Elemental Carbon Levels at a Potash Mine

ABSTRACT

A survey of worker exposures in a potash mine and mill was undertaken to evaluate diesel exposures during the feasibility effort of an epidemiologic study. Nineteen occupations were sampled. Results were averaged into six underground groups and three surface groups. Elemental carbon exposures, a diesel surrogate measure, were similar among all surface occupations; however, miners working in the auto shop were the most heavily exposed. Underground, ramcar drivers had the highest mean exposures over a 3-day sampling period, and those miners working in the underground diesel shop and warehouse had the lowest exposures. A multiple comparison of means test in SAS yielded three statistically different exposure groups when compared at the alpha = 0.1 significance level. The results of the survey demonstrate that exposure to elemental carbon varies considerably among mining occupational groups. This should prove useful for dose-response analyses in a future epidemiologic study.

METHODS

The sampling plan included both personal and area monitoring for elemental carbon and other diesel-related contaminants. Area sampling packages also included submicrometer, total and respirable dust monitoring, an eight-stage impactor, Palmes passive dosimeters for nitrous oxide and nitrogen dioxide, and long-term colorimetric tubes for carbon monoxide, carbon dioxide, and sulfur dioxide. This article will focus on the elemental carbon findings and present submicrometer or diesel particulate matter (DPM) and nitrogen dioxide concentrations for comparison purposes.
Area samples were collected in three locations on the surface: the auto shop, where diesel equipment is repaired; an office, to evaluate employees working indoors in laboratory and office environments; and outdoors near the mine exhaust shaft, to estimate maximum ambient concentrations. 

Underground area samples were collected in the overhaul shop, on a continuous haulage section, and on a ramcar section.

Personal samples were collected from all miners working on two representative production sections. The two sections selected included one ramcar section and one continuous haul- age section. Exposures from other underground miners were measured as well. These included three foremen (the supervisor, a shift foreman, and a section foreman) and four workers in the maintenance and warehouse areas. A sample of workers from all of the surface locations was also included. Table 1 notes the occupations sampled. Thirty-one miners, representing a majority of the occupations and exposures in this potash mine and mill, were sampled for 2 to 3 days each. All jobs were grouped into nine occupational groupings (Table 1) based on the similarities of their exposure-related tasks.

Elemental Carbon

Elemental carbon estimates were collected using a Mine Safety AppliancesCo. (MSA)cassette modified to collect particles less than 1.1 um in aerodynamic diameter at an air flow of 1 L/min according to NIOSH Analytical Method 5040. Multiple samples were collected from three areas on the surface during two consecutive day shifts. Multiple samples were collected during three day shifts in three underground locations.

Samples were analyzed at Sunset Laboratories using a thermal-optical method designed to obtain speciation of elemental and organic fractions in the deposited aerosol.

Submicrometer or Diesel Particulate Matter

Submicrometer particulate was measured in area samples only, using a dichotomous sampler from a design modified by the Mine Safety and Health Administration. The impactor is an expanded version of the MSA respirable dust cassette. A spacer, inserted between the cassette halves, holds the impaction plate. The inlet of the impactor is fitted with a brass insert which has a 1.O-mm diameter nozzle. A 10-mm nylon cyclone precedes the impactor to remove the larger particles. At a flow rate of 1.7 L/min, the respirable particulate which passes through the impactor inlet is separated into two fractions. The submicrometer particulate (0.9 pm) passes through the holes in the impaction plate and is deposited on a filter. The remainder of the respirable particulate is deposited on the impaction plate. The impaction plate is greased to better collect this particulate and prevent bounce.

RESULTS


Elemental Carbon and DPM

There are no current recommended exposure levels for ele mental carbon. Comparisons with submicrometer or DPM concentrations collected side by side are presented for reference. In 1995 the American Conference of Governmental Industrial Hygienists added diesel exhaust to their Notice of Intended Changes at a proposed threshold limit value of 150 ug/m3.

Area-Wide Sampling Results

Elemental carbon levels on the surface ranged from 0 to 27 ug/m3. Underground, the area-wide exposures ranged from 17 to 606 ug/m3. On average, over the 3-day sampling period, exposure in the ramcar section was the highest (453 ug/m3). The lowest average concentration measured underground was in the diesel and overhaul shops (60 ug/m3), but even this concentration was more than twice as high as concentrations seen on the surface.

By comparison, DPM concentrations ranged from 116 to 279 ug/m3 at the surface locations and from 104 to 1035 ug/m3 in the underground locations. Average concentrations at all sampled locations are shown in Table 2.

While the Pearson correlation coefficient between elemental carbon and DPM is very high (r2 = 0.98), the ratio of the two contaminants is variable and increases as the contaminant concentrations increase. This may be due to a variety of reasons; primary among them is the fact that DPM measures all particulate less than the sampler cutpoint (approximately 0.9 p m with this sampler), while the elemental carbon analysis measures only the carbon particles in that size range. This makes the elemental carbon measure much more specific to diesel exhaust, because in addition to the elemental carbon particles, which are produced almost solely by diesel combustion, DPM can include organics (such as cigarette smoke), oil mists, and ore particles in the submicrometer size range.

Personal Sampling Results

Ramcar drivers had the highest average personal exposures to elemental carbon (345 ug/m3; Table 3). The average expo-sures of the continuous miner operators and the belt crew were almost identical (225 and 222 ug/m3, respectively). Underground mechanics, along with others working in the shop areas, had the lowest average underground exposures (53 ug/ m3). On the surface, the mechanics had the highest exposures (31 ug/m3). Again, these exposure levels were approximately half of those seen underground.

Comparison of Average Exposure

To determine whether or not there were any significant differences between the mean occupational exposures to elemental carbon, multiple comparison t-tests (Least Significant Difference) and Duncan’s multiple range tests were performed using SAS. Table 4 reports the results of the LSD test comparing elemental carbon exposure among the occupational groups at both the alpha=0.05 and alpha=0.10 significance levels. Results of the test at the alpha = 0.10 results yield three distinct and logical occupational categories, while the results of the test at the 0.05 significance level yield mixed groupings. The categories are differentiated by the alpha characters in the “Group” columns. Groups with the same alpha character (e.g., A, B, C, or D) have group mean results which are not significantly different from one another.  However, if the group alpha characters are different from one another, there is a statistically significant difference between the group means. 

The results (Table 4) indicate that the ramcar drivers were in the most highly exposed category. Four occupational groups fell into the moderate exposure category: continuous miner operator, belt crew, miner (unspecified), and foremen. The underground maintenance and warehouse workers and all of the surface workers were in the lowest exposure group.

A comparison of the mean exposures between the area samples was also carried out (Table 5). Here again, three exposure categories were found. The ramcar section had the highest exposure. The continuous haulage section was in the middle exposure group. The underground and surface diesel shops had exposures that were in the lowest exposure group, along with the office area and the outdoor samples near the shaft




