Exposure to diesel exhaust has been linked to various adverse health outcomes [1, 2]. In 2012, the International Agency for Research on Cancer (IARC) categorized diesel engine exhaust as a carcinogen to humans (Group 1) [3]. Exposure to diesel particulate matter (DPM) is especially concerning for underground miners. Since underground miners work alongside diesel equipment in a confined environment, they can be exposed to some of the highest levels of diesel exhaust in the U.S [4–7]. Therefore, the Mine Safety and Health Administration (MSHA) promulgated a rule to limit exposures of metal/nonmetal underground miners to DPM to an eight-hour time-weighted average (TWA) of 160 µg/m3 total carbon (TC) [6, 7]. This compliance limit is based upon feasibility of compliance with the goal being to reduce a carcinogen, in this case DPM, to the lowest levels possible.
Since this rule went into effect, DPM exposures in mining have decreased, but they are still above levels of other occupations in other industries, such as agriculture, construction, manufacturing, etc., and in some cases above the personal exposure limit (PEL) for mining [5, 8]. A review of MSHA compliance data between 2009–2014 shows that approximately 20–25% of underground miners are out of compliance each year. Further analysis shows that 46% of the out-of-compliance samples are from blasters, load haul dump drivers, and scalers, with blasters being one of the highest exposed professions in mining. Workers in these professions usually work outside of cabs in areas where diesel equipment is operating. The average DPM exposures for blasters are about 156 µg/m3 TC, with exposures at times above 500 µg/m3 TC. Mines can have a difficult time ventilating the areas due to the large energy requirements to move air through the large underground openings where these miners commonly work. Some mines use administrative controls to avoid miners’ exposure to DPM by having them work on off-schedules or upstream of diesel vehicles [9]. However, these types of solutions are not always feasible or practical.
Blasters can spend a substantial amount of their time at the face loading explosives into blastholes. The face can be quite large, requiring the use of a high lift. The ANFO loader is a machine that incorporates a high lift where a canopy is usually located above the mine worker. Therefore, one potential control technology to help reduce their exposures may be using the canopy air curtain (CAC) attached to the canopy of the ANFO loader.
As demonstrated in Fig. 1, the CAC delivers clean air over the operator’s breathing zone. A fan draws in air through a filter to capture the dust and then supplies clean air beneath the canopy where a miner is working. The development of the canopy air curtain (CAC) dates back to the 1970s, starting with the initial development of the CAC by the Donaldson Company, Inc. under contract from the U.S. Bureau of Mines [10]. This CAC was originally developed for continuous miner operators when continuous mining machines had cabs. The need for a CAC on the continuous miner was eliminated when the cab was removed from the machine design. However, CAC development progressed to include designs for a roof bolting machine to protect roof bolters from respirable coal mine dust [11–13]. The original design of the CAC by the National Institute for Occupational Safety and Health (NIOSH) provided even airflow across the entire canopy and results from laboratory testing demonstrated reductions of respirable dust concentrations ranging from 67–75% in ventilation airflow ranging from 10–120 fpm [11].
Fletcher Mining Co. incorporated the CAC into their roof bolting machines but revised the design to have the airflow just around the perimeter of the canopy. Laboratory testing showed that this design only reduced the respirable dust concentrations by 17–24% [13]. In order to increase its effectiveness, Fletcher re-designed the air canopy based upon recommendations from NIOSH to provide airflow over the entire canopy, using a staggered perimeter airflow design with a higher flow rate that prevented contaminated air from entering the CAC protection zone. Laboratory testing demonstrated that this change increased the reduction from 17–24% to approximately 50% [14]. Field testing the roof bolter CAC in underground coal mines with this design showed respirable dust reductions ranging from 3–60% [15]. The variations in respirable dust reduction were due to differing ventilation airflows, differing heights the canopy was above the operator, differing times the operator actually worked underneath the CAC, and the low respirable dust concentrations encountered during the study.
The next step was to redesign the CAC for shuttle cars. This CAC is square in shape and utilizes uniform airflow across the canopy. Laboratory testing at NIOSH demonstrated reductions in respirable dust with different ventilation rates ranging from 74–83% at a ventilation rate of 0.61 m/s (120 fpm), 39–43% at a ventilation rate of 2.0 m/s (400 fpm), and 6–16% at a ventilation rate of 4.3 m/s (850 fpm). The higher the interference ventilation airflow, the lower the percent reduction in respirable dust [16].
1.1 Design of Diesel CAC for ANFO Loader
Since the CAC is successful for reducing respirable dust exposures, it was thought that it could be used to prevent DPM overexposures. The basic design of the shuttle car CAC was modified by researchers to use on an ANFO loader. However, there are some extra challenges when trying to reduce DPM instead of dust. The particles of DPM are smaller (submicron and nanometer) than dust particles (greater than 1 micron) and act more like a gas than particles. Therefore, one of the first adjustments is for the filtration system to capture submicron particles. A MERV 13 filter is currently used in the CAC for shuttle cars, but this filter is only designed to capture 50–75% of submicron particles. This capture efficiency is too low. Therefore, the initial tests were performed with a higher-rated filter: MERV 16. A MERV 16 rated filter was selected based upon past experience [17–19]. Higher-rated filter media such a HEPA grade can increase backpressure resulting in decreased airflow and induce leaks around the filter, thereby reducing the amount of protection from DPM to the miner. The MERV 16 rated filter has the ability to capture DPM particles at a high efficiency, while still allowing the required airflows to prevent contaminated air from entering the miner’s breathing zone. MERV 16 filters are designed to reduce submicron and nanometer particles by over 95% and have been shown to be effective for use in enclosed cabs [17–19].
Since blasters are one of the highest DPM-exposed working groups in mining, the CAC was designed to filter DPM and fit onto an ANFO loader. A diesel CAC was developed using the plenum design based on the one developed for the shuttle car CAC as seen in Fig. 2. This CAC was rescaled to fit under the canopy of an ANFO loader. This was accomplished by fabricating two 0.91-m by 0.91-m (3-ft by 3-ft) plenums, each with a series of openings at uniform spacing. Each plenum contains its own blower (rated at 1,800 cfm) and operates independently. Both plenums would be attached side by side on the canopy of the ANFO loader to cover the whole area of the ANFO platform/basket. The diesel CAC was designed to produce airflow similar to the shuttle car CAC (1.16 m/s at the borehole). Besides the size, there were a few other modifications from the shuttle car CAC. The shuttle car CAC was fabricated from metal, but the diesel CAC was made from a durable flame-retardant plastic for lighter weight. As mentioned earlier, the filtration was another difference in the diesel CAC, for a MERV 16 filter was used instead of the MERV 13. The blower passes air through a V-bank filter setup with a MERV 16 filter and 15.24 cm (6 in) tubing into the CAC.