frackingHydraulic fracturing or “fracking”  is the process of injecting large volumes of water, sand, and chemicals into the ground at high pressure to break up shale formation allowing more efficient recovery of oil and gas. This form of well stimulation has been used since the late 1940s, but has increased substantially during the past 10 years with the advent of horizontal drilling technology that greatly improves access to gas deposits in shale. 

Approximately 435,000 workers were employed in the U.S. oil and gas extraction industry in 2010; nearly half of those workers were employed by well servicing companies, which includes companies that conduct hydraulic fracturing (BLS).1
To date, most of the attention on the safety and health implications of hydraulic fracturing has been related to impacts on the environment, primarily the potential for ground water contamination by hydraulic fracturing fluids. 

Although worker safety hazards in the oil and gas extraction industry are well known, there is very little data regarding occupational health hazards during hydraulic fracturing operations; for example, whether workers are exposed to toxic chemicals at hazardous concentrations. 

To investigate potential worker health hazards in this rapidly expanding industry and address the existing lack of information on occupational dust and chemical exposures associated with hydraulic fracturing, NIOSH initiated the NIOSH Field Effort to Assess Chemical Exposures in Oil and Gas Extraction Workers. Initial hazard assessments identified exposure to crystalline silica during hydraulic fracturing as the most significant known health hazard to workers and this has been the focus of the NIOSH study to date. 

Crystalline silica, in the form of sand (“frac sand”), plays a major role in the hydraulic fracturing process. Each stage of the fracking operation typically involves hundreds of thousands of pounds of “frac sand.” The sand is used as a proppant to hold open the fissures created by hydraulic fracturing and allow the gas to flow out of the shale into the well.  Moving, transporting and refilling thousands of pounds of sand onto and through sand movers, along transfer belts, and into blenders generates considerable dust, including respirable crystalline silica, to which workers can be exposed. 


Inhalation of fine dusts of respirable crystalline silica can cause silicosis.2 Silicosis is an incurable but preventable lung disease. Mortality statistics undercount silicosis cases. Still, death certificates document that an average of 162 individuals died annually from or with silicosis in the U.S. over the period 2000-2005.3 The disease typically develops after long periods of exposure and progresses gradually. However, rapidly fatal cases of acute silicosis resulting from very intense exposures over only a few months or years are well documented among sandblasters, tunnelers, miners, and some other occupational groups.2 Crystalline silica has also been determined to be an occupational lung carcinogen4,5 and there is evidence that inhaling respirable silica dust causes chronic obstructive pulmonary disease (COPD), chronic renal (kidney) disease and various autoimmune diseases. Individuals with silicosis are known to be at higher risk of tuberculosis and several other respiratory infections. 

Silica Dust Levels

NIOSH collected 116 air samples at 11 different hydraulic fracturing sites in five different states (AR, CO, ND, PA and TX) to evaluate worker exposure to crystalline silica.  At each of the 11 sites, full-shift personal-breathing-zone (PBZ) exposures to respirable crystalline silica consistently exceeded relevant occupational health criteria (e.g., the Occupational Safety and Health Administration (OSHA) Permissible Exposure Limit (PEL), NIOSH Recommended Exposure Limit (REL), and the American Conference of Governmental Industrial Hygienist’s (ACGIH) Threshold Limit Value (TLV®)).   At these sites, 54 (47%) of the 116 samples collected exceeded the calculated OSHA PELs; 92 of 116 (79%) exceeded the NIOSH REL and ACGIH TLV.  The magnitude of the exposures is particularly important; 36 of the 116 (31%) samples exceeded the NIOSH REL by a factor of 10 or more. The significance of these findings is that even if workers are properly using half-mask air-purifying respirators, they would not be sufficiently protected because half-mask air-purifying respirators have a maximum use concentration of 10 times the occupational health exposure limit. 

Based on these results, NIOSH concluded that an inhalation health hazard existed for workers exposed to crystalline silica at the evaluated hydraulic fracturing sites. NIOSH notified company representatives of these findings and provided reports with recommendations (listed below) to control exposure to crystalline silica. We recommend that all hydraulic fracturing sites evaluate their operations to determine the potential for worker exposure to crystalline silica and implement controls as necessary to protect workers. 

Based on workplace observations at each of the 11 hydraulic fracturing sites, NIOSH researchers identified seven primary points of dust release or generation from hydraulic fracturing equipment or operations. These included the following locations or equipment:  

Dust emitted from “thief” hatches (open ports on the top of the sand movers used to allow access into the bin)

Dust ejected and pulsed through side fill ports on the sand movers during refilling operations

Dust generated by on-site vehicle traffic, including sand trucks and crew trucks, by the release of air brakes on sand trucks, and by winds

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

Dust released from the top of the dragon’s tail (end of the sand transfer belt) on sand movers

Protecting Workers

Given the magnitude of silica-containing, respirable dust exposures measured by NIOSH, personal respiratory protection alone is not sufficient to adequately protect against workplace exposures. A combination of product substitution (where feasible), engineering, administrative, and personal protective controls, along with worker training, is needed to control workplace exposure to respirable silica during hydraulic fracturing. Working with industry partners, NIOSH researchers have identified the following controls, some simple, and some more complex, that can be implemented in a variety of ways.

Use a less hazardous non-silica proppant (e.g., ceramic) where feasible.

Use local exhaust ventilation for capture and collection. Cyclones dust collectors and a portable baghouse connected to thief hatches  can capture dusts as they are generated. NIOSH researchers have developed two conceptual phase controls for this source of dust generation. The first is a mini-baghouse assembly that could be retro-fitted over the existing thief hatch openings. The baghouse takes advantage of the positive pressure generated by sand filling which inflates the bag and dust control is achieved as a filter cake develops on the inside the baghouse fabric. The design is envisioned to be self-cleaning as the filter cake would fall back into the sand container as the fabric collapses when air pressure is released after bin filling.

Use passive enclosures at points of dust generation.  Install stilling curtains (also called staging curtains) around the bottom sides of the sand movers to limit dusts released from belt operation. Stilling curtains can be made of clear thick plastic (including heavy plastic strips) or other appropriate materials to contain dusts. Enclosures can also be considered along and at the ends of the sand transfer belt (dragon tail).

Minimize distances between the dragon tail and T-belts and blender hoppers. Minimizing the distance that sand falls through the air can help minimize dust generation.

Replace transfer belts with screw augers on sand movers. This involves Prevention-through-Design considerations for engineers and equipment designers when new sand movers are manufactured or are rebuilt and will require more extensive engineering and mechanical retrofitting. NIOSH has an active program that encourages Prevention-through-Design considerations so that occupational health and safety aspects (such as dust control) are built into equipment during the design phase.

Use amended water (e.g., containing chloride and magnesium salts) to reduce dust generation on roads into and at the well site. Do not use well brines for dust control.

Mandate use of cam-lock caps for fill ports on sand movers. When sand mover bins are being filled, sand dust is pulsed from the fill port on the opposite side of the sand mover. Mandating that cam lock caps be secured in place can help minimize dust generation.

Use administrative controls. Limit the number of workers, or the time workers must spend, in areas where exposure to high concentrations of silica can occur.  Consider options for remote operations to remove employees from areas where exposures can occur.

Provide worker training. Hydraulic fracturing workers should be trained on the hazards of crystalline silica and the steps they should take to limit dust generation and reduce the potential for exposure.

Monitor workers to determine their exposure to crystalline silica. Conduct PBZ air sampling on workers engaged in activities where “frac” sand is used.  Documenting worker exposures is important to verify the need for controls, determine the efficacy of controls that have been implemented, and ensure that the appropriate respiratory protection is used as an interim control until engineering controls can be implemented.  This information is also useful for worker training and informing workers about their exposures. Employers should consult with an occupational safety and health professional trained in industrial hygiene to ensure an appropriate sampling strategy is used.

Use appropriate respiratory protection as an interim measure until engineering controls are implemented. As discussed above, a half-mask air-purifying respirator may not provide sufficient protection. As an interim measure until engineering controls are implemented and evaluated, a higher level of respiratory protection should be used. Employers should consult with an occupational safety and health professional (industrial hygienist) to determine the appropriate respirator to be used. Employers should establish a comprehensive respiratory protection program that adheres to OSHA regulations (CFR 29 1910.134) and ensure that workers who wear respiratory protection are medically cleared, properly trained and fitted, and are clean shaven each day. The NIOSH policy on respiratory protection for crystalline silica can be found at: NIOSH guidance for selecting respirators can be found at

The NIOSH document Best Practices for Dust Control in Metal/Nonmetal Mining discusses dust control in underground mining operations. Research results from this document have direct relevance for minerals handling operations in hydraulic fracturing operations.

Help Wanted

As noted above, NIOSH is designing conceptual engineering controls to minimize exposure to silica during hydraulic fracturing. NIOSH hopes to have a working prototype in the next month and is looking for industry partners to help us test this engineering control. If you are interested, please contact us via the blog comment box below or by e-mail at NIOSH is also looking for additional partners in drilling and well servicing to work with us to evaluate worker exposures to other chemical hazards and develop controls as needed. Other potential workplace exposures can include hydrocarbons, lead, naturally occurring radioactive material (NORM) and diesel particulate matter which have not been fully characterized. Please refer to the document NIOSH Field Effort to Assess Chemical Exposure Risks to Gas and Oil Workers for details and contact us if you have questions or wish to participate. 


BLS, Quarterly Census of Employment and Wages:

Davis GS [1996]. Silica. In: Harber P, Schenker MB, Balmes JR, eds. Occupational and environmental respiratory disease. 1st ed. St. Louis, MO: Mosby—Year Book, Inc., pp. 373–399.

National Occupational Respiratory Mortality System (NORMS).

NIOSH Hazard Review, Health Effects of Occupational Exposure to Respirable Crystalline Silica.

National Toxicology Program [2012]. Report on carcinogens 12th ed. U.S. Department of Health and Human Services, Public Health Service.