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 282-17a, 2011 Sep; :1-81
The process of milling asphalt- and concrete-paved surfaces undergoing maintenance or rehabilitation produces dust that usually contains crystalline silica, a natural constituent of most asphalt- and concrete-pavement mixes. Inhalation of respirable crystalline silica is a well-documented workplace hazard, with chronic overexposures causing silicosis and increasing the risk of lung cancer. Preliminary NIOSH field research from 2004 through 2006, in which industrial hygiene surveys of asphalt pavement-milling operations on highway resurfacing jobs were conducted, suggested that milling machines' existing water-spray dust-suppression systems are not consistently effective enough to adequately control exposures among workers conducting asphalt milling. Based on these findings, NIOSH researchers and the Silica/Milling-Machines Partnership, coordinated by the National Asphalt Pavement Association (NAPA), determined a need for improved dust emission-control systems for pavement-milling machines. In 2007, NIOSH mining engineers used their experience with related coal mining equipment to recommend preliminary design guidelines for improved emission-control systems, and the milling-machine manufacturers in the Partnership then developed prototypes incorporating these preliminary guidelines. In 2008, the current field study was conducted at a former commercial airport in Marquette, Michigan, to evaluate the dust emission-reduction performance of these prototype systems. Four manufacturers brought machines equipped with their prototype dust emission-control systems to this site for testing, two in June 2008 and two in September 2008. Runways of this former airport site provided a controlled test environment since no other activities were occurring nearby at the time, and milling tests could be conducted as needed for the study. The tests consisted of numerous, replicate short-term milling trials (nominally about 10 minutes each in duration) . During a trial, a test milling machine removes approximately 2 inches of depth of the asphalt surface of the runway while operating either its existing, production water-spray system (the "baseline configuration") or one of its modified test emission-control configurations. During the trials, respirable-dust concentrations were measured at ten selected locations around each mill using continuous "real-time" data-logging dust monitors. The trials involving each test mill were divided into sets, with each set including one trial of the mill's baseline configuration and one each of its modified configurations. The results from six key monitoring locations (among the ten) together are considered to best represent relative dust-emission rates, because they surround the relatively low-to-the-ground dust-generating areas of the machine where the modified emission controls are located. Trial-mean concentrations from these "lower six" locations were averaged together to obtain a single lower-six-location average for that trial. The average lower-six result for the baseline dust-control configuration in a set was compared with that for each of the modified test configurations in that set, and dust-emission reductions computed for each test configuration versus the baseline in that set. Average reductions across all sets for that machine were computed, along with their statistical confidence intervals. Promising results were obtained for some modified dust emission-control systems. One test configuration, which included additional water-spray nozzles oriented counter to the material flow in the primary conveyor area near the cutter-housing discharge, yielded an estimated, statistically significant reduction of 43% to 55% (depending on data-set selection) in respirable-dust concentrations at the lower-six locations compared to those for the baseline configuration on the same mill. Another, which used a fan and ductwork to place the cutter housing and its discharge area under negative static pressure and evacuate dust-laden air to the top of the secondary conveyor boom, yielded an estimated, statistically significant reduction in dust emissions of about 60%. In most cases, these test configurations did not result in statistically significant reductions of a similar magnitude in dust concentrations at the remaining four monitoring locations, which were evaluated as two location groups (two conveyor-top locations and two operator-bridge locations). However, those four locations are farther from the dust-generating areas targeted by the test emission controls, and concentrations at those locations generally are appreciably lower. These facts possibly make differences in dust levels less evident. They also raise the possibility that differences in dust concentrations at these locations may not be as directly affected by changes in dust-emission rates as those at the lower-six locations, and affected relatively more by changes in ambient dust levels and ambient conditions like wind. NIOSH researchers and Partnership members concluded from these results that further optimization of the dust emission-control systems followed by an additional set of field tests should be undertaken. This will allow for confirmation of the effectiveness of the most successful design elements and the opportunity to further improve and evaluate dust emission-control performance. Following successful completion of these additional controlled field tests, plans call for conducting industrial-hygiene field surveys to measure workers' full-shift personal breathing-zone exposures to respirable crystalline silica during the use of each manufacturer's optimized dust emission-control system.
Region-5; Control-technology; Engineering-controls; Respirable-dust; Dust-control; Construction; Road-construction; Road-surfacing; Machine-operation; Machine-operators; Silica-dusts; Concretes; Control-methods; Control-systems;
Author Keywords: Asphalt-milling; Respirable-dust; Respirable-silica; Crystalline silica; Quartz; Road construction
National Institute for Occupational Safety and Health, Division of Applied Research and Technology, Engineering and Physical Hazards Branch, Mail Stop R-5 ,4676 Columbia Parkway, Cincinnati, OH 45226-1998