A total of 779 full-shift personal REC measurements was taken at the seven mining facilities on workers who spent the entire shift underground (). The average REC exposure level measured underground by facility ranged from 40 μg m−3 at Facility G to 384 μg m−3 at Facility A, an ~10-fold range across the facilities. Only 16 (2%) of the REC measurements on underground workers were below the LOD.
Measured personal respirable elemental carbon exposure levels (μg m−3) for underground, mixed, and surface workers by mining facility: full-shift time-weighted average concentrations
A total of 265 full-shift personal REC measurements was taken on workers who spent the entire shift on the surface (). The average REC exposure level measured for surface workers ranged from 2 μg m−3 at Facilities G and H to 6 μg m−3 at Facility A. Sixty-three percent of the measurements taken on the surface were below the LOD. There were 101 measurements taken on workers who worked both underground and on the surface during the work shift and these means ranged from 3 to 160 μg m−3 across facilities.
The distribution of the exposure measurements taken underground and on the surface by facility is displayed in . Average REC levels underground ranged from 20 to 64 times higher than the average levels measured on the surface across the seven facilities, and the highest levels measured underground were >100 times higher than on the surface. There was an ~10-fold variation between the facilities in the average exposure levels for the underground jobs monitored. An analysis of variance on the log-transformed underground measurements demonstrated a statistically significant difference (P < 0.001) between facilities.
Fig. 1. Personal respirable elemental carbon measurements (μg m−3) for surface and underground jobs by mining facility (A–I). Full-shift time-weighted concentrations. s, surface, u, underground. The boxes display the 25th and 75th percentiles, (more ...)
There was much less contrast between the ROC levels measured on the underground and surface workers, as compared with REC (). The AM of the ROC levels measured underground ranged from 64 to 195 μg m−3, while the AM of the ROC levels measured on the surface ranged from 38 to 71 μg m−3 by facility. The average ROC levels measured underground were approximately twice the levels measured on the surface; however, at one facility (G), the AM of the ROC concentrations on the surface (70 μg m−3) was somewhat higher than the ROC AM underground (64 μg m−3), despite a 20-fold difference in the average REC level on the surface (2 μg m−3) compared with that underground (40 μg m−3) at this facility.
Measured personal ROC exposure levels (μg m−3) for underground, mixed, and surface workers by mining facility: full-shift time-weighted average concentrations
The average levels of NO and NO2 by facility are displayed in . The average levels underground were ~15 times higher than on the surface. The AM of the NO concentrations measured underground ranged by facility from 0.20 to 1.49 ppm, compared with a range of 0.02 to 0.11 ppm by facility on the surface. The AM of the NO2 concentrations measured underground ranged from 0.10 to 0.60 ppm, compared with a range of 0.01–0.06 ppm on the surface.
Measured personal NO and NO2 exposure levels (ppm) for underground, mixed, and surface workers by mining facility: full-shift time-weighted average concentrations
The measurements of REC, ROC, NO, and NO2 taken underground on the same workers on the same days were significantly correlated (i.e. P value < 0.05), indicating a consistent pattern in exposure levels among the particulate and gaseous components of DE monitored. The Pearson correlation coefficients between the log-transformed concentrations of REC with ROC, NO, and NO2 were 0.62, 0.71, and 0.62, respectively, based on just the measurements taken underground. contains scatter plots of both the underground and surface measurements that display the distribution of the NO and NO2 samples and the corresponding REC samples by facility. Stronger correlations were seen at the mines with the higher REC concentrations (A, B, and E) compared with the mines with lower REC concentrations (G, H, and I).
Fig. 2. Personal NO and NO2 (in ppm) versus REC (in μg m−3) exposure levels measured underground and on the surface by mining facility. Full-shift time-weighted average concentrations. Solid circles are underground and open circles are surface (more ...)
The AM of the REC measurements for all underground jobs with five or more measurements are presented in . The highest exposed jobs were located primarily in the face area, while maintenance and support jobs that spent less time at the face, in general, had lower AMs. The highest REC exposure levels were measured at Facility A for workers located in the production areas near the face, where average exposure levels ranged from 313 μg m−3 for the drill operator to 488 μg m−3 for the loader operator, with GSDs for these jobs that ranged from 1.3 to 2.1, indicating relatively homogeneous exposure levels. The mean REC exposure levels at the potash facilities ranged from 118 to 263 μg m−3 at Facility B and from 48 to 216 μg m−3 at Facility D. The GSDs for two of the jobs in Facility B were relatively high compared to other jobs (i.e. 5.6 and 6.9), indicating a greater range in the daily average exposure levels for these jobs. The REC levels at the salt mine, Facility E, ranged from 29 to 140 μg m−3 with GSDs that ranged from 1.4 to 4.3. The lowest REC levels underground were measured in the three trona mines, with AMs that ranged from 31 to 58 μg m−3 at Facility G, from 62 to 116 μg m−3 at Facility H, and from 42 to 146 μg m−3 at Facility I, with GSDs that were mostly <3.0. The SEMs were generally ≤20% of the AM value, indicating reasonably good precision in the exposure estimates by job title.
Measured personal respirable elemental carbon exposure levels (μg m−3) by mining facility and job title for underground jobs with five or more measurements: full-shift time-weighted average concentrations
There was much less variation between jobs with each facility and across facilities in the surface REC measurements. The highest exposed job with measurements among the surface workers at the seven facilities was the crusher operator at Facility B (29 μg m−3, N = 3), followed by the shift truck driver at Facility A (17 μg m−3, N = 1). The AMs for all other surface jobs monitored were ≤10 μg m−3. The results by job title for measurements on surface workers are not presented since the measurements were not used as exposure estimates.
The area samples taken underground and on the surface to measure REC are summarized in by face, haulage and travel ways, underground shop and office areas, and surface. The averages of the area samples display a similar gradient in REC air levels as the personal samples. The highest concentrations were measured underground at the face, with AM concentrations ranging from 50 to 661 μg m−3 by facility. These concentrations were ~30–80% higher than the AM concentrations in the haulage and travel ways. The results for the shop and office samples were substantially lower for all but Facilities A and I, but the number of measurements in all facilities was small. The area measurements were not used for estimation of personal exposure levels, but nonetheless provided an indication of the range of REC concentrations to which the underground workers at these facilities were potentially exposed. The AMs of the area samples taken at the face generally were equal to or greater than the respective AMs of the personal samples taken on workers in production jobs primarily at the face.
Measured area respirable elemental carbon concentrations (μg m−3) by mining facility and sampling site
After grouping of the underground jobs based the number of measurements, 40% of the underground exposure-years (0–68% by facility) were assigned exposure estimates based on standardized job titles (U1); 40% (0–63%) were based on time spent in four underground areas (U2), and 6% (0–27%) were from groups formed based on similar CO air concentrations (Stewart et al., 2010a) (U3). Only 12% (0–71%) of the exposure-years were assigned estimates based on the average of all underground measurements (U4), and only 1% (<1–2%) were assigned an estimate based on <5 measurements (U5). For surface jobs, 69% (36–90% by facility) of the exposure-years were based on a mean from exposure group A (minimal contact), 23% (4–46%) were based on an exposure group B (light equipment or bystander) mean, and 4% (0–21%) were based on an exposure group C (heavy equipment) mean. A total of 75% of the surface exposure-years were attributed to the S1 estimation group (facility-specific).