratios and RSDs for these cases are shown in Table 7. These values also fail both the 25% and the 50% criteria. The Federal #2 samplers were very close to each other. The fixed-location samplers at the cab (LI) were only 460 mm (18 in.) from the personal sampler worn by the operator in the cab (El). In addition, the fixed-loca tion samplers at each bolter (L2 and L3) were only 760 mm (30 in.) from the corresponding personal sampler (E2 and E3). Yet the RSDs for ratios Ll/El, L2/E2 and L3/E3 were 0.27,0.32 and 0.39, respectively. The average RSD for this two-mine subset was 0.39. These results show a surprisingly wide variation in dust levels between samplers located within a few feet of each other. Variance from the samplers The high RSD values for the fixed-location/personal sampler ratios warranted an analysis of the samplers themselves. In the two-mine study, each fixed location value was the average of a three- sampler package in which the cyclone inlets were only 75 to 125 mm (3 to 5 in.) apart. In the Federal #2 Mine study, 11 shifts of dust samples were taken at five fixed-location sites, for 55 val ues. In the Baker Mine study, 11 shifts of dust samples were taken at seven fixed-location sites, for 77 values. The mean sampler-to-sampler RSD for the 132 fixed-lo cation values in both mines was 0.12. An RSD value of 0.12 for samplers only 75- to 125-mm (3- to 5-in.) apart accounts for part of the poor correlation observed for samplers separated by greater distances. Conclusions The results of this work show that fixed-location area samples cannot predict the shift dust exposure of a machine operator, even if the best fixed location is sought, the fixed location is quite close to the operator and the bias due to the concentration gradient is cor rected. The average RSD from the literature survey was 0.58. From the two-mine comprehensive study, the value was 0.37, and, from the subset of area samplers within 760 mm (30 in.) of the worker, the value was 0.39. A part of this variability is due to the samplers themselves, which had an RSD of 0.12. Acknowledgments The authors gratefully acknowledge the important contribution of those who collected the data for this analysis: NIOSH employees Bruce Cantrell, Calvin Garbowsky, Gregory Green, Thomas Mai, Shawn Vanderslice and Jeanne Zimmer; MSHA employees Karl Ahlen, David Atchison. Ken Fields, Everett Gerbec and Brian Murphy. References Babbitt. C,, Jankowski. R,, Burnett. J.. and Rajan. 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NTIS # PB81-236762 Sun, G., Ramani, R.V., Grygiel, E.W., and McNider, T.E., 1997, "Relationship between area concentrations and personal exposures along longwall faces." Applied Occupational and Environmental Hygiene.Vol. 12.No. 12.pp. 991-998.