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.
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.
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.
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