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In-depth survey report: respirable crystalline silica exposure during pavement milling using a Caterpillar milling machine equipped with a local exhaust ventilation system.
Hammond DR; Shulman SA; Hein MJ
Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, EPHB 282-26a, 2014 Sep; :1-31
Between July 23rd and September 25th, 2013, National Institute for Occupational Safety and Health (NIOSH) researchers and the Silica/Milling-Machines Partnership coordinated by the National Asphalt Pavement Association (NAPA) conducted field testing of a local exhaust ventilation (LEV) system on a Caterpillar PM200 cold milling machine. The tests included nine days of air sampling across three different highway construction sites in Minnesota. At each site, full-shift personal breathing zone samples for respirable crystalline silica were collected from the operator and ground man during normal work activities of asphalt pavement milling. The data were analyzed two ways, (1) assuming the data were normally distributed and (2) assuming that they were lognormally distributed. For each distribution, a 97.5% upper confidence limit for the arithmetic mean respirable crystalline silica exposure for each occupation was calculated and compared to the NIOSH recommended exposure limit (REL) of 0.05 mg/m3. For results on either scale, 97.5% upper confidence limits are chosen for each occupation in order that the combined confidence is 95%. For the normal distribution analysis, the arithmetic mean respirable crystalline silica exposure for the operator was 0.052 mg/m3 with an upper 97.5% confidence limit of 0.071 mg/m3. The arithmetic mean respirable crystalline silica exposure for the ground man was 0.037 mg/m3 with an upper 97.5% confidence limit of 0.055 mg/m3. For the lognormal distribution analysis, the arithmetic mean respirable crystalline silica exposure for the operator was 0.052 mg/m3 with an upper 97.5% confidence limit of 0.083 mg/m3. The arithmetic mean respirable crystalline silica exposure for the ground man was 0.038 mg/m3 with an upper 97.5% confidence limit of 0.061 mg/m3. The 95% upper confidence limits for the arithmetic means for the operator and ground man are above the REL. For either analysis method it cannot be stated that the arithmetic mean exposures were below the REL for the population of sites from which those studied were chosen. Based on the results of this study, NIOSH researchers recommend that Caterpillar consider refining their design to prevent clogging of the duct system before conducting additional field testing of the LEV dust controls. A possible solution to prevent clogging would be to further increase the open area at the intake to the LEV system so that the air intake velocity is lower without reducing the total volumetric flow-rate of air through the system. A lower intake air velocity should reduce the number of particles larger than the respirable size range of 10 microm from being drawn into the LEV system while keeping the drum housing and primary conveyor under negative pressure. With these suggestions or other modifications to prevent clogging of the LEV dust controls, NIOSH researchers recommend that Caterpillar consider conducting additional field testing to verify that their final dust control design will reduce worker exposures below the NIOSH REL. The recommendations in this report are based on past successful dust control studies and would not prevent Caterpillar from pursuing other technologies, ideas, or inventions to reduce silica exposures on asphalt milling machines.
Region-5; Control-technology; Engineering-controls; Exhaust-ventilation; Ventilation-systems; Construction-equipment; Air-sampling; Road-construction; Road-surfacing; Breathing-zone; Silica-dusts; Respirable-dust; Exposure-assessment; Exposure-levels; Exposure-limits; Employee-exposure; Equipment-design; Machine-operation; Machine-operators; Air-flow; Dust-control; Dust-control-equipment; Volumetric-analysis; Asphalt-industry; Milling-industry; Author Keywords: Silica; Asphalt; Engineering Control
Field Studies; Control Technology
NTIS Accession No.
National Institute for Occupational Safety and Health
Page last reviewed: April 12, 2019
Content source: National Institute for Occupational Safety and Health Education and Information Division