Workers' Silica Exposure at Fracking Sites Far Exceeds OSHA Limit, NIOSH Study Finds

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By Robert Iafolla  

The concentration of silica in the air workers breathe exceeded occupational health criteria at all 11 hydraulic fracturing sites tested by the National Institute for Occupational Safety and Health, the American Industrial Hygiene Association announced July 31.

Researchers measured the silica levels of more than 100 personal breathing zone samples at fracking sites in five states, finding levels above the Occupational Safety and Health Administration's permissible exposure limit (PEL), NIOSH's recommended exposure limit (REL), and the American Conference of Governmental Industrial Hygienists threshold limit value (TLV).

In some instances, the samples exceeded OSHA's PEL by a factor of 10 or more. That level of exposure is enough to overwhelm the maximum use concentration ratings for the half-mask, air-purifying respirators that workers typically wore, the study said.

“Although effective engineering controls for crystalline silica are well established in other industries, controls to limit silica-containing dust generation during hydraulic fracturing are only now emerging due to the relatively recent understanding of the hazard and magnitude of exposure risks,” the study authors wrote.

The study appears in the Journal of Occupational and Environmental Hygiene, published jointly by AIHA and ACGIH.

Silica Exceeds Limits

NIOSH first flagged the occupational hazard of silica exposure at fracking sites in a June 2012 Hazard Alert. The journal article describes the research that informed that alert (42 OSHR 576, 6/28/12).

Silica sand is a crucial and common component in many fracking operations. Silica is often mixed with the water and chemicals injected into shale formations during fracking, with silica acting as a “proppant” to keep the underground fractures open to allow oil or natural gas to flow. Approximately 28 million metric tons of silica sand was used in fracking during 2012, according to the U.S. Geological Survey.

The heavy use of silica is reflected in the air contamination at fracking sites. Just over half of the 111 samples that NIOSH tested exceeded the PEL for silica, while 69 percent exceeded the REL, and 84 percent exceeded the TVL.

OSHA's PEL is calculated from the percent of crystalline silica in the respirable dust; the PEL for 100 percent silica would be 0.1 milligram per cubic meter of air. NIOSH's REL is 0.05 milligram per cubic meter, and the ACGIH's TVL is 0.025 milligram per cubic meter.

OSHA's proposed regulations for silica could lower the PEL to 0.05 milligram per cubic meter, although that has been met with fierce resistance from industry. The proposal has been on hold at the White House Office of Management and Budget since February 2011.

Five Shale Formations

Researchers took samples at fracking sites at five shale formations: Bakken in North Dakota, DJ Basin in Colorado, Eagle Ford in Texas, Fayetteville in Arkansas, and Marcellus in Pennsylvania. The fracking operations at those locations used silica as the proppant, with the exception of the Bakken site, where 60 percent of the proppant was a ceramic material.

None of the 10 samples taken at Bakken exceeded OSHA's PEL. Two-thirds of the samples at Marcellus, 54 percent at Fayetteville, 53 percent at DJ Basin, and 50 percent at Eagle Ford went over the PEL.

Typical fracked gas or oil wells go through 12 to 20 stages, with each stage requiring hundreds of thousands of pounds of sand, creating airborne dust at the sites, the study said.

Sand is delivered from tractor trailers and pumped with compressed air through fill ports into on-site storage and delivery vehicles known as “sand movers.” Those sand movers use motor-driven belts called “dragon tails” to supply sand to blender trucks, with larger loads requiring multiple vehicles moving sand across a transfer belt. Blender hoppers pump the sand-as-proppant through a manifold, piping, and into the well.

Workers operating sand movers and transfer belts are exposed to the highest levels of silica, the study said.

Sites of Dust Generation

Overall, the study found seven points of dust generation that were common at all 11 sites:

• from “thief hatches” on the tops of sand movers during filling,

• from the sand mover belt,

• from the momentum of falling sand below the dragon tail at the blender hopper,

• from transfer belts when sand is deposited onto the belt and conveyed to the blender,

• from sand leaving the end of the dragon tail,

• from fill ports of sand movers during refilling operations, and

• from truck traffic at the fracking site.

The study said that about 435,000 workers were employed in the U.S. oil and gas extraction industry during 2010, with nearly half employed by well servicing companies, including companies that use fracking.

By Robert Iafolla  

The study “Occupational Exposures to Respirable Crystalline Silica During Hydraulic Fracturing” is available at

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