Annals of Occupational Hygiene Advance Access originally published online on July 2, 2007
Annals of Occupational Hygiene 2007 51(6):509-516; doi:10.1093/annhyg/mem027
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Exposure to Refractory Ceramic Fibres in the Metal Industry
1 Finnish Institute of Occupational Health, P.O. Box 93, FI-70701 Kuopio, Finland
2 Finnish Institute of Occupational Health, Topeliuksenkatu 41 a A, FI-00250 Helsinki, Finland
3 University of Kuopio, P.O. Box 1627, FI-70211 Kuopio, Finland
4 Ministry of Social Affairs and Health, P.O. Box 33, FI-00023 Government, Finland
* Author to whom correspondence should be addressed. Tel: +358 30 474 7232; fax: +358 30 474 7474; e-mail: markku.linnainmaa{at}ttl.fi
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSION AND CONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Refractory ceramic fibres (RCF) are used in thermal isolation in the metal industry where high temperatures are regularly employed. Asbestos materials were earlier commonly used for these purposes. In this work, two Finnish steel plants, three foundries and a repair shop were studied for the ceramic fibre exposure of their workers under normal production and during the replacement of oven insulation. Personal and stationary sampling was used together with a novel nasal lavage sampling for the evaluation of personal exposure. Fibres were counted with optical and electron microscopy and they were identified using an energy-dispersive X-ray analyser. Ceramic fibres were found in most production phases [range <0.01–0.29 fibres per cubic centimetre (f cm–3)]. Considerably higher fibre counts were obtained during the maintenance work (range <0.01–14.2 f cm–3). Nasal sampling was found to correlate with the airborne fibre concentrations at the group level. The mean fibre concentrations varied from 34 to 6680 f ml–1 of lavage liquid. Use of personal respiratory protectors diminished the exposure on the average as analysed in the lavage specimens, but the effect of respirator use did not appear clearly in the results. Because of the heat conditions, the workers used the respirators for a strict minimum period. A considerable exposure to RCF occurs in the studied plants. Its risk should be evaluated and managed more closely in view that the material is carcinogenic. Use of personal respiratory protectors should be encouraged. Their effective use could be verified by the nasal sampling for fibres after the work shift.
Keywords: exposure • foundries • nasal sampling • refractory ceramic fibres • respirators • steel plants
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSION AND CONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Refractory ceramic fibres (RCF) are used in thermal insulation applications where temperature exceeds 350°C. Asbestos materials were earlier commonly used for these purposes, although their temperature resistance is not as good as that of ceramic fibres. RCF are aluminium silicates containing mainly alumina and silicon dioxide and small amounts of other oxides. These can be used at temperatures close to 1500°C. Ceramic fibres are thin and only slowly dissolve in the lungs. RCF have induced tumours in experimental animals exposed by inhalation. Therefore, the International Agency for Research on Cancer (IARC, 1988, 2002) and the European Union (Commission Directive 97/69/EC that is the 23rd amendment of Council Directive 67/548/EEC) have classified RCF as possibly carcinogenic to humans. This view is supported by the findings that the risk of urinary organ cancers may be increased in workers occupationally exposed to RCF (LeMasters et al., 2003). In addition, formation of cristobalite is possible at temperatures >1100°C (Gantner, 1986). Cristobalite is a well-known inducer of pneumoconiosis and cancer (IARC, 1997). The evidence which the IARC classification decisions are based on has been discussed in detail by Baan and Grosse (2004). A comprehensive document on occupational exposure to RCF and its potential health risks has been published recently by the National Institute for Occupational Safety and Health in the US (NIOSH, 2006).
NIOSH (2006) proposes a recommended exposure limit (REL) for RCF of 0.5 fibres per cubic centimetre (f cm–3). In addition, an action level of 0.25 f cm–3 is presented to help employers determine when exposures are approaching the REL. The threshold limit value of the American Conference of Governmental Industrial Hygienists (ACGIH, 2006) is 0.2 f cm–3. In Europe, separate occupational exposure limits (OELs) have usually not been given for RCF but the same OELs are used for all man-made mineral fibres (MMMF). This OEL is 0.5 f cm–3 in Germany, while the Netherlands, Sweden and the UK have OELs of 1 f cm–3 (NIOSH, 2006). In Finland, new OELs for MMMF and RCF are 1 and 0.2 f cm–3, respectively (STM, 2007). The European OELs are based on optical microscopy and the WHO (1997) definition of a fibre (length >5 µm, diameter <3 µm, aspect ratio 
3:1). Slightly different fibre definitions are used in the US (NIOSH, 2006).
It is possible that the shorter fibres (<2 µm) are ingested by pulmonary macrophages and thereby transported to other organs and eventually removed from the body, e.g. in the urine (Savolainen et al., 1996). In fact, ceramic fibres were identified and quantitated in urine samples of workers engaged in the manufacturing of friction materials (Savolainen et al., 1996). The average fibre length in the urine was 0.97 µm. Their occurrence in the urine of the exposed workers could be associated with the detected risk of the urothelial cancers.
Alkaline earth silicate (AES), also called calcium–magnesium silicate (CMS), fibres are more soluble in vivo. AES fibres have been exonerated from the European carcinogen classification because of their low biopersistence. According to the Commission Directive 97/69/EC, fibres containing alkali and alkali earth metal oxides >18% are not considered to be carcinogenic. Therefore, they are increasingly used as substitutes for RCF, especially in Europe. The directive is based on experimental data indicating that biopersistence is a good indicator of fibre pathogenicity (Bernstein et al., 2001). AES fibres do not tolerate as high temperatures as RCF. However, the difference between the maximum-use temperatures is decreasing due to extensive AES product development fostered by the carcinogen classification. It should be noted that AES fibres have not been evaluated by IARC or by US authorities. RCF and CMS materials have been found to produce very similar exposure levels during manufacture, use and in laboratory tests (Class et al., 2001).
Only a few reports on occupational exposures to dust and fibres in the metal industry have been published in English. These reports are rather old and RCF have not been identified in them. Total dust concentrations were 3.7–15.5 mg m–3 and the concentrations of fibres thinner than 3 µm were 0.39–5.2 f cm–3 in measurements done during mounting of thermal insulation on ovens in the UK (Head and Wagg, 1980). The exposures to total and fine dust were 0.5–35.8 mg m–3 and 0.12–16.9 mg m–3, respectively, during removal of oven insulations in the US (Gantner, 1986). The airborne dust contained 4–15% cristobalite. A somewhat newer report has been published in Swedish. In this study (Krantz et al., 1994), very high concentrations of dust (up to 600 mg m–3) and fibres (up to 200 f cm–3) were detected in the ovens during removal of the insulation. The fibre concentrations were 2 f cm–3 and 1–5 f cm–3 during removal and mounting of the insulation of oven doors. This report has also been summarized in the new NIOSH document (2006).
In Finland, where workers occupationally exposed to carcinogenic agents must be registered (Act 717/2001), the pressure to replace carcinogenic products is especially high. The substitution is also technically possible for most applications of RCF. However, there is still a need to use RCF in metal industry due to the high temperatures encountered in many operations, especially in foundries and steel plants. There were 93 workers registered because of exposure to RCF in 2003. The aim of the present study was to elucidate the use of RCF and the dangerous tasks caused by it in the Finnish metal industry. The effectiveness of the exposure control methods and the spreading of fibres to working environment were also examined. In addition, the usefulness of the nasal lavation method in exposure assessment was investigated.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSION AND CONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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The plants studied included two steel plants (A and B) and three foundries (an iron, a steel and a copper foundry). In addition, the repair of the insulation of rollers used in the cold-rolling stand of Plant B was investigated in a small service company. In steel plants, measurements were done both during the normal production and in repair operations during a service break. Only normal production was studied in the foundries. Normal production sometimes contained short-term tasks including handling of ceramic fibre materials. All the repairmen were employed by subcontractors. Fibre samples were collected at the breathing zones of 55 workers (29 production workers and 9 repair workers in the steel plants, 1 worker in the repair shop, 3 workers in the iron foundry, 8 workers in the steel foundry and 5 workers in the copper foundry). Samples were also collected at fixed locations to study the spreading of the fibres. Concentrations of inhalable dust (based on the Comité Européen de Normalisation–American Conference of Government Industrial Hygienists–International Organization for Standardization definition) were also determined. The measurements were conducted in 2004–2005.
Air samples for the optical microscope (OM) counting of fibres were collected onto cellulose acetate filters (Millipore AAWP, diameter 37 mm, pore size 0.8 µm) and samples for electron microscope (EM) investigation onto polycarbonate filters (Nuclepore, diameter 37 mm, pore size 0.2 µm). The sampling rate was usually 2 l min–1 (SKC 224 pump), although in some stationary sites 10 l min–1 (Reciprotor 406 G pump) was also used. The sampling volume was 100–700 l. The inhalable dust samples were collected with IOM samplers onto cellulose acetate filters (Millipore, diameter 25 mm, pore size 0.8 µm).
The samples for optical microscopy were made transparent with a 1:1 mixture of dimethyl phthalate and diethyl oxalate. The fibres were counted with a phase-contrast microscope (Leitz Laborlux Pol 12) with x400 magnification. Walton–Beckett graticule with a diameter of 100 µm was used to define the area of the field of view. The number of fibres counted was at least 100. No more than 100 graticule areas were inspected.
The polycarbonate filters were taken into an ultrasound bath for 1–2 s in distilled water–ethanol mixture. Because fibres were partly attached on the internal walls of the polycarbonate filter cassettes during sampling, the interiors of the cassettes were washed by the distilled water–ethanol mixture. Then these two water–ethanol mixtures were combined and filtered through polycarbonate filters (Millipore ATTP, diameter 37 mm, pore size 0.8 µm). The filters were then covered with gold (Bal-Tec SCD005 Sputter Coater).
The scanning EM (Jeol JSM-6400) counting was done with x1000 magnification. The RCF results only include fibres with diameters <3 µm. The fibres were identified with an energy-dispersive X-ray analyser (Thermo Noran Quest) based on their relative elemental peak intensities.
In the steel plants, the size distributions of the airborne fibres were also studied by electron microscopy. The fibre dimensions were measured using EM scale bar as a tool. Fibres were selected randomly; fibres in the centre point of different fields were selected. The operator always used temporarily lower magnification with EM to ensure that no fibre was measured twice.
The weighing of inhalable dust samples with a microbalance (Mettler Toledo AT261) was done at constant climatic conditions, and static charge elimination was employed.
In the nasal lavage sampling, each nostril was washed with 4.5 ml of sterile 0.9% saline solution as described by Paananen et al. (2004). The mucus in the lavage samples was dissolved by adding 100 µg of protease enzyme (Proteinase K, Merck & Co.; 10 mg ml–1). The samples were incubated first at 55°C for 5 h and then overnight at room temperature. Then the samples were filtered through a cellulose ester membrane filter (Millipore white-gridded AAWG, diameter 25 mm, pore size 0.8 µm). After drying at 40°C for 1–2 h, the filters were made transparent with acetone vapour (Millipore Acetone Vaporiser). The fibres were counted with a polarized light microscope (Leica DMLP) equipped with phase-contrast optics with x100 to x400 magnification. Light microscopy was used because its detection limit (even 0.1 f ml–1) is remarkably lower and the sample preparation more straightforward than in electron microscopy, where the detection limit was reported to be 375 f ml–1 by Paananen et al. (2004). With the maximum sample volume of 9 ml, the detection limit of the light microscopy method was 0.1 f ml–1, when the whole sample was analysed. Furthermore, most MMMF are large enough to be counted with light microscope. Only fibres longer than 20 µm were counted because the total number of fibres was so large that counting them all would have been difficult and time consuming. It is also worth noting that nearly 80% of the fibres were longer than 20 µm, and therefore it was logical to set the limit to 20 µm.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSION AND CONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Airborne fibres in the steel plants
The levels of MMMF counted by OM were below the new OELs (0.2 and 1 f cm–3) in the personal samples taken in Plant A during the normal production (Table 1). Higher numbers of fibres were detected by EM compared to OM. The EM results show that only part of the airborne MMMF was RCF.
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The fibre concentrations were low at stationary sampling points. The mean concentrations of MMMF and RCF were
0.03 f cm–3, even close to the sources. Also, the maximum concentrations remained low even though the number of samples was rather high (n = 39). The highest concentrations were 0.05 f cm–3 in the OM counts and 0.02 f cm–3 in the EM counts.
The fibre concentrations detected in the personal samples in Plant B (Table 1) were low during the normal operation. Because the RCF concentrations were below the detection limit even during the furnace lid insulation, which was considered to involve the highest exposure, no further EM counting was performed. The MMMF concentrations were 0.02 f cm–3 at the stationary sampling points.
As could be expected, the fibre concentrations were higher during the maintenance break while removing and installing thermal insulation. The mean concentration of RCF clearly exceeded the OEL (0.2 f cm–3) in almost all the breathing zone samples in a furnace of Plant A (Table 2). The highest concentrations were >4 f cm–3 in individual samples. The OEL of 1 f cm–3 for MMMF was also generally exceeded. On the other hand, the samples taken at fixed points did not violate the above-mentioned standards except some samples inside the furnace (Table 3). The fibre concentrations were very low in the samples collected outside the furnace.
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The concentrations of RCF were also clearly above the OELs in removal of furnace insulation in Plant B during the maintenance break (Table 2). The exposure was much lower during changing of sealing and valve repair than during the other operations. Lower fibre concentrations were again detected at the stationary sampling sites (Table 3). The concentrations of RCF were, however, quite high inside the furnace and at the door opening. Some contamination was also found in the surrounding areas, although the concentrations were low.
The size distributions of the fibres were similar in both plants; therefore, the results were combined. There were a lot of long fibres in the air samples taken in the steel plants, 47% of the fibres were >50 µm. The lengths of the shorter fibres were quite evenly distributed between 1 and 50 µm. The diameter distribution was bimodal (Fig. 1) with modes in the size classes of 1.0–1.5 µm and 3.5–4.0 µm.
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Four samples of used insulation with the largest fraction of devitrified fibres observed in the counting with OM were examined with X-ray diffraction. Cristobalite was identified only in one of the samples. Its content was a few percent.
The concentrations of inhalable dust were 0.6 and 5.4 mg m–3 (n = 2) in the personal samples and from 0.5 to 2.3 mg m–3 (n = 4) at stationary sampling sites during casting in Plant A. The concentration of inhalable dust exceeded the Finnish OEL of 10 mg m–3 in two personal samples out of three taken during repair and installation of ladles and turning table sensors during the normal production. The highest concentration was 26.4 mg m–3. On the other hand, the concentrations of inhalable dust were low, 
2 mg m–3, in the samples collected during removal and installation of insulation in the furnace during the maintenance break. Sampling of inhalable dust was not carried out during the normal production in Plant B. The concentration was 7.9 mg m–3 during demolition of a furnace and 0.3 mg m–3 in the smoking area in Plant B during the service break. There was no correlation between the concentrations of fibres and inhalable dust (r = 0.35).
Airborne fibres in the repair shop
The concentration of MMMF exceeded the OEL of 0.2 f cm–3 in all the personal samples in the repair shop, where virtually all the airborne fibres were RCF. The concentration was especially high during emptying the waste container (14 f cm–3). However, the duration of this task is short. The concentration was 1.3 f cm–3 during removal of old insulation from the rollers and 0.35 f cm–3 during grinding of the rollers. The concentrations were 0.21–0.29 f cm–3 (n = 3) in the area samples. However, the concentration of inhalable dust was only 0.8 mg m–3 in the personal sample collected during removal of insulation.
Airborne fibres in the foundries
The concentrations of fibres were measured in the foundries only during the normal production. The results of the personal samples, which are summarized in Table 4 indicate that exposure to fibres was lower in the foundries than in the steel plants and no excessive exposure levels were detected.
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In the foundries, the fibre concentrations were below the detection limit of 0.01 f cm–3 in almost all the area samples (n = 42); even the highest concentration was as low as 0.02 f cm–3.
The concentrations of inhalable dust were also low in the foundries. The concentration varied from <0.1 to 3.8 mg m–3 (n = 13) in the area samples.
Nasal lavage samples
The mean concentrations of fibres varied from 55 to 430 f ml–1 in the nasal lavage samples taken in steel Plant A and from 34 to 930 f ml–1 in Plant B during the normal production (Table 5). Neither the use of respirator nor its type had any clear effect on the concentration. A typical example of a photomicrograph of a nasal lavage sample is shown in Fig. 2.
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The concentrations were clearly higher in the service shop and during the service breaks in the steel plants even though respirators were generally worn (Table 6).
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The concentrations of fibres were generally low (16–314 f ml–1, n = 15) in the nasal lavage samples taken in the foundries. However, two workers gave samples with exceptionally high concentrations (2660 and 10 700 f ml–1). None of the workers wore a respirator.
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DISCUSSION AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSION AND CONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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The exposure to MMMF and RCF usually remained quite low in the steel plants even in repair tasks during their normal production in spite of a few rather high concentrations of inhalable dust detected at the same time. The concentration of inhalable dust had no predictive value for fibre concentrations. The fibre counts obtained with OM were usually clearly lower than those determined with EM. Some reasons for this are the better resolution of EM and different magnifications used for optical and EM, x400 and x1000, respectively. Therefore, very thin fibres were detected only by the EM. Polycarbonate filter cassettes for EM counting were washed with ethanol and distilled water, as mentioned above, so fibres attached on the internal walls of cassettes were also caught. Moreover, the microscope slide preparation for optical microscopy is slightly three dimensional, so that the microscopist must focus constantly, leading to a risk that some thin fibres were not detected. In the EM counts of MMMF, the fraction of RCF varied widely, from <10% to
100%. The steel plants investigated were modern and provided with effective ventilation. The level of general hygiene was also good. This was also seen as very low fibre concentrations in the area samples. Respirators were not systematically worn while performing repair and other tasks involving elevated exposure risk during the normal production. However, this would be desirable because of the carcinogenic potential of RCF. Very high airborne concentrations of RCF were detected during the maintenance break. As expected, removal and installation of furnace linings resulted in high exposures. High airborne concentrations of RCF were also measured during sealing and drilling/cutting operations. The fibres did not spread widely but their concentrations were low at the stationary sampling points outside the working areas. This is probably partly a consequence of the large size of the fibres; about half of them were >50 µm. In addition, the under-pressure in the furnaces effectively prevented the release of the fibres to the actual working areas. Respirators were almost always used during the high-risk tasks.
High concentrations of RCF were found in the personal samples taken in the service shop during the maintenance of the rollers used in the cold-rolling stand of steel Plant B. The machine was provided with local exhaust ventilation and the concentrations of fibres and inhalable dust were quite low in the area samples. Subcontracting is continuously increasing in the large companies. This also includes dangerous tasks as the roller maintenance investigated in this study. In such situations, it would be important that the large companies would see their responsibility and make sure that sufficient precautions have been taken by the subcontractor.
The nasal lavage sampling showed that considerable fibre loads can be accumulated in the upper respiratory tract with possible deleterious effects on health. The mean concentrations varied from 34 to 930 f ml–1 in the nasal lavage samples taken in the steel plants during the normal production and from 230 to 4410 f ml–1 during the maintenance break operations. The concentration was 3770 f ml–1 in the service shop and 16–314 f ml–1 in the foundries (in addition, two exceptionally high concentrations were determined in the foundries). These levels were of the same order of magnitude as those measured earlier in the prefabricated house industry, where nasal lavage sampling was applied for the assessment of personal fibre exposures at low MMMF levels (Paananen et al., 2004). At the group level, the nasal lavage values follow nicely the airborne concentrations of fibres. Both methods indicate that exposures were high during maintenance operations in the steel plants and in the service shop, lower in the steel plants during the normal operation and lowest in the foundries. However, the correlation was not good at the individual level, where the effect of respirator use did not appear clearly in the lavage results. This is probably only partly due to the inherent problems of the nasal lavage sampling; i.e. possibility of fibre accumulation in the nostrils and removal of fibres while blowing the nose during the working day, because the protective effect of respirators could clearly be seen in the prefabricated house industry study. The inconsistency was more likely mainly due to incorrect use and maintenance of respirators. The respirators were taken off too early while staying still in the contaminated area. Poor cleaning of reusable respirators was also possible.
In summary, the study confirmed that workers handling RCF are exposed to fibres especially in the steel industry including their maintenance subcontractors. This was mainly due to deficiencies in the use of respirators. Therefore, strict surveillance of the selection, use and maintenance of respirators is crucially important.
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FUNDING |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSION AND CONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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Finnish Work Environment Fund (103320).
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSION AND CONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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The authors appreciate technical assistance by Juhani Piirainen and Antero Lisko. Thanks are also due to Markku Seuri and Markku Pavela for active participation in planning of the study. We also wish to express our gratitude to the representatives and workers of the plants where the measurements were conducted.
Received November 27, 2006; in final form April 27, 2007
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSION AND CONCLUSIONSFUNDINGACKNOWLEDGEMENTSREFERENCES |
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