The respirable dust and DPM concentrations (average over the entire shift and real-time) inside and outside of two cabs were measured while the vehicles were operating in a limestone mine. Samples were collected on 17 days over a nine-month period with sampling at least once a month. The pressure inside the cab was also monitored to determine when the cab doors were opened. A positive pressure indicated the cab was sealed (the door was closed), while a pressure reading of zero indicated the cab door was open.

The average concentrations inside and outside of the cab over the entire shift were used to calculate the total percentage of respirable dust and DPM reduced by the cab system (reduction efficiency). This total reduction efficiency could be influenced by the opening of a cab door periodically throughout the day. Therefore, the real-time data for concentrations inside and outside of the cab when positive cab pressure existed were averaged and used to determine the reduction efficiency when the cab was sealed. Both types of reduction efficiencies were compared to evaluate the influence of work practices on the concentration of dust and DPM inside the cab. In addition, the concentrations of respirable dust and DPM inside the cab were measured to determine the operators’ exposure level while performing their routine work practices.

In this study, face drill and roof bolter mining machines were equipped with newly designed filtration and pressurization systems as shown in Fig. 1 and evaluated at an underground limestone mine. Air was drawn into the system from the outside and filtered (intake filter). This filtered air flowed down into the main HVAC unit, located on the outside wall of the enclosed cab. Simultaneously, air was drawn through the recirculation filter and was combined with the intake air in the main HVAC unit. This conditioned air flowed through the final filter and was circulated into the enclosed cab. The recirculation and final filter were identical on the face drill and roof bolter machines. The recirculation filter had the following dimensions: 7.6 cm (3 in.) width, 40.6 cm (16 in.) length, 5.1 cm (2 in.) depth, and used filter media with a dust capture efficiency similar to the American Society of Heating, Refrigerating and Air-Conditioning Engineer's (ASHRAE) MERV between 8 and 9. The final filter was 29.9 cm (11.375 in.) wide, 44.45 cm (17.5 in.) long, 9.53 cm (3.75 in.) deep, and had a MERV 16 filter rating. Both vehicles were relatively new, with good cab integrity (good gaskets, good molding around door, etc.).

The only difference between the filtration and pressurization unit on the drill and the one on the roof bolter was the intake filtering unit. The intake unit on the face drill was a Donaldson system, which used a non-fan-powered filter housing referred to in this report as a static filter unit. For this design, the outside air was drawn through the intake filter by the main fan on the HVAC unit, which can be operated at three different fan speeds. Because of this, the amount of intake airflow was completely dependent on the pressure and filter loading components of the entire system, which consisted of the intake, recirculation and final filters.

The intake unit of the roof bolter was a fan-powered Sy-Klone International RESPA-SD unit. The RESPA-SD unit uses a design that brings the outside air into the unit and causes it to travel through two powered air precleaners in series. Each precleaner unit delivers approximately 1.13 m³/min (40 cfm) of air, making the total makeup air quantity about 2.27 m³/min (80 cfm). These precleaners use a centrifugal design to spin off the larger dust particles (> 5.0 μm). After going through the centrifugal precleaner units, the air then passes through a canister filtering cartridge 33 cm (13 in.) long and 20.3 cm (8 in.) in diameter. The centrifugal precleaning technique reduces the amount of dust loading on the intake filter, potentially increasing the time between filter changes. Once the intake air passes through the intake canister filter (MERV 16), the air then combines with the recirculated air at the main HVAC unit, as with the face drill unit.

For DPM measurements, a sampling package was inserted inside the cabs of the roof bolter and face drill mining machines with an identical sampling package placed outside of the cabs. The sampling package contained three SKC DPM cassettes with quartz fiber filters for elemental carbon (EC) and total carbon (TC) analysis and an EC monitor developed by NIOSH (either the NIOSH prototype or the FLIR Airtec (Bugarski et al., 2011)) attached to a submicron impactor for real-time EC measurement. The flow rates of all pumps and instruments were checked and recorded. The SKC DPM cassettes were attached to 10-mm Dorr-Oliver cyclones with tubing extending from the cassette to MSA Elf pumps operated at 1.7 L/min (0.06 cfm). The SKC DPM cassette is the standard method for collecting DPM (MSHA, 2001; Noll et al., 2007) and retrieves particulate at a 0.8-μm cut point at 1.7 L/min (0.06 cfm) onto quartz fiber filters to collect DPM in the presence of dust. This sample is used to determine EC and TC concentrations via NIOSH method 5040 (Birch, 2004). The EC monitor measures real-time EC concentrations via laser absorption (Noll and Janisko, 2007).

EC and TC were measured since they are both used as surrogates for determining DPM exposures in underground mines (MSHA, 2001, 2006; Noll et al., 2006). In fact, for compliance sampling, MSHA first uses TC concentrations taken from a personal sample for determining DPM exposures (MSHA, 2001, 2006). However, TC can be prone to interferences from sampling artifacts, cigarette smoke and oil mist (Noll et al., 2006). A dynamic blank, as described by Noll and Birch (2008), can be used to correct for some sampling artifacts and is part of the SKC DPM cassette. However, the influence of cigarette smoke and oil mist cannot be corrected for with a dynamic blank and cannot always be avoided in the mine environment. Contrary to TC, EC is not prone to these interferences and is still a major portion of the DPM sample. Therefore, in addition to measuring TC from a personal sample, MSHA also uses the EC portion of the sample for compliance purposes. EC is converted to equivalent TC concentrations with a conversion factor as described by MSHA's Program Policy Letter No. P08-IV-01 (MSHA, 2008).

Since EC is not prone to interference and can be measured in real time, it was used to determine reduction efficiencies of the cab systems. TC measurements inside the cab were collected to determine the potential exposure of miners to DPM, since the final PEL is expressed as TC. TC concentrations were determined in two ways: (1) the TC from the NIOSH method 5040 sample was used after being dynamic blank corrected and, (2), the EC concentration was multiplied by a conversion factor of 1.3, since this factor is commonly observed in underground metal/nonmetal mines (Noll et al., 2007).

Determining the difference in respirable dust levels in the operator cabs was accomplished by monitoring two sampling locations, one inside the enclosed cab and the other on the outside of the cab, using standard respirable dust measurement techniques as described by Colinet et al. 2010. The inside location provides the potential dust exposure levels of the equipment operator and is compared to the dust concentrations measured outside the enclosed cab.

All sampling instrumentation was placed on a sampling rack for each sampling location. Three gravimetric samplers located side-by-side on the sampling rack provided an average respirable dust concentration at each of the sampling locations. Escort Elf (Zefon International Inc., Ocala, FL) sampling pumps were used and calibrated to a flow rate of 1.7 L/min (0.06 cfm) before each field survey (the required flow rate established by the American Conference of Governmental Industrial Hygienists for the metal/nonmetal industry (MSHA, 1990)). Respirable dust samples were drawn through a 10-mm (0.4-in.) Dorr-Oliver cyclone, which classifies the respirable portion of dust and, then, were deposited on a polyvinyl-chloride 37-mm (1.5-in.) filter (SKC Inc., Eighty-Four, PA). Filters were pre- and post-weighed to the nearest 0.001 mg on a microbalance in a temperature/humidity-controlled weighing room at NIOSH's laboratory in Pittsburgh, PA. All sampling pumps were post-calibrated to ensure that an acceptable flow rate of 1.7 L/min (+/− 0.015 L/min) (0.06 cfm) was maintained throughout testing. For every 10 gravimetric filters used in the field, a blank cassette was used to determine a correction factor for the filter weighing process, which was applied to all the field gravimetric measurements.

Instantaneous respirable dust measurements were taken with personal Data RAM (pDR 1000) instruments (Thermo Fisher Scientific Corp., Waltham, MA). This real-time dust monitor measures the respirable aerosol concentration based upon the light scatter of particles that pass through an internal sensing chamber. This instrument usually requires a correction factor using a gravimetric sampler for acceptable accuracy (Colinet et al., 2010), which was accomplished by first comparing the average respirable dust concentrations measured by the three gravimetric samplers to the pDR's instantaneous respirable dust concentration for a defined sampling time period. A correction factor was then calculated by dividing the pDR average concentration value into the gravimetric value. This calculated correction factor was then multiplied by all of the individual aerosol measurements taken with the pDR instrument and recorded in an Excel spreadsheet.

All cab pressure measurements were taken with DP-CALC micromanometers, Model 5825 (TSI Inc., Shoreview, MN). These pressure measurements were taken every minute and recorded on the unit's internal datalogger. After each day of testing, the data was downloaded to a laptop computer and stored as an Excel data file. The pressure measurement provided the necessary data to determine when the door on the enclosed cab was opened for any significant time period.

The samplers were turned on before the miners had to begin their shift so that setting up the experiment did not delay the miners’ work. At the end of the shift, the samplers were turned off and removed from the vehicles. The quartz filters from the SKC DPM cassettes were analyzed for EC and TC at the NIOSH laboratory in Pittsburgh, PA using NIOSH method 5040 (Birch, 2004). The gravimetric filters were sealed and weighed at NIOSH, Pittsburgh, PA. Data from the EC real-time monitors were downloaded each day. A data point was collected every minute and used to calculate an eight-hour time-weighted average (TWA) along with 5-, 10- or 15-minute rolling average concentrations. The real-time respirable aerosol levels were recorded on an internal data logger every 10 seconds and were downloaded to a laptop computer at the end of each day of testing. This procedure was repeated for 17 days throughout a nine-month test period, with an attempt to test at least once a month to obtain a long-term evaluation.

Unfortunately, there was not a complete data set for each day; instrument malfunctions occurred with the pressure monitor on one day when evaluating the drill and on four days when evaluating the bolter. Six data sets from the EC monitors (some of which were prototypes) had issues with tubing, kinked line and/or damage occurring during sampling. These problems prohibited the calculation of the efficiency of the cab system when the cab was sealed, since it could not be obtained without pressure and real-time data. Also, on some days, only one vehicle was operating, which limited the number of data points for each vehicle.

In this study, NIOSH method 5040 EC results using Eq. (1) were used to determine the reduction efficiency of the cab system for DPM: (1)Reduction efficiency=ECoutside−ECinsideECoutside×100 where EC_outside = the EC concentration measured by NIOSH method 5040 outside of the cab, and EC_inside = the EC concentration measured by NIOSH method 5040 inside of the cab.

Triplicate samples were collected inside and outside of the cab, but due to pump error on some days of testing, only duplicate EC samples were available for analysis. These multiple samples were used to calculate replicate efficiencies for each experiment. This was accomplished by inserting the data for the highest concentration outside the cab and the lowest concentration inside the cab into Eq. (1). The next reduction efficiency was calculated using the lowest concentration outside the cab and the highest concentration inside the cab. When triplicate samples were analyzed, the samples left were used to determine the third efficiency. This method for determining efficiencies ensures the largest standard deviation between efficiencies. If the EC concentration inside the cab was below the limit of detection (LOD), the LOD value was inserted as the EC inside concentration in Eq. (1). The reduction efficiency was then recorded as being greater than the calculated value. The two or three replicate reduction efficiencies calculated for each experiment were averaged to obtain the overall value for that cab on that day. The same procedure and equation were used to calculate the respirable dust reduction efficiencies, but the gravimetric data outside and inside the cab were used instead of the EC values.

The experimental error for measuring reduction efficiencies was determined by calculating the relative standard deviation (RSD) of the reduction efficiencies for each experiment. Instead of determining a confidence interval for each set of measurements, the RSDs from each day for each cab were pooled to achieve a much stronger statistical determination of error, since each experiment only possessed two to three data points. The pooled RSD was multiplied by the 95% confidence student t factor (1.96 with the degrees of freedom in this case) to obtain the error of this analysis at the 95% confidence level.

The respirable dust results using gravimetric analysis and EC results using NIOSH method 5040 provided information on the efficiency of the cab system for the entire shift and were affected by the opening of the cab doors. Therefore, in order to determine an efficiency when the cab was sealed, the reduction efficiency was calculated by first averaging the real-time concentrations of the Airtec and pDR at the times of the day when the cab door was closed (shown by the pressure data). These concentrations were then used in Eq. (1) to determine the reduction efficiency of the cab for DPM and respirable dust. On some of the sampling, the concentrations measured with the pDRs outside of the cab when the door was closed were low (less than or equal to 0.21 mg/m³), and an accurate efficiency of the cab system for respirable dust when the cab was sealed could not be determined with these data points. In fact, at these concentrations, the measurements could be strongly influenced by other aerosols such as DPM and, since the pDRs are not calibrated for these aerosol particle sizes, erroneous results from the pDRs could result.

An RSD could not be calculated for the efficiencies when using the pDRs and Airtecs, since only single measurements were taken. However, the gravimetric data was used to calibrate the pDR results, and the Airtecs have been shown to be equivalent to NIOSH method 5040 (Noll and Janisko, 2007). Therefore, RSDs similar to the gravimetric and NIOSH method 5040 samples would be expected when determining the efficiencies with these instruments.

The RSDs of the duplicate or triplicate TC measurements inside the cab for each day were pooled as discussed for the EC measurements above and, then, multiplied by the 95% confidence student t factor to determine the experimental precision for the TC measurements.

As seen in Table 1, the enclosed cab systems in the face drill and roof bolter mining machines reduced DPM between 46% and 95% under normal operating conditions. A large range of efficiencies were evident because the reduction efficiency depended upon the amount of time the cab door was open for that test. This can be substantiated by the concentration of EC inside the cabs being above 39 μg/m³ only when the pressure was near zero, indicating the cab door was open (see an example of this in Fig. 2). In addition, the range of reduction efficiencies was much tighter when the influence of the open door was eliminated. The reduction efficiency was primarily more than 90% (Table 2) when a positive pressure existed in the cab. Even though there were some days that data was not collected, at least six days of sampling per cab were achieved, resulting in 13 data points between the two cabs. This number of samples is not extensive, but is enough to provide valuable information on the characteristics of the cabs.

The reduction efficiency for respirable dust was similar to the reduction efficiency for DPM with these enclosed cabs. As can be seen in Table 3, a large range (51-99%) of reduction efficiencies for respirable dust were observed, which was probably due to the influence of the operator's opening and closing of the door. As seen in Fig. 3, the concentration of respirable aerosols did not exceed 0.4 mg/m³, unless the pressure was zero, indicating that the door was open. As with the DPM results, the reduction efficiencies were tighter when evaluating the pDR only when the cab was sealed, always resulting in reduction efficiencies above 91%, and above 95% for most days of testing. The tests results indicate that cab systems with a MERV 16 rated filter can reduce DPM and dust in real mining conditions by more than 90% when sealed. Even though there were some days that data was not collected, at least seven days of sampling per cab were achieved, resulting in 21 data points between the two cabs. Again, this number of samples is not extensive, but is enough to provide valuable information on the characteristics of the cabs.

These DPM and dust reduction efficiencies with the unidirectional designed cab system and the MERV 16 filter were similar to results observed when using a HEPA filter in other field studies of cab systems, even though the HEPA was shown to be slightly more efficient than the MERV 16 filter in laboratory tests (more than 99% efficient for capturing DPM with the HEPA filters compared to 96% with the MERV 16) (Noll et al., 2012, 2008). This similarity could be due to the MERV 16 filters providing equivalent protection from DPM and dust as the HEPA when employed in the cab systems under real mining conditions. In other words, the MERV 16 and HEPA filters provide enough protection (more than 96%) so that the filtration system is no longer a limiting factor to the reduction efficiency of the cab system. Instead, other factors such as cab integrity and pressurization are limiting the reduction efficiency of the cab system operating in the mine.

Another scenario that could cause similarities is that the cab systems tested with the HEPA filters were not as effective as the cabs tested in this current study, resulting in the newly designed cabs providing the same protection with slightly less efficient filters. The cab systems tested with the HEPA filters could have had more leaks, less pressurizations or some other flaw not present in the cabs tested in the current study. At this time, it is not known which scenario is correct, offering an opportunity for future studies.

As seen in Tables 1-3 and discussed earlier, the opening of the cab door on average reduced the reduction efficiencies of DPM and respirable dust from more than 90% to about 80%. In fact, for most samples (about 47 out of 58), the reduction efficiencies were at or above 80% for DPM and dust. The other 11 samples had reduction efficiencies as low as approximately 46%. It is not known why the door was open for longer periods for the days with the lower reduction efficiencies.

Even though opening the door can have a major effect on reduction efficiencies, routine work practices dictate that the opening of the cab door will continue to occur periodically throughout the day. However, minimizing the time the door is open will enhance the protection of the miner. Since 80% reduction efficiency was achieved for most days, miners may be able to adapt their work practices to achieve at least this efficiency for every day. At the mining operation where this study occurred, an 80% and greater efficiency resulted in DPM concentrations inside the cab of less than or equal to 117 μg/m³ TC (see Table 1) and, as shown in a previous publication, less than 0.6 mg/m³ respirable dust concentrations (Cecala et al., 2012).