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Worker Exposure to Silica during Hydraulic Fracturing

Why is silica a concern for workers during hydraulic fracturing? Recent NIOSH field studies identified overexposure to airborne silica as a health hazard to workers. Silica dust clouds from delivery trucks loading into sand movers Photo credit: NIOSH Silica dust clouds from delivery trucks loading into sand movers. Photo credit: NIOSH Large quantities of silica sand are used during hydraulic fracturing. Sand is delivered via truck and then loaded into sand movers, where it is subsequently transferred via conveyer belt and blended with other hydraulic fracturing fluids prior to high pressure injection into the drilling hole. Transporting, moving, and refilling silica sand into and through sand movers, along transfer belts, and into blender hoppers can release dusts containing silica into the air. Workers can be exposed if they breathe the dust into their lungs. NIOSH identified seven primary sources of silica dust exposure during hydraulic fracturing operations: ◾Dust ejected from thief hatches (access ports) on top of the sand movers during refilling operations while the machines are running (hot loading). ◾Dust ejected and pulsed through open side fill ports on the sand movers during refilling operations ◾Dust generated by on-site vehicle traffic. ◾Dust released from the transfer belt under the sand movers. ◾Dust created as sand drops into, or is agitated in, the blender hopper and on transfer belts. ◾Dust released from operations of transfer belts between the sand mover and the blender; and ◾Dust released from the top of the end of the sand transfer belt (dragon’s tail) on sand movers. An Overview of the "Fracking" Process The process known as "fracking" has long been used to extract oil from depleted wells. It is now widely used across the country to tap previously unreachable oil and natural gas locked within deep rock formations. 1. Well may be bored using directional drilling, a method that allows drilling in vertical and horizontal directions to depths of more than 10,000 ft. (3,048 m) 2. Large amounts of water, sand and chemicals are injected into the well at high pressure, causing fissures in the shale 3. Sand flows into the fissures, keeping them open so that the oil or natural gas from the shale can flow up and out of the well Graphic: Doug Stevens Copytright 2012, Los Angeles Times Reprinted with permission. Fracturing fluid is made up of a base fluid, proppant, and chemical additives. Water accounts for about 90 percent of the fracturing mixture and sand accounts for about 9.5 percent. Chemicals account for the remaining one half of one percent of the mixture. The base fluid applies pressure to the formation and delivers the proppant to the fractures. The base fluid is usually water, but can include methanol, liquid carbon dioxide, and liquefied petroleum gas. Proppant consists of particles that hold open the fractures created by hydraulic fracturing, allowing the oil and gas to flow out of the formation and into the well bore. Silica sand is frequently used as a proppant. Other proppants can include sintered bauxite or ceramics, and resin-coated sand. Chemical additives include friction reducers, scale inhibitors, solvents, acids, gelling agents, and biocides that are added to protect equipment, reduce pumping requirements, and maintain the integrity of the oil or gas formation. The OSHA general industry PEL for quartz, the common form of crystalline silica found in sand, is an 8-hour time-weighted average exposure to respirable dust calculated from the following formula. 10 PEL = (% Silica) + 2 The PEL is approximately equal to 0.1 mg/m3 for pure quartz silica. The PEL is outlined in 29 CFR 1910.1000 Table Z-3. If other forms of crystalline silica are present, the PEL calculation must be modified per Table Z-3. The NIOSH REL is a fixed value of 0.05 mg/m3. NIOSH Findings on Worker Exposures to Silica In cooperation with oil and gas industry partners, NIOSH collected 116 full shift air samples at 11 hydraulic fracturing sites in five states (Arkansas, Colorado, North Dakota, Pennsylvania, and Texas) to determine the levels of worker exposure to silica at various jobs at the worksites. Many air samples showed silica levels for workers in and around the dust generation points above defined occupational exposure limits.i Of the 116 samples collected: ◾47% showed silica exposures greater than the calculated OSHA PEL. ◾79% showed silica exposures greater than the NIOSH REL of 0.05 milligrams per cubic meter (mg/m3). ◾9% of all samples showed silica exposures 10 or more times the PEL, with one sample more than 25 times the PEL. ◾31% of all samples showed silica exposures 10 or more times the REL, with one sample more than 100 times the REL. Sand Transfer Operation Photo credit: NIOSH Silica dust by worker conducting sand transfer operations. Photo shows sand mover and transfer system. Photo credit: NIOSH Determining worker exposure levels is important for selecting the right type of control measures, including engineering controls and respiratory protection. For example, half-face respirators are not protective for silica levels over 10 times the exposure limit.

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