Exposure to Dust and Particle-associated 1-Nitropyrene of Drivers of Diesel-powered Equipment in
Underground Mining

ABSTRACT

A field study was conducted in two mines in order to determine the most suitable strategy for ambient exposure assessment in the framework of a European study aimed at validation of biological monitoring approaches for diesel exhaust (BIOMODEM). Exposure to dust and particle-associated 1-nitropyrene (1-NP) was studied in 20 miners of black coal by the long wall method (Czech Republic) and in 20 workers in oil shale mining by the room and pillar method (Estonia). The study in the oil shale mine was extended to include 100 workers in a second phase (main study). In each mine half of the study population worked underground as drivers of diesel-powered trains (black coal) and excavators (oil shale). The other half consisted of workers occupied in various non-diesel production assignments. Exposure to diesel exhaust was studied by measurement of inhalable and respirable dust at fixed locations and by personal air sampling of respirable dust. The ratio of geometric mean inhalable to respirable dust concentration was approximately two to one. The underground/surface ratio of respirable dust concentrations measured at fixed locations and in the breathing zones of the workers was 2-fold or greater. Respirable dust was 2- to 3-fold higher in the breathing zone than at fixed sampling locations. The 1-NP content in these dust fractions was determined by gas chromatography–mass spectrometry/mass spectrometry and ranged from 0.003 to 42.2 ng/m3 in the breathing zones of the workers. In mine dust no 1-NP was detected. In both mines 1-NP was observed to be primarily associated with respirable particles. The 1-NP concentrations were also higher underground than on the surface (2- to 3-fold in the coal mine and 10-fold or more in the oil shale mine). Concentrations of 1-NP in the breathing zones were also higher than at fixed sites (2.5-fold in the coal mine and 10-fold in the oil shale mine). For individual exposure assessment personal air sampling is preferred over air sampling at fixed sites. This study also suggests that particle-associated 1-NP much better reflects the ambient exposure to diesel exhaust particles than dust concentrations. Therefore, measurement of particle-associated 1-NP is preferred over measurement of dust concentrations by gravimetry, when linking ambient exposure to biomonitoring outcomes such as protein and DNA adducts and excretion of urinary metabolites of genotoxic substances.

MATERIALS AND METHODS

Populations studied

The pilot studies were conducted in Ostrava (Czech Republic) at a black coal mine and in Kohtla-Järve (Estonia) at an oil shale mine. Black coal was extracted using a long wall technique, on five production days per week in a three shift system (one maintenance shift followed by two production shifts). In the oil shale mine the room and pillar method was used. The study sample for the pilot study comprised 40 male workers, of whom 20 (10 at each mine) were underground miners and operators of diesel-powered engines (‘underground workers’) and 20 (10 at each mine) were workers engaged in various production assignments above ground that were not associated with the use of diesel-powered engines (‘surface workers’). At the coal mine the 10 surface workers were operators or mechanics in a coal processing plant. At the oil shale mine the 10 surface workers were employed as drivers of non-diesel vehicles, turners and locksmiths in a metal workshop, as operators in an oil shale processing plant or mechanics involved in the maintenance of electrically powered water pumps. The underground workers were drivers of diesel-powered locomotives during the maintenance shift (black coal mine) or drivers of diesel-powered excavators during the production shift (oil shale mine). The underground workers were a priori expected to have ‘high’ exposure to diesel exhaust, whereas the surface workers were anticipated to experience ‘low’ (background) levels of exposure determined by general air quality in the workplace and in the region where they lived.

The main study was conducted in Kohtla-Järve (Estonia) at the oil shale mine. In this study 50 underground workers and 50 surface workers were studied. The oil shale workers came from the same jobs as in the pilot study.

Diesel equipment

In the oil shale mine bulldozers, excavators and loaders were used for transportation of oil shale rock from the blasting site to a chain conveyor belt. Technical specifications of this equipment are shown in Table 1. In the coal mine, trains running on a track and trains hanging from a rail were used for transportation of coal, construction materials and personnel over long distances (1–10 km). Diesel-powered locomotives, which were also equipped with a crane, towed the wagons (for technical specifications see Table 2).

Study period

Data collection for the pilot study was carried out between 15 and 18 March 1999 and 22 and 25 March 1999 (coal mine), and 12 and 14 April 1999 (oil shale mine). Data collection for the main study was carried out between 5 and 22 June 2000, at which time the oil shale mine was operating 4 days/week. Underground workers were studied during the morning, afternoon or night shift on a Monday, Tuesday and Wednesday (production shift in the oil shale mine) and during the morning shift on a Monday and Thursday (maintenance shift in the coal mine). Workers on the surface were studied during the day shift on a Monday, Tuesday and Wednesday for the oil shale mine and on a Tuesday and Wednesday for the coal mine. During the pilot study the oil shale mine was operating only on Monday, Tuesday and Wednesday. In the main study the workers were followed on Monday, Tuesday, Wednesday and Thursday.

Air samples at fixed locations

Samples of airborne particulate matter (‘particles’) were collected on the surface and underground at fixed sampling locations. At each sampling location two samples of inhalable dust and two samples of respirable dust were collected at a height of ∼1.5 m above the floor. During the main study one sample of inhalable dust and two samples of respirable dust were collected on each day of the study. Inhalable dust (according to EN 481) was collected using a sampler head developed by the Institut für Gefahrstoff Forschung der Bergbau Berufsgenossenschaft (IGF), Bochum, Germany. Respirable dust (also generally according to EN 481, but using the Johannesburg convention) was collected using an elutriator pre-separator (type MPGII; IGF, Bochum, Germany). Particles were collected on polystyrene membrane filters with a Teflon® coating (type TE38; Schleicher & Schüll, Dassel, Germany). If overloading of the filters occurred during a measurement period the loaded filter was removed and replaced by a new filter. These samples were treated for gravimetrical and 1-NP analysis as separate samples.

Personal air sampling

All of the surface and underground workers who participated in the pilot study were asked to carry personal air sampling equipment during two shifts in the same working week. With an air sampling pump (GSA 2000 or Gilian; GSA, Messgerätebau, Neuss, Germany and Gilian Instruments, Wayne, NJ) operated at an electronically controlled flow of 2.0 l/min, respirable dust was collected in the breathing zones of the workers. The airflow was set by dry calibration before and after air sampling. The dust was collected on the TE38 filters (see above). In the main study, personal air sampling was carried out during just one shift for each worker.

Gravimetry of air samples

The weight of the membrane filters was determined before the start of the air sampling campaign, using a Mettler analytical balance (Sartorius, Göttingen, Germany). Before air sampling, the membrane filters were stored in the weighing room for 24 h to allow adjustment to the temperature and humidity conditions of this room prior to weighing of the filters. This procedure was repeated for the loaded filters and unused blank filters. The procedure fully complies with ISO/DIS 15767 (International Standards Organization, 2002).

Analysis of EC

A sample of oil shale dust was analysed for its EC content using a coulometric method as described by Dahmann et al. (1996).

Analysis of 1-NP

The 1-NP content of the membrane filters was analysed following the method described in Scheepers et al. (1994) with some modifications. The extracts were fractionated on silica cartridges (Bond-Elut LRC; Varian, Harbor City, CA). Nitro-PAHs were reduced using sodium hydrosulphide hydrate (Fluka, Buchs, Germany). The amino analogues were extracted and derivatized using heptafluorobutyric acid (Acros, Geel, Belgium) prior to gas chromatography–mass spectrometry/mass spectrometry (GC-MS/MS) analysis (conditions described by Van Bekkum et al., 1997). Nine-fold deuterated 1-NP (d9-1-NP) was used as an internal standard and was added prior to extraction. SRM 2975 (National Institute for Standardisation and Testing, Gaithersburg, MD) was analysed in duplicate with each series.


Calculations

In a few cases during one sampling period more than one filter was collected (usually if filters were overloaded during the course of sampling they were replaced by new ones). The dust and 1-NP concentrations were calculated by multiplying each concentration by the corresponding sampling time. The total of these products was then divided by the sum of sampling periods. In this way a time-weighted average was determined from reconstitution by calculation of results of gravimetrical and 1-NP determinations of separate samples. The within-worker and between-worker variance ratios (wR0.95 and bR0.95) were calculated from the variance components of the 97.5th and 2.5th percentiles of the log-normally distributed exposures for each group. Significance tests accompanying Pearson correlations were derived from analysis of variance (ANOVA). All analysis was performed with the aid of Stata 7.0 software (Stata, College Station, TX).


RESULTS

Prior to commencement of the study, raw materials from the coal mine and the oil shale mine were analysed for EC and 1-NP. Black coal and oil shale contained substantial amounts (several weight%) of EC. 1-NP was not detected by GC-MS/MS at a limit of detection of 5 pg on column. Based on these findings it was decided that an exposure assessment for exposure to diesel exhaust would be based on the determination of 1-NP associated with respirable dust.

Concentrations of dust and 1-NP at fixed locations

The results of air sampling at fixed sites are presented in Table 3. Geometric mean (GM) concentrations of dust were all <1 mg/m3, except underground in the oil shale mine (pilot study). In the pilot studies the ratio of GM respirable to inhalable dust levels was approximately 0.4–0.5. In the main study this ratio remained the same on the surface but increased to 0.8 in underground workplaces. GM concentrations of inhalable and respirable dust in the coal mine were higher in underground workplaces than in workplaces on the surface. At the oil shale mine this difference was ~5-fold in the pilot study, but in the main study it was reduced to a factor of less than two.

GM concentrations of 1-NP were higher in underground workplaces than surface locations: approximately 3-fold or more in both mines (pilot phase). In the main study this difference was much greater (two orders of magnitude). In spite of the differences in respirable and inhalable particle concentrations, the GM concentrations of 1-NP associated with the respirable particle fraction were similar to the 1-NP values observed derived from air sampling of the inhalable particle fraction.

Personal exposures to respirable dust and 1-NP

Concentrations of respirable dust observed in the breathing zones of individual workers are presented in Table 4. The measurements of respirable dust in individuals during the first and second shifts were significantly positively correlated (Fig. 1). Respirable particle concentrations observed in the coal mine (maintenance shift) were similar to those obtained in the oil shale mine (production shift). Also, breathing zone concentrations of respirable dust did not change much in the oil shale mine from the pilot phase to the main study. In all of the three studies the observed breathing zone concentrations were roughly 2-fold higher for underground workers, compared with surface workers.

In the oil shale mine the concentrations of respirable dust observed in the breathing zone were similar to concentrations observed at fixed site sampling locations (differences were <2-fold). In contrast, in the coal mine breathing zone concentrations were 3-fold higher than concentrations at fixed locations.

1-NP associated with the collected respirable dust showed some interesting patterns (see Table 5). In the coal mine 1-NP levels in the breathing zones of the drivers of trains were ~2-fold higher than the levels observed for surface workers. However, in the oil shale mine underground workers appeared to have 10-fold (main study) to at least 20-fold (pilot study) higher concentrations in their breathing zones than surface workers. Results obtained during the first and second shifts (pilot studies) were similar. The correlation coefficient was similar to that observed for respirable dust (0.47), but this was strongly influenced by a single exceptionally high measurement in an underground oil shale miner during the first shift monitored (Fig. 2).

In the coal mine concentrations of particle-associated 1-NP observed in the breathing zones were 2.5-fold (underground) to 4-fold (surface) higher compared with values observed at fixed locations (1-NP associated with respirable dust). During the pilot study in the oil shale mine, underground breathing zone values were observed to be 10-fold higher than values observed by fixed site sampling. However, on the surface the situation was the reverse: breathing zone values were lower than values obtained by fixed site sampling at surface locations. The concentrations of 1-NP in the breathing zones of the surface workers during the main study were 20-fold higher compared with the observed values at fixed sites. In underground workplaces this difference was smaller (less than a factor of two).

A positive correlation (r = 0.39) was observed between the concentrations of respirable dust and 1-NP (Fig. 3a). For conditions on the surface, respirable dust and 1-NP were not significantly correlated (Fig. 3b).
