Evaluation of dust exposure to truck drivers following the lead haul truck

Haul trucks have the potential to generate large amounts of respirable dust. This respirable dust has been shown to be a health hazard to personnel, especially if it has a high silica content. Lack of dust sampling data from haul trucks prompted the completion of a study to quantify respirable dust concentrations. These field studies were conducted measuring instantaneous respirable dust at stationary locations from passing haul trucks at a stone quarry and at a coal mine site. Fugitive dust levels from haul trucks were analyzed to characterize dust dispersion. This analysis indicates safe following distances for haul trucks and other heavy equipment to avoid overexposure of respirable dust from the lead haul truck. Introduction Almost all surface mining operations use mining haul trucks. These trucks are used to move material within or from the mining property. Past research, using the U.S. EPA’s emissions factors for unpaved haul roads, has shown that haul trucks generate the majority of dust emissions from surface mining sites, approximately 78% to 97% of total dust emissions (Cole and Zapert, 1995; Amponsah-Dacosta and Annegarn, 1998; Reed et al., 2001). Observations of dust emissions from haul trucks show that if the dust emissions are uncontrolled, they can be a safety hazard by impairing the operator’s visibility. Reducing the operator’s visibility increases the probability for haul truck accidents. However, the greatest long-term health hazard of dust generated from hauling operations is due to inhalation of respirable dust, which contains dust particles whose median diameter is 4.0 μm and PM10, which is the size fraction whose medium diameter is 10 μm (Soderholm, 1989; ISO, 1993; Lippmann, 1995; U.S. GPO, 2002). Exposure to respirable dust has long been considered a health hazard at surface mining operations, especially if silica dust is present. Respirable dust containing silica has caused more than 250 deaths annually (MSHA, 1997). There is also the potential for 106,000 to 182,000 mine production, development and exploration workers to be exposed to respirable dust from surface mining operations (U.S. Census Bureau, 2001; NIOSH, 2002). The large range in the number of workers results from differences in the statistical databases from the U.S. Census Bureau and the Mine Safety or Health Administration (MSHA). An evaluation of a recent MSHA database of respirable dust samples containing silica from 1996 through 2000 shows the exposure for truck drivers and road grader operators. At surface coal mines, approximately 10% of the dust samples taken exceeded the exposure limit for silica dust for truck drivers. For road grader operators, that rate was approximately 5% (Hale, 2002). For stone mining operations, the number of respirable dust samples taken that exceeded the exposure limit for silica dust was approximately 5% for truck drivers. The overexposure rate was approximately 29% for road grader operators (Hale, 2002). At nonmetal mines, the rates were approximately 5% and 14% for truck drivers and road grader operators, respectively (Hale, 2002), and the rates for metal mining operations were approximately 8% and 3% for truck drivers and road grader operators, respectively (Hale, 2002). Although the health and safety concerns for dust overexposure focus on respirable dust, PM10 has also been shown to represent a health hazard. Many epidemiologic studies have been completed that show that PM10, by itself, causes harm to humans. A 50 μg/m3 increase in the 24-hour average PM10 concentration was statistically significant in increasing mortality rates by 2.5% to 8.5% (U.S. EPA, 1996). Long-term effects from PM10 are dependent on the exposure to PM10 over the life of the worker. Of particular concern is the use of haul trucks outside of the mining industry. There are many off-road and over-the-road haul trucks that are used in the construction industry. Many of the over-the-road haul trucks enter quarry sites to obtain material to deliver it to construction sites, and these trucks generally use the mine site haul roads. Some of these quarry sites have a high traffic volume of over-the-road trucks. These operators can be exposed to high respirable silica dust concentrations during their time at the quarry site. Even though their time at the quarry site may be minimal, they may return several times during the day to the quarry for material thus,