Exposure risks to worker health at unconventional oil and gas extraction sites can include chemicals and inhalation of respirable crystalline silica (RCS). National Institute for Occupational Safety and Health (NIOSH) researchers have identified concentrations of RCS at hydraulic fracturing sites that can be 10-50 times higher than the Occupational Safety and Health Administration (OSHA) Permissible Exposure Limit (PEL). NIOSH research has identified at least 7 point sources where silica dust is generated. Engineering controls have been proposed by NIOSH to limit dust generation at some of the known locations of most concern. One of these controls is a NIOSH-developed mini-baghouse retrofit assembly (NMB), to be mounted on the thief hatches on top of the sand mover, one of the largest sources of dust generation. This report details the results of a trial of the 3rd generation of the NMB that occurred at Southwestern Energy (SWN) Sand Company in North Little Rock, Arkansas, May 19 - 21, 2015. This trial is a follow-up to the 2013 test of the 2nd generation of the technology [Alexander et al. 2016

NIOSH 2015]. Area air samples were collected at 12 locations on and around a sand mover with and without the mini-baghouse retrofit assembly in place. Samples were analyzed for respirable dust and RCS. Improvements made to the design of the mini-baghouse retrofit assembly enhanced performance relative to the generation 2 version. Results indicate that the mini-baghouse retrofit assembly effectively reduced both respirable dust and RCS downwind of the thief hatches. Reductions of airborne respirable dust at the three locations with the highest uncontrolled concentrations were all greater than 99% (all controlled concentrations were less than the limit of detection), and reductions in airborne RCS ranged from 98% to greater than 99%. Measurements of the static pressure inside the bags remained low throughout filling of the sand mover, avoiding the need to suspend sand transfer and manually shake filter bags. Analysis of a bulk sample of the dust collected by the baghouse assembly during this trial showed the presence of silicon, silica monoxide and the silica dioxide radicals which are indicators of freshly fractured quartz, a particularly hazardous form of RCS. Design enhancements are proposed to provide weather resistance and ease of clamping of the mini-baghouse. Future trials are planned to determine the efficacy of these design changes.