WORKER EXPOSURE TO CRYSTALLINE SILICA DURING HYDRAULIC FRACTURING
Hydraulic
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.
Silicosis
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: http://www.cdc.gov/niosh/docs/2008-140/.
NIOSH guidance for selecting respirators can be found at http://www.cdc.gov/niosh/docs/2005-100/default.html.
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 nioshblog@cdc.gov. 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.
References
BLS, Quarterly Census of Employment and Wages: http://www.bls.gov/cew/.
BLS, Quarterly Census of Employment and Wages: http://www.bls.gov/cew/.
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). http://webapp.cdc.gov/ords/norms.html
NIOSH
Hazard Review, Health Effects of Occupational Exposure to Respirable
Crystalline Silica. http://www.cdc.gov/niosh/docs/2002-129/pdfs/2002-129.pdf.
National
Toxicology Program [2012]. Report on carcinogens 12th ed. U.S. Department of
Health and Human Services, Public Health Service. http://ntp.niehs.nih.gov/?objectid=03C9AF75-E1BF-FF40-DBA9EC0928DF8B15
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