Document ID: OSHA-2010-0034-1628
Agency: osha
Document Type: Supporting & Related Material
Title: 
Posted Date: 2013-07-11T04:00Z

PEA Appendix X: Technological Feasibility of the Proposed Rule on Crystalline Exposure in the Gas and Oil Extraction Industry (Hydraulic Fracturing)
This appendix on hydraulic fracturing in the gas and oil extraction industry is a supplement to the Preliminary Economic Analysis for OSHA's proposed rule on occupational exposure to crystalline silica. It covers respirable crystalline silica, hereinafter termed "silica." 
Methodology
Defining "Silica" Data
Unless specifically indicated otherwise, all silica exposure data, samples, and results discussed in this technological feasibility analysis refer to measurements of personal breathing zone (PBZ) respirable crystalline silica. The term "respirable crystalline silica" is used as defined in the proposed rule (see "Definitions").
Data Sources and Source Characteristics
General information on the data sources and source characteristics are discussed in Section IV.A -- Methodology of the Preliminary Economic Analysis. Details regarding the data sources used for this supplemental appendix are presented below under the heading "Baseline Conditions and Exposure Profile."
Methods to Assess Feasibility of Control Technology  
Exposure profiles were developed by job category. OSHA analyzed the distribution of silica exposure data for each job category involved in hydraulic fracturing operations, drawing information from sources such as NIOSH site visits, trade and industry organizations, site visits, and peer reviewed journals, and supplemental sources. 
All results in the general industry exposure profiles are 8-hour time-weighted average (TWA) PBZ samples collected over periods of 360 minutes or more (for the purposes of this analysis, defined as "full-shift"). To determine an 8-hour TWA, the exposure level for the period sampled is assumed to have continued over any unsampled portion of the shift. OSHA has preliminarily determined that this sample criterion is valid because workers in general industry are likely to work at the same general task or repeating set of tasks over most of their shift; thus, unsampled periods generally are likely to be similar to the sampled periods.
For additional information on the methodologies used for this analysis, please consult Section IV.A -- Methodology of the Preliminary Economic Analysis.  
Gas and Oil Extraction Industry (Hydraulic Fracturing)
Description
Hydraulic fracturing, sometimes called "fracking," is a process used to extract natural gas and oil deposits from shale and other tight geologic formations. The process begins once well drilling is complete. Workers in the oil and gas industry (NAICS 213112 -- Support Activities for Oil and Gas Operations) pump fracturing fluid, composed of base fluid (usually water), a proppant (usually sand), and chemical additives, into the new well bore under extremely high pressures (e.g., 7,000 psi to 9,000 psi) (Esswein, 2012). The high pressure fractures the shale or rock formation, allowing the natural gas trapped in the formation to flow into the well. Workers mix large quantities of sand into the fracturing fluid. The sand acts as a proppant to hold the fractures in the shale formation open after the pressure is released. Use of this process has increased significantly in recent years due to new horizontal drilling and multistage hydraulic fracturing technologies that improve access to natural gas and oil deposits.
When silica sand is used as a proppant, high concentrations of respirable silica dust can become airborne as workers deliver, convey, and mix the sand with fracturing fluid. An enormous quantity of proppant is involved in the hydraulic fracturing process; each lateral drilling zone radiating from the vertical well bore requires 190,000 to 300,000 lbs of sand. A vertical well might serve several horizontal zones, each of which is treated sequentially by hydraulic fracturing (involving approximately a half-day of active pumping per zone). More than one vertical well can be drilled at one well pad, and the amount of personnel, sand, equipment, and activity at a site increases when multiple crews hydraulically fracture multiple wells at the same time.
The hydraulic fracturing process generally proceeds as follows. Sand truck drivers deliver sand to the site and pneumatically pump it from trucks into large pieces of equipment (sand movers) that store sand. Workers regulate the flow of sand out of the sand mover onto a series of associated conveyor belts, which carry the sand to a hopper from which the sand is metered into a blender. The sand, water, and chemical additives are mixed together in the blender before the sand-laden fracturing fluid is pumped through a high-pressure manifold into the well. This final step does not contribute to worker silica exposures because sand is both wet and contained in an enclosed system by this stage; however, up to this step, respirable silica emissions occur at numerous points as the dry sand is moved from the trucks to the sand movers to the conveyor belts to the blender hopper. 
Hydraulic fracturing crews work as a team that travels from well site to well site. Individual workers are specialized and have defined roles. Those whose jobs keep them in the central area near the sand-handling equipment can experience extremely high levels of respirable silica exposure. Ancillary workers who have work locations on the perimeter can experience elevated silica exposures, although they are not in the immediate vicinity of the dust emissions. Workers whose jobs take them into the central work area only intermittently or not at all experience variable exposure depending on the amount of time they spend near dusty activities. Table IV.C-A1 provides information on these job categories and their sources of exposure.
Baseline Conditions and Exposure Profile
OSHA reviewed the best available exposure monitoring data, consisting of six NIOSH reports on hydraulic fracturing sites. Between 2010 and 2011, NIOSH visited 11 hydraulic fracturing worksites in five states (seven sites in Colorado and individual sites in Arkansas, North Dakota, Pennsylvania, and Texas) (OSHA-NIOSH Alert, 2012). NIOSH collected full-shift air samples to determine the levels of worker exposure to respirable silica on the work sites.
NIOSH spent three days at each of five sites to obtain PBZ air samples and some area samples (OSHA-NIOSH Alert, 2012). Conditions varied between the sites. NIOSH collected samples in all four seasons, with temperatures ranging from 30º to 113º Fahrenheit and at elevations ranging from 246 feet to 4,813 feet.  Well sites included single stage "re-fracs" (rejuvenating old wells), multistage hydraulic fracturing, and "zipper-fracs" (multiple parallel wells) (OSHA-NIOSH Alert, 2012). 
Respirable dust at these sites contained a relatively high percentage of silica. Among the 88 samples for which this information is available, more than half had greater than 41 percent silica in the sample (with a range of 6 to 100 percent silica). Exposure controls were largely absent during the monitoring periods, representing what are characterized here as baseline conditions. An exception was NIOSH's Site 6, at which a granular ceramic media containing 1 percent silica replaced half of the silica sand proppant (NIOSH HF-Site 6, 2011). 
The following sections describe the baseline conditions, and Table IV.C-EP summarizes the exposure information for the affected job categories. 
Because few controls were in use at the time of the NIOSH visits, and industry work practices have been modified somewhat since that time, OSHA seeks additional information to update both the exposure profile and information related to controls.

                                 Table IV.C-A1
     Job Categories, Major Activities, and Sources of Exposure of Workers
in the Industry Providing Support Activities for Oil and Gas Operations (NAICS 213112)
                                       
Job Category*
Major Activities and Sources of Exposure
Fracturing Sand Workers, in the Central Area (e.g., sand mover operator, conveyer belt tender, blender tender, water operator, pump truck operator)
 Operate and tend equipment in the central sand-handling area on hydraulic fracturing sites 
Dust ejected from the thief hatches on the top of the sand movers. 
Dust released from the conveyor belt under the sand movers. 
Dust created as sand drops into or is agitated in the blender hopper.
Dust released from conveyor belt operation. 
Sand released at the top of the end of the sand belt on the sand movers. 
Dust ejected from the side fill ports on the sand movers. 
Ancillary Support Workers
(e.g., chemical truck operator, hydration unit operator)
 Operate or tend equipment that is at a fixed location on the perimeter or slightly removed from the central sand-handling area, such as chemical trucks and hydration units.
Dust disbursed from processes operated by fracturing sand workers in the central sand-handling area.
Sand and aggregate on the ground, crushed by heavy equipment and disturbed by passing vehicles.
Accumulated dust in vehicle and equipment cabs occupied by drivers and operators.
Remote/Intermittent Support Workers   (e.g., roving operator, ground guide, sand coordinator, mechanic, QA technician, fueler, wire-line crew)
 Active over a wide area of the site, primarily outside the central sand handling area, but may include brief, occasional excursions into the central sand-handling area. These workers may spend time at a primary base location (truck, trailer) away from sand-handling.
Dust disbursed from processes operated by fracturing sand workers in the central sand-handling area.
Sand and aggregate on the ground, crushed by heavy equipment and disturbed by traffic on the site.
Dust released inside trailer while QA/QC techs sieve sand to check sand quality (QC technician only).
*Job categories are intended to represent job functions; actual job titles may differ and responsibilities may be allocated differently, depending on the facility.

Sources: NIOSH HF-Site 1, 2010; NIOSH HF Site-2, 2011; NIOSH HF Site-3, 2011; NIOSH HF Site-4, 2011; NIOSH HF Site-5, 2011; NIOSH HF Site-6, 2011.

Baseline Conditions for Fracturing Sand Workers
OSHA reviewed 51 exposure results for fracturing sand workers from the six NIOSH reports on hydraulic fracturing sites. The exposure profile, provided in Table IV.C-EP, shows a full-shift mean exposure of 464 ug/m[3], a median of 330 ug/m[3], and range of 10 to 2,570 ug/m[3] for this group of workers. Seventy-five percent of the sample results in this job category exceed the current PEL of 100 ug/m[3] and more than half (27 of 51 samples) exceed 250 ug/m[3]. Within this most highly exposed group, 19 samples (37 percent of the 51 total samples) are greater than 250 ug/m[3], but do not exceed 820 ug/m[3] (820 ug/m[3] is the highest exposure level of any worker in the other job categories at hydraulic fracturing sites).
One of the highest sample results (2,000 ug/m[3]) was collected on a worker at the bottom operator station on a sand mover at a site where "hot loading" occurred and where sand contained a high percentage of silica (most respirable dust samples in which silica was detected contained 50 to 100 percent quartz) (NIOSH HF-Site 3, 2011). The worker with the highest full-shift sample result (2,570 ug/m[3]) tended sand conveyer belts in hot, dry, breezy weather at a location where respirable dust samples contained 30 to 65 percent quartz (NIOSH HF-Site 1, 2010). At both sites, extremely high silica exposure levels were associated with worker positions immediately down-wind of points from which sand dust was released (e.g., thief hatches, conveyers, sand hoppers). These fracturing sand workers wore either filtering facepiece or half-facepiece respirators.
NIOSH documented baseline conditions for fracturing sand workers, which included largely uncontrolled work processes using dry sands from various sources. The work typically occurs at sites that frequently contain numerous trucks, sand movers, and related large equipment that block natural breezes that might otherwise create some air exchange in the area where dust is released in the highest concentration (Esswein, 2012; Rader, 2012). An alternative proppant (e.g., ceramic media) is used occasionally at sites where conditions benefit from the proppant's unique properties (e.g., strength, shape, size, uniformity). The exposure profile represents fracturing sand worker exposure on sites operating under these baseline conditions.
Baseline Conditions for Ancillary Support Workers
The six NIOSH reports on hydraulic fracturing sites also contain exposure data (six samples) for ancillary support workers. Half of the samples exceeded the current PEL of 100 ug/m[3], while the remaining samples were 50 ug/m[3] or less. The median exposure level for this job category is 142 ug/m[3], with a mean of 243 ug/m[3] and range of 9 ug/m[3] to 820 ug/m[3].
Ancillary support worker baseline conditions are also documented by the NIOSH reports. Workers in this job category work at fixed positions just outside the central sand-handling area. The primary sources of exposure for ancillary support workers are the processes controlled by the fracturing sand workers (Esswein, 2012). Variable wind and weather conditions carry airborne silica from the central work area, where controls are largely absent, causing bystander exposure for ancillary support workers (Esswein, 2012). Silica dust accumulated in the vehicle cabs and silica-containing sand and aggregate crushed on the ground by passing heavy equipment contribute to ancillary support worker exposure whenever these sources are disturbed. The exposure profile, based on NIOSH's reports, represents ancillary support worker exposure on sites operating under these baseline conditions.
Baseline Conditions for Remote/Intermittent Support Workers   
The six NIOSH reports provide 26 sample results for remote/intermittent support workers, who typically had lower daily exposures compared to fracturing sand workers and ancillary support workers. The remote/intermittent support worker exposures are characterized by a median of 51 ug/m[3], a mean of 88 ug/m[3], and a range of 6 ug/m[3] to 630 ug/m[3]. Overall, 11 samples (42 percent) are 50 ug/m[3] or less, another 11 (42 percent) are greater than 50 ug/m[3] but no greater than 100 ug/m[3], and four samples (16 percent) exceed 100 ug/m[3]. Among the workers in this job category, only those serving as ground guides periodically experienced exposures greater than 100 ug/m[3] (4 samples, or 18 percent of the 22 samples for ground guides). Although their exposure is intermittent, their duties take them near moving vehicles (which disturb dust) and into the central sand-handling area as they guide sand delivery trucks into positions near sand movers. The single sample for a QA technician was less than 25 ug/m[3], as was one of the three samples obtained for mechanics (the other two sample results for mechanics were between 50 ug/m[3] and 100 ug/m[3]).
NIOSH also documented baseline conditions for remote/intermittent support workers, which included the largely uncontrolled work processes of workers in another job category (fracturing sand workers). Sand and aggregate crushed on the ground by passing heavy equipment contribute to remote/intermittent support worker exposure whenever these materials are disturbed. Similar to the other job categories, the exposure profile for remote/intermittent support workers is based on NIOSH's reports and therefore represents the exposure of workers in this job category operating under these baseline conditions.

Table IV.C.EP -- Respirable Crystalline Silica Exposure Range and Profile for Hydraulic Fracturing During Support Activities for Oil and Gas Operations (NAICS 213112)

                               Exposure Summary
                                Exposure Range
                                       
                               Exposure Profile
 Job Category
                                       N
                                     Mean
                                  (μg/m[3])
                                    Median
                                  (μg/m[3])
                                      Min
                                  (μg/m[3])
                                      Max
                                  (μg/m[3])
                                       
                               <25 (μg/m[3])
                                   >=25 and
                                      50
                                  (μg/m[3])
                                  >50 and
                                      100
                                  (μg/m[3])
                                >100 and 250
                                  (μg/m[3])
                                    >250
                                  (μg/m[3])
Hydraulic Fracturing Workers
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
  
Sand Mover Operators
                                      34
                                      548
                                      488
                                      10
                                     2,000
                                       
                                    1
2.9%
                                    2
5.9%
                                    3
8.8%
                                    5
14.7%
                                   23
67.6%
  
Conveyer Belt Tenders
                                       2
                                     1,495
                                     1,495
                                      420
                                     2,570
                                       
                                    0
0.0%
                                    0
0.0%
                                    0
0.0%
                                    0
0.0%
                                   2
100.0%
  
Blender Tenders
                                      15
                                      135
                                      110
                                      40
                                      485
                                       
                                    0
0.0%
                                    2
13.3%
                                    5
33.3%
                                    6
40.0%
                                    2
13.3%
  
Subtotal for Hydraulic Fracturing Workers
                                      51
                                      464
                                      330
                                      10
                                     2,570
                                       
                                    1
2.0%
                                    4
7.8%
                                    8
15.7%
                                   11
21.6%
                                   27
52.9%
Ancillary Workers
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       

Ancillary Workers
                                       6
                                      243
                                      142
                                       9
                                      820
                                       
                                    1
16.7%
                                    2
33.3%
                                    0
0.0%
                                    1
16.7%
                                    2
33.3%
Remote/Intermittent Support Workers
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       
                                       

Ground Guides
                                      22
                                      96
                                      51
                                       6
                                      630
                                       
                                    6
27.3%
                                    3
13.6%
                                    9
40.9%
                                    2
9.1%
                                    2
9.1%

Mechanics
                                       3
                                      59
                                      68
                                      23
                                      87
                                       
                                    1
33.3%
                                    0
0.0%
                                    2
66.7%
                                    0
0.0%
                                    0
0.0%

Q/A Technicians
                                       1
                                      10
                                      10
                                      10
                                      10
                                       
                                   1
100.0%
                                    0
0.0%
                                    0
0.0%
                                    0
0.0%
                                    0
0.0%

Subtotal for Remote Support Workers
                                      26
                                      88
                                      51
                                       6
                                      630
                                       
                                    8
30.8%
                                    3
11.5%
                                   11
42.3%
                                    2
7.7%
                                    2
7.7%
Total
                                      83
                                      330
                                      121
                                       6
                                     2,570
                                       
                                   10
12.0%
                                    9
10.8%
                                   19
22.9%
                                   14
16.9%
                                   31
37.3%

Table x.EP -- Respirable Crystalline Silica Exposure Range and Profile for Hydraulic Fracturing During Support Activities for Oil and Gas Operations (NAICS 213112)
Notes: All samples are PBZ results for durations of 360 minutes or more and represent 8-hour time-weighted average (TWA) exposures with the assumption that exposure continued at the same level during any unsampled portion of the shift. 

This exposure profile assumes that the distribution of the available exposure samples represents the distribution of actual workers and facilities in this industry. OSHA seeks additional information to better describe the distribution of exposures in this industry.

Sources: NIOSH HF-Site 1, 2010; NIOSH HF Site-2, 2011; NIOSH HF Site-3, 2011; NIOSH HF Site-4, 2011; NIOSH HF Site-5, 2011; NIOSH HF Site-6, 2011.

Additional Controls
Additional Controls for Fracturing Sand Workers
As indicated in the exposure profile, OSHA estimates that 10 percent of fracturing sand workers currently have exposures below 50 ug/m[3]. For the remaining workers, additional controls will be required to reduce exposures below current levels.
Silica is emitted from several points on equipment operated by fracturing sand workers. NIOSH identified the following seven primary sources of silica emissions affecting workers engaged in hydraulic fracturing. NIOSH prioritized the sources (for control) as follows: 
   1. Dust ejected from thief hatches on top of the sand movers.
   2. Dust released from the conveyor belts under the sand movers.
   3. Dust generated on site by truck traffic (road dust).
   4. Dust created as the sand drops into, or is agitated in the blender hopper.
   5. Dust released from the conveyer belt operation.
   6. Sand released at the top end of the sand belt (associated with the sand movers).
   7. Dust ejected from the fill ports on the side of the sand movers.
(Source: NIOSH HF Site 6, 2011)
 Table IV.C-Ax shows how these seven primary and two lesser sources of exposure relate to the three job categories. 
Table IV.C-Ax  -  Sources of Worker Exposure to Silica at Hydraulic Fracturing Sites

Job Category
                       Potential Silica Exposure Sources

 Thief hatches  - sand mover top
 Conveyor belt under  sand movers
                             Dust raised by traffic
                                 Blender hopper
                            Conveyor belt operation
                 Transfer point from sand belts on sand movers 
 Sand Fill Ports
                        Sand sieve (QC laboratory only)
                              Dust in vehicle cabs
Hydraulic Fracturing Worker (Central Zone)
                                      **
                                      **
                                       *
                                      **
                                      **
                                      **
                                      **
                                      --
                                      --
Ancillary Support Workers (Nearby)
                                       *
                                       *
                                      **
                                       *
                                       *
                                       *
                                       *
                                      --
                                      **
Remote/Intermittent Support Workers
                                       *
                                       *
                                       *
                                       *
                                       *
                                       *
                                       *
                                      ** 
                                       *
*   = Exposure is primarily as bystander; silica dust originates with other workers' activities.
**  = Exposure is directly associated with the workers' activities and equipment.
Sources: NIOSH HF-Site 1, 2010; NIOSH HF Site-2, 2011; NIOSH HF Site-3, 2011; NIOSH HF Site-4, 2011; NIOSH HF Site-5, 2011; NIOSH HF Site-6, 2011.

To limit worker exposure to silica, emissions must be reduced from each of these major sources, a process that will require a combination of control methods. Beneficial control methods include local exhaust ventilation (LEV), wet methods, enclosure, work practices, and substitution.
Local Exhaust Ventilation (LEV)
Control equipment that encloses and ventilates emission points is used broadly to control silica dust in both general industry and the construction industry. This control method is highly effective when designed to capture dust at the release point and with sufficient suction (pressure and volume) to overcome competing forces, such as turbulence, leakage, other sources of air flow, and dust particles in motion. Captured air released in the work area needs to be treated with an appropriate air-cleaning device to prevent respirable particles from recirculating back into workers' breathing zones. LEV with a tight-fitting or partially enclosing hood is a control option for all the major sources of dust released from sand-handling equipment in hydraulic fracturing work zones. 
OSHA identified two commercial providers offering powered LEV systems built for the purpose of controlling dust emissions from dust sources associated with filling sand movers (FracSandDC, 2012; NOV, 2012). One is an add-on retrofit option for sand movers (3,200 cubic feet per minute ([cfm]). The other is reportedly available installed on new sand movers, retrofit on existing sand movers, or as a dust control service package providing trailer-based equipment (45,000 cfm) and personnel to set it up and operate it on a per-job basis. Both draw air from the sand mover to control dust released while sand trucks pneumatically fill the sand mover (STEPS, 2012; JJBodies, 2011; NOV, 2012; FracSandDC, 2012). Captured dust is held in containers until disposed of (in accordance with local requirements) (STEPS, 2012). One of the systems also can be configured to provide LEV at the transfer belt and conveyers (STEPS, 2012; JJBodies, 2011). The manufacturer reports that preliminary test results suggest substantial reductions in airborne dust exposure; however, workers in the area continue to require respiratory protection (STEPS, 2012).
Separately, NIOSH has designed and tested a prototype mini-baghouse passive dust collection system that fits over individual thief hatches and deposits collected sand back into the sand mover (STEPS, 2012; Esswein, 2012; NIOSH HF Site 6, 2011). NIOSH recommends that "baghouse material should be selected to control respirable particles in the size range of 3-5 microns" (i.e., the size of respirable dust) (NIOSH HF Site 6, 2011). NIOSH reports that the design is promising and may be commercially available in the future (Esswein, 2012). 
OSHA notes that with LEV systems that focus on controlling dust from thief hatches or the sand mover in general, other control methods (e.g., additional LEV, wet methods) still will be needed to manage dust released from conveyers, transfer belts, and hoppers.
OSHA has not identified studies or data to demonstrating the effectiveness of LEV for controlling silica exposure of fracturing sand workers. However, the sections of this technological feasibility analysis covering foundries, pottery, and construction industry activities such as milling, rock and concrete drilling, and rock crushing discuss examples of beneficial ventilation systems currently in use for other large-scale operations involving sand and other silica-containing materials. Although these industries do not handle the same quantity of sand on a daily basis as that used by the hydraulic fracturing industry, several of the industries do use notable amounts of high-silica sands and have achieved marked reductions in silica exposure using LEV systems. The following examples from the foundry industry demonstrate that appropriately designed and maintained LEV systems can have a great influence on silica exposure levels. The reader is directed to Section IV.C -- Technological Feasibility of the Preliminary Economic Analysis for additional examples from the industries mentioned above.
The foundry industry includes facilities that handle large quantities of silica sands. Although foundries do not use the extreme tonnage of sand encountered at hydraulic fracturing sites, the handling processes are similar, including extensive use of conveyer belts under hot and dry conditions to transport dusty sands returned from the shakeout area for reuse in molds for metal casting. Foundry workers in the sand systems operator job category manage the flow of sand through hoppers and bins before blending it with clay (another silica-containing material) in equipment called mullers. In the ferrous sand-casting foundry industry, exposure monitoring data obtained by OSHA at a foundry showed an 83 percent reduction in sand systems operator silica levels (from 231 ug/m[3] to 40 ug/m[3]) after the foundry installed LEV and repaired leaks in the mixer (OSHA SEP Inspection Report 122040488). Published standards for sand mixer and mullers, bins, hoppers, and screens specify that equipment be well enclosed and exhausted at a minimum rate of 150 cfm (200 cfm in the case of screens) per square foot of opening (ACGIH, 2010; AFS, 1985).
Both OSHA and NIOSH showed that controlling dust from foundry sand-handling equipment could reduce silica exposures. An exposure of 11 ug/m[3] (limit of detection [LOD]) was obtained for a sand systems operator who was controlling a muller with both muller belts and sand elevator fully enclosed (OSHA SEP Inspection Report 108772377). NIOSH reported exposures less than 30 ug/m[3] at a facility where a sand systems operator monitored a pneumatic transport system that moved sand to the mixing equipment. In addition, this facility used specifically sized (A50-grain), pre-washed lake sand for casting, which likely helped reduce exposures (NIOSH ECTB 233-107c, 2000). Pre-washing sand can remove fine respirable-sized particles that might otherwise become airborne when workers use the sand.
Although conditions in foundries are substantially different than hydraulic fracturing sites, the principles of enclosure and exhaust ventilation apply equally to both. A well-designed ventilation system associated with an appropriate process enclosure or enclosing hood is highly effective for capturing silica dust before it spreads through the workplace. While no documentation exists showing to what extent the commercial systems currently available or under development control respirable silica exposure, the available evidence suggests that each of those systems likely reduces dust emissions from thief hatches (one of the greatest sources of dust at these sites). Photographs and videos of hydraulic fracturing worksites suggest that thief hatches account for at least half (and likely more than half) of the visible dust released at these sites over the course of a day (FracSand DC, 2012). Visible dust is not a measure of respirable dust concentration, but it is a marker for airborne dust in general, of which respirable dust is typically one component (OSHA 3362-05, 2009). The LEV systems currently available or under development for hydraulic fracturing sites are unproven, but OSHA finds that available information on these and similar types of equipment suggest that this type of technology has potential as an effective control for thief hatch emissions. If so, the exposures of all workers in the central fracturing sand-handling area could be reduced by half (based on the visual impression that 50 percent of total emissions are contributed by thief hatch emissions, as noted above). Air monitoring will be required to confirm the actual extent of the exposure reduction.
The supplier of at least one commercially available ventilation system also applies LEV to other dust sources associated with hydraulic fracturing equipment, including conveyer belts, transfer belts, hoppers, and drop points (STEPS, 2012). The available information is insufficient for evaluating the effectiveness of these controls. An analogous situation exists, however, in a study of rock-crushing equipment used to crush pure quartz stone in the Iranian quartz powder production region (Bahrami et al., 2008). Like hydraulic fracturing equipment in the United States, the crushers initially were completely without controls, operating in an extremely high-silica environment (the stone contained 98 percent silica). These investigators compared area samples obtained at uncontrolled, small, mechanized crushing machines to similar samples obtained for equivalent machines fitted with LEV at hoppers, rotary grinders, screeners and conveyor belts (LEV system not further described by the investigators). They found that airborne respirable dust concentrations were higher (levels of 111,000 ug/m[3] to 179,000 ug/m[3]) for uncontrolled equipment compared to those fitted with LEV, which were 99 percent lower (Bahrami et al., 2008). This study is described in more detail in the Preliminary Economic Analysis Section IV.C -- Technological Feasibility discussion of rock crushing machine operators and tenders. Although hydraulic fracturing sand equipment is markedly larger scale, and worker exposure levels tend to be correspondingly higher, the Iranian experience offers insight into the degree of control that might be available from basic LEV installed on previously uncontrolled equipment, where silica can constitute 100 percent of the respirable dust. If the hydraulic fracturing silica emissions from conveyors, drop points, and hoppers were also reduced by the same 99 percent reported by Bahrami et al. (2008), the current maximum hydraulic fracturing worker silica exposure (2,570 ug/m[3]) might be reduced by a corresponding amount to 26 ug/m[3] (or half this amount if eliminating emissions from thief hatches resulted in a 50 percent decrease in total exposure). OSHA acknowledges that the large scale of hydraulic fracturing equipment might make it more difficult to control than the small Iranian rock crushers (e.g., markedly greater cfm required, temporary equipment might not fit as well). If instead, the conveyer, drop points, and hoppers were only controlled by 66 percent (instead of 99-percent reduction), hydraulic fracturing worker exposures would still be reduced to 848 ug/m[3]. When combined with a 50-percent reduction due to control of exposure from thief hatches, the highest fracturing sand worker exposure would be 424 ug/m[3]. This exposure level is considerably higher than 50 ug/m[3] but is within the maximum use concentration (MUC) for respirators with an assigned protection factor (APF) of 10 (e.g., an elastomeric half-facepiece respirator fitted with P-100 filters).
Wet Methods
Wet dust suppression methods have proven effective for controlling silica dust in a wide variety of settings. Water spray, or amended water spray (including additives to extend the functional benefit of the water spray), is widely used to control dust in outdoor storage yards in both general industry and the construction industry. Although OSHA does not have information demonstrating the effectiveness of this method for controlling road dust at hydraulic fracturing sites, numerous examples exist in other industries, where heavy equipment operates constantly on what otherwise would be dusty driving areas. For example, in the structural clay industry (i.e., manufacturing bricks and concrete block from clay and concrete that contain silica), front-end loaders and other heavy equipment constantly move back and forth on the site. As at hydraulic fracturing sites, spilled sand and related silica materials at structural clay sites are crushed by vehicles and become airborne when disturbed. Workers in the material handler job category are exposed to silica when they operate this equipment; however, wet methods also can reduce exposure levels for these workers. Dust suppressants or frequent wetting using a water spray truck can limit the amount of dust that becomes airborne. For example, a brick manufacturing facility described in NIOSH ECTB 233-124c (2000) sprayed the yard (product storage area) with water five times per day. Five of the six results obtained for material handlers operating in the area were below the LOD (16 ug/m[3] in this case), while one result was 43 ug/m[3] (NIOSH ECTB 233-124c, 2000). 
A study by Addo and Sanders (1995) offers additional support for the application of dust suppressants to work areas and storage yards. The study examined three chemical dust suppressants (lignosulfate, calcium chloride, and magnesium chloride) applied to an unpaved roadway for four and a half months and found that, compared to an untreated roadway, the suppressants reduced fugitive dust emissions by 50 to 70 percent.
Dust suppressants, such as foam sprays, can also be applied to conveyers to prevent silica dust from becoming airborne as raw materials are transferred between work areas. 
As noted elsewhere (under baseline conditions for the material handler - loader operator subcategory in the structural clay industry), dust suppressants applied to the storage yard helped limit exposure use at a structural clay facility visited by NIOSH and is associated with a silica result of 56 ug/m[3], despite visible dust accumulation in the loader cab (NIOSH ECTB 233-124c, 2000).  This example highlights the importance of examining all potential sources of exposure, including dust in vehicle cabs. OSHA has preliminarily determined that more frequent cleaning of this loader cab would minimize dust and reduce the operator's silica exposure to a level of 50 ug/m[3] or less (a reduction of at least 6 ug/m[3], or 12 percent). NIOSH has identified dust from floors and surfaces in cabs as a source of operator exposure in the hydraulic fracturing industry as well. OSHA aniticpates that the same efforts to clean vehicle cabs will work equally well in the hydraulic fracturing industry as in the structural clay industry.
Wet methods and dust suppressants, such as foam sprays can also be applied to process equipment and conveyers to prevent silica dust from becoming airborne as raw materials are transferred within a work area. Road milling machines, which process and convey large quantities of silica-containing asphalt road surface, make use of wet methods during milling (at the cutting drum) and increasingly are applying water spray to the recyclable asphalt product (containing sand and silica rock aggregate) on conveyer belts as a dust control measure. In a study conducted in the Netherlands, a novel wet dust emission suppression system reduced the PBZ respirable quartz exposure of asphalt milling machine drivers to a mean of 20 ug/m3 (n = 4), with a range of 9 ug/m[3] to 30 ug/m[3] (Van Rooij and Klaasse, 2007). The system consists of 24 spray nozzles (located at the picks drum, collection conveyer, and loading conveyer), which spray aerosolized water containing an additive (likely a foam, based on the product name) onto the milled asphalt material (Van Rooij and Klaasse, no date, 2007). The additive foam causes the dust to become tacky and aggregate, and the foam expands rapidly to encompass small particles generated by the tool's aggressive action. This technology can offer more effective dust suppression than plain water. Milling machine tenders benefitted equally from the system, having a mean PBZ respirable quartz exposure of 8 ug/m[3] (n = 4), with a range of 4 ug/m[3] to 12 ug/m[3]. Compared with a standard milling machine, which uses only cooling water (not aerosolized) on the blade, the use of the aerosolized water and foam system reduced the mean exposure for drivers and tenders combined by 97 percent. Without the added controls (i.e., cooling water only), mean exposure was 418 ug/m[3] (n = 2) for drivers and 509 ug/m[3] (n = 1) for tenders. 
OSHA recognizes that gas and oil companies must use great care to limit the number and type of materials introduced into the well hole; therefore, additives might not be suitable for sands destined for hydraulic fracturing. Investigators Van Rooij and Klaasse (2007) also reported results of using aerosolized water without the additive foam. Aerosolized water alone provided a substantial benefit, resulting in PBZ respirable quartz exposures of 42 ug/m[3] and 57 ug/m[3] for milling machine drivers and 56 ug/m[3] and 104 ug/m[3] for tenders. Aerosolized water reduced the mean exposure for drivers and tenders combined by 86 percent compared with cooling water only; however, three of four exposures remained above the proposed PEL of 50 ug/m[3]. The authors did not report individual sample durations, but the average sampling time for all 15 results was 254 minutes (range: 60 to 388 minutes). The investigators concluded that exposure results were lower when the additive was used in the spray water. NIOSH, in cooperation with an industry group, is evaluating control methods, including water spray and LEV, for road milling machines in the United States.
Wet dust suppression systems can also reduce general dust levels across a worksite when other, more local methods only partially control the emissions source. NIOSH describes this control method as it is used in the mining industry, which, like hydraulic fracturing, handles large quantities of silica materials as sand, rock and ore during processes that, if uncontrolled, generate substantial dust:
      Wet suppression systems are probably the oldest and most often used method of dust control at mineral processing operations. In the vast majority of cases for mineral processing operations, the wet suppression system used is a water spray system. Although the use of water sprays is a simple technique, there are a number of factors that should be evaluated to determine the most effective design for a particular application. There are two methods to control dust using water sprays at mineral processing operations: 
            :: Preventing dust from becoming liberated and airborne by directly spraying the ore. 
            :: Knocking airborne dust down by spraying the dust cloud and causing the particles to collide with water droplets and fall out of the air.
            Most operations use a combination of both methods in the overall dust control plan. When considering the use of a wet suppression system, some general considerations and guidelines apply: 
            :: The effectiveness of water spray application is dependent on nozzle type, droplet size, spray pressure, spray pattern, spray angle, spray volume, spray droplet velocity, and spray droplet distribution. 
            :: Each ore type and application point is a unique situation and needs to be evaluated separately to achieve the optimal design. 
            :: Water evaporates and needs to be reapplied at various points throughout the process to remain effective. 
            :: Water freezes and its use is limited during certain times of the year and in certain climates. 
            :: Wet suppression cannot be used with all ores, especially those that have higher concentrations of clay or shale. These minerals tend to cause screens to bind and chutes to clog, even at low moisture percentages. 
            :: Over application in the volume of moisture is a problem in all operations and can impact the equipment as well as the total process. In most cases, a well-designed suppression system will not exceed 0.5% moisture application, which is roughly equivalent to one gallon per ton of ore. 
            :: The suppression system should be automated so that sprays are only used during times of production when ore is actually being processed. For dust knockdown, a delay timer may be incorporated into some applications to allow the suppression system to operate for a short time period after a dust-producing event. 
      When considering sprays, one of the primary aspects is the droplet size. When wetting the ore to keep dust from becoming airborne, droplet sizes above 100 microns should be used. In contrast, when the goal is to knock down existing dust in the air, the water droplets should be in size ranges similar to the dust particles. The intent is to have the droplets collide and attach themselves to the dust particles, causing them to fall from the air. In these cases, droplets in the range of 10 to 50 microns have been shown to be most effective. [From NIOSH IC 9521, 2010]
As discussed in the section on construction rock crushing, an international report on wet dust control methods for rock crushers in India offers evidence that water mist reduces silica for rock crushing and conveying operations. At several small, tightly clustered rock crushing machine sites in India, five initial respirable quartz results obtained during dry crushing operations ranged from 60 ug/m[3] to 360 ug/m[3], with a median of 290 ug/m[3] and a mean of 246 ug/m[3] (Gottesfeld et al., 2008). Although the stationary (movable, but apparently not mobile) crushers were mechanized (powered), the workers loaded the crusher hopper manually and carried off the crushed material by hand in sacks. None of the crushing machines was equipped with an operator's booth. Among the sites evaluated for this study, the bulk stone quartz content ranged from below 4 percent to 27 percent, with an additional 3 to 6 percent cristobalite at some sites. 
Results were markedly lower when water spray systems were installed. Of the 150 small Indian crushing mills in the study area, 40 subsequently agreed to install atomizing water spray dust suppression systems. The 18 follow-up breathing zone and area samples collected during the monsoon season range from 5 ug/m[3] to 55 ug/m[3], with a median of 11 ug/m[3] and a mean of 14 ug/m[3] (sampling durations not reported). A second set of follow-up samples was collected during the dry season. These 27 post-control dry season samples (15 PBZ and 12 area samples), obtained over approximately 2 to 5 hours, range from 10 ug/m[3] to 630 ug/m[3], with a median of 20 ug/m[3] and a mean of 63 ug/m[3]. Gottesfeld et al. (2008) note that the higher sample results observed after spray systems were installed (29 percent exceeded 50 ug/m[3]) might have been due to one or more spray nozzles that did not function and neighboring rock crushing mills that did not have dust control equipment (dust drifted between neighboring operations). Although the wide exposure range indicates that elevated exposure occurred occasionally, both the median and the mean were dramatically lower after the control system was installed. Respirable dust levels dropped by 63 percent. 
A general mist system of the type described above (see Gottesfeld et al. 2008) could provide supplemental dust control for emissions sources at hydraulic fracturing sites where LEV has not yet completely controlled workers' silica exposure. As discussed in Section IV.A -- Methodology, employers will benefit from expert advice in selecting a water mist system. The size of the droplets is at least as important as the type and volume of the spray.
Additional exposure reductions can be achieved by moistening the proppant on conveyer belts and at drop points. This method is recommended by NIOSH and typically involves adding 0.1 percent to 1.5 percent water to the proppant (NIOSH HF Site 6, 2011; NIOSH RI 9689, 2012). Hydraulic fracturing sites can account for the amount of moisture added as dust suppressant to materials on conveyer belts approaching the blender hopper, so the fluid balance in the fracturing slurry remains predictable. OSHA recognizes that adding moisture at the early stages of the process (e.g., in the truck before sand is delivered) is less practical, as it could interfere with the truck's pneumatic sand delivery system. Because they fully enclose the sand, pneumatic transport systems are a highly effective dust control method, providing the dust controls are available on the receiving vessel (in this case, the sand mover). OSHA also acknowledges that it might be more difficult to account for water added as a dust suppressant between the delivery truck and final conveyers, since more of the water would evaporate under warm and dry conditions than during cool or humid conditions.
Enclosure
Enclosure limits emissions from areas under positive pressure (e.g., fill ports and unused thief hatches on sand movers) and areas of turbulence (e.g., conveyers, sand drop points from the ends of conveyers). Enclosures used with LEV improve ventilation effectiveness so engineers can design systems with smaller, more energy-efficient fans.
NIOSH noted that the fill ports (nozzles) on the sides of the sand movers can be a primary source of silica exposure for all fracturing sand workers in the area during the periods when the sand movers are refilled by the sand delivery truck drivers (NIOSH HF Site 1, 2010). Sand delivery typically involves just one or two of the several nozzles on each sand mover. One component of silica management at hydraulic fracturing sites involves preventing silica release from those fill nozzles that are not in use. Fill ports not intended for pressure relief and should be closed with manufacturer-provided or replacement end caps (NIOSH HF Site 6, 2011). Tight closure with a cap will prevent silica emissions from this source. Tight closure by valve or cap is a typical design feature wherever unused ports are present in pneumatic sand transport system receiving vessels (i.e., tanks, rail cars, trucks, and process equipment -- including sand movers) (Smith and Voges, no date; Dynamic Air, 2011; Bhatia, no date). Replacement port caps are commercially available (NOV, 2012). Installing a leak-proof gasket and closing unused thief hatches will also help ventilation systems function more efficiently and reduce opportunities for exposure.
Exposure reduction can be enhanced by enclosing conveyors and particularly conveyor drop points. NIOSH advocates reducing and enclosing drop points: "Some methods to perform this are through the use of rock ladders, telescopic chutes, spiral chutes, and bin-lowering chutes" (NIOSH RI-9689, 2012). These options are applicable to hydraulic fracturing sites, for which NIOSH recommended shrouding or skirting at the end of the sand belt to limit dust released as material falls from the belt (NIOSH HF Site 6, 2011).
Work Practices and Administrative Controls
Work practices and administrative controls provide workers with standard operating procedures that help workers cover fill ports and close any thief hatches that do not need to be open during sand mover filling and hydraulic fracturing processes, require workers to stand back from dust emission points unless necessary, minimize hot-loading unless adequate controls are in place to protect workers, and limit personnel in the areas where greatest exposure tends to occur. 
Minimizing the occurrence of hot-loading and limiting the number of personnel in the area when hot-loading must occur are both additional options for reducing fracturing sand worker exposures associated with these periods of peak emissions. OSHA recognizes that the practice of hot-loading reduces otherwise unproductive time spent refilling the sand mover.
Another work practice control involves adjusting equipment to minimize the height from which proppant falls from conveyer belts during transfers (to other conveyors or to the blender hopper). Procedures that help reduce the drop distance minimize the influence of competing air currents and reduce the amount of dust that becomes airborne as proppant transfers between conveyors or from conveyor to blender hopper. Design "VS-50-20" in ACGIH (2010) recommends that drop distances be less than 3 feet. For ventilated systems, additional ventilation is required to compensate for dust released during greater falls. NIOSH also recommends that fall heights for materials be minimized whenever possible (NIOSH RI-9689, 2012). 
Combination of Controls
The massive quantities of sand and high silica content mean that a combination of controls likely will be necessary to reduce silica dust at fracturing sites. Control methods such as LEV, general misting wet methods, road wetting with amended water, full enclosure (sealing unused side ports), and work practice/administrative controls are not mutually exclusive and can be used in any combination. 
As determined in the discussion of LEV above, exposure levels can be reduced considerably by installing effective ventilation controls both on thief hatches (estimated 50 percent reduction) and on conveyors, transfer belts, drop points and hoppers (additional estimated 66 percent reduction). This combination of controls will reduce the exposure level of all workers who currently experience levels of 250 ug/m[3] or less to below 42 ug/m[3]. Furthermore, using these methods, the highest fracturing sand worker exposure would be 424 ug/m[3]. A site water misting/fogging system could further reduce airborne silica levels for these most highly exposed workers by an additional 63 percent (Gottesfeld et al., 2008). The resulting exposure level for the most highly exposed worker in this job category would be 157 ug/m[3]. 
Substitution
Substitution is another option for reducing silica exposures at hydraulic fracturing sites. Oil and gas extraction worksites present two opportunities for substitution: work zone surfacing materials and proppant. 
NIOSH reported that spilled silica sand and aggregate crushed by heavy equipment in the work zone contribute to worker silica exposures. This source of exposure can be reduced by covering the work zone with substitute materials such as low-silica or granite aggregate (which contains silica, but is very hard so less subject to crushing).
The second substitution option involves the proppant. Hydraulic fracturing requires a granular media proppant -- typically sand. To function as a proppant, the sand must stand up to considerable pressure in the well, and the physical properties of quartz make this type of sand particularly useful. However, alternate media are available and widely used for this purpose under certain circumstances. Commercially available alternatives include sand of other mineral content (reduced silica sand, usually mined from a different source than pure silica sand), coated sand (resin over sand grains to improve durability), and low-silica clay or ceramic granules. NIOSH observed a hydraulic fracturing crew using a ceramic sand containing less than 1 percent silica (NIOSH HF Site 6, 2011). Substituting such a proppant for silica sand would reduce silica exposure levels by up to 99 percent or more (depending on the amount of silica in the alternative proppant) compared to pure silica sand.
OSHA acknowledges that these substitute materials are more costly than natural sands. Due to their cost, alternate proppants tend to be reserved for special circumstances (particularly high-pressure wells) where the special characteristics (increased durability, uniformity, or more spherical) are needed to help extend well life. 
Low-silica alternate media can also be used in combination with (high-quartz) natural sand media. NIOSH obtained PBZ samples at a site that used a mixture of natural sand and ceramic proppant (58 percent of the total proppant used that day was the low-silica ceramic proppant, while the remaining 42 percent was silica sand). PBZ samples indicated that the silica content of the samples was lower (3 to 25 percent silica) than at sites using only high-silica sands (typically between 50 and 100 percent silica) (NIOSH HF Site 6, 2011). Although reducing the silica content of the proppant does reduce the silica in the airborne dust, worker exposures can still be significant; at this NIOSH site 9 of the 11 PBZ samples exceeded 50 ug/m[3]. None exceeded 100 ug/m[3].
In an example from the foundry industry, which also processes, conveys, and blends quantities of high-silica sand, substituting non-silica granular media (that is less toxic than silica) for silica sand used for molds and cores can virtually eliminate the silica exposures of all foundry sand system operators. A report from the Industrial Commission of Ohio shows that exposures dropped below the LOD for all workers when the foundry used a non-silica substitute: olivine sand (ERG # OH-1460). Another aluminum foundry reported respirable dust levels of 300 to 1600 u/m[3] but no exposure to silica when using olivine sand (Foundry Engineering Group Project  -  Case History H, 2000). This example from the foundry industry supports NIOSH's findings showing marked reductions in respirable dust silica content at a hydraulic fracturing site using a low-silica alternate media as a portion of the proppant (NIOSH HF Site 6, 2011).
Before using an alternate material, employers must evaluate the health hazards associated with it and take any necessary steps to protect workers from the hazards.
Additional Controls for Ancillary Support Workers
The exposure profile, presented in Table IV.C.EP, provides information on ancillary support workers, including OSHA's estimate that half (50 percent) of the workers in this job category are currently exposed to silica levels of 50 ug/m[3] or less. The additional controls necessary to reduce the most highly exposed fracturing sand workers to 157 ug/m[3] also will reduce the exposure of all ancillary support workers to 50 ug/m[3] or less. 
Ancillary support workers primarily are exposed to dust drifting into their work areas from the central fracturing sand zone. Following the control process outlined above for fracturing sand workers, the highest exposure in this job category (820 ug/m[3]) will be reduced by an estimated 50 percent (to 410 ug/m[3]) by effective LEV on thief hatches, and by another estimated 66 percent (to 135 ug/m[3]) by LEV applied to conveyors, transfer belts, drop points, and hoppers. An additional 63-percent reduction (to 50 ug/m[3]) will occur when site misting is applied. No additional controls are necessary for ancillary support workers. 
In the event that any workers in this job category do remain exposed above the proposed PEL of 50 ug/m[3], other control methods are available, including improved closure and housekeeping in vehicle cabs to prevent tracked or settled dust from becoming a source of exposure. NIOSH recommends several cab design features and emphasizes the importance of maintenance and cleanliness (NIOSH 2009-123, 2009). Cabs employing several of these recommendations regularly achieve exposure reductions (inside versus outside the cab) exceeding 90 percent (Cecala et al., 2005; NIOSH 528, 2007). 
Furthermore, ancillary support workers will also benefit from yard dust controls, as discussed for remote/intermittent support workers.

Additional Controls for Remote/Intermittent Support Workers   
The exposure profile, summarized in Table IV.C.EP, presents OSHA's estimate that 42 percent of remote/intermittent support workers have current exposures of 50 ug/m[3] or less. Additional controls will be needed to reduce the exposure levels of the remaining 58 percent of workers in this job category.
Controlling silica emitted from fracturing sand-handling equipment also will reduce most exposure experienced by remote/intermittent support workers, who must occasionally enter the central fracturing sand zone or are exposed to dust drifting into their work areas. 
Like the ancillary support workers, the remote/intermittent support workers primarily are exposed to dust drifting into their work areas from the central fracturing sand zone. Following the control process outlined above for fracturing sand workers, the highest exposure in this job category (630 ug/m[3]) will be reduced by an estimated 50 percent (to 315 ug/m[3]) by effective LEV on thief hatches, and by another estimated 66 percent (to 104 ug/m[3]) by LEV applied to conveyors, transfer belts, drop points, and hoppers. An additional 63-percent reduction (to 39 ug/m[3]) will occur when site misting is applied. Based on this information, OSHA preliminarily concludes that no additional controls are necessary for remote/intermittent support workers. However, additional potential sources of exposure exist for these workers and if employers that exposure levels remain elevated, they should consider options for reducing dust disturbed by passing vehicles on the site.
Certain remote/intermittent support workers (e.g., Q/A technicians who sieve sand as part of quality testing) handle silica-containing materials in a manner that could be a potential source of exposure if performed on a large scale. However, no evidence exists that these workers experience measurable exposure from the small-scale short-term testing activities in which they are involved at hydraulic fracturing sites. As indicated in Table IV.C.EP, the single sample that NIOSH obtained for a Q/A technician (who sifted sand samples) had a result of 10 ug/m[3] (below the LOD). 
Suppress Yard Dust
Wet dust suppression methods for yard dust are described above in the discussion of wet methods for controlling fracturing sand worker exposures. To reiterate, water spray or amended water spray (including additives to extend the functional benefit of the water spray) are widely used to control dust in outdoor storage yards in both general industry and the construction industry. As noted previously, Addo and Sanders (1995) examined three chemical dust suppressants (lignosulfate, calcium chloride, and magnesium chloride) applied to an unpaved roadway for four and a half months and found that, compared to an untreated roadway, the suppressants reduced fugitive dust emissions by 50 to 70 percent.
Feasibility Finding
Feasibility Finding for Fracturing Sand Workers
Based on the best available information, OSHA estimates that 90 percent of fracturing sand workers require additional controls. OSHA preliminarily concludes that silica levels of 50 ug/m[3] or less can be achieved for 47 percent of the workers in this job category (those with current exposures that do not exceed 250 ug/m[3]). These levels can be achieved by sealing fill ports; installing a fully effective LEV system on thief hatches, which alone will reduce exposure levels by an estimated 50 percent; and installing LEV on conveyors, transfer belts, drop points, and hoppers, as described in the discussions of LEV and combinations of controls for fracturing sand workers (for an additional 66 percent exposure reduction). A study of LEV by Bahrami et al. (2008) demonstrated a 99-percent difference between controlled and wholly uncontrolled exposure associated with small-scale, high-silica rock crushing, conveying, screening, and hopper operations. OSHA has preliminarily estimated 66 percent effectiveness rather than 99 percent for the larger scale, but otherwise similar conveying and hopper operations at largely uncontrolled high-silica hydraulic fracturing worksites.
For the 37 percent of workers in this job category that are currently exposed above 250 ug/m[3], but no greater than 820, the same controls (i.e., LEV on the thief hatches and LEV on conveyors, transfer belts, drop points, and hoppers) will reduce exposure levels to an estimated range of 51 ug/m[3] to 135 ug/m[3]. A site water misting/fogging system will further reduce airborne silica levels for these workers by an additional 63 percent, to 50 ug/m[3]. Gottesfeld et al. (2008) reported an average 63 percent reduction in silica concentrations when water misting/fogging systems were installed an Indian rock crushing site. 
OSHA finds that the available information presented in this analysis suggest that, using these control methods, levels of 50 ug/m[3] or less might not be achieved for the 16 percent of fracturing sand workers (8 out of 51 samples) that currently have exposures in excess of 820 ug/m[3].  The resulting exposure level for the most highly exposed worker in this job category would be 157 ug/m[3]. Although above the proposed PEL of 50 ug/m[3], this level is well within the MUC for respirators that have an APF of 10 (e.g., a half-face piece elastomeric respirator with P-100 filters). OSHA preliminarily concludes that the proposed PEL of 50 ug/m[3] can be achieved for 47 percent of fracturing sand workers. The remaining 16 percent (with exposures above 820 ug/m[3] and up to 2,570 ug/m[3]) will require respirator protection until such time as enhanced controls are available for this operation. 
Where practical, further reductions can be achieved by using 0.1 percent to 1.5 percent water to moisten the proppant on conveyer belts and drop points (NIOSH HF Site 6, 2011; NIOSH RI 9689, 2012). However, additional information is needed to confirm that this method does not interfere with the water ratio in the hydraulic fracturing slurry. As an alternative, an exposure level of 50 ug/m[3] can be achieved for all fracturing sand workers by using an alternate non-silica proppant instead of silica sand.
Feasibility Finding for Ancillary Support Workers
Based on the best available information, OSHA estimates that the proposed PEL of 50 ug/m[3] or less will be achieved for all ancillary support workers. For the 50 percent of ancillary support workers who currently experience elevated exposures, this level will be achieved when employers implement the additional controls described above (those which reduce the exposure of the most highly exposed fracturing sand worker from 2,570 ug/m[3] to 157 ug/m[3]). Ancillary support workers primarily are exposed to dust drifting into their work areas from the central fracturing sand zone, as shown in Table IV.C-Ax. OSHA estimates that the steps employers take to control silica concentrations in the fracturing sand zone will affect ancillary support workers similarly, reducing their highest exposure level from 820 ug/m[3] to 50 ug/m[3]. Following the control process outlined in the feasibility finding for the fracturing sand worker job category, the highest exposure for ancillary support workers (820 ug/m[3]) will be reduced by 50 percent (to 410 ug/m[3]) by effective LEV on thief hatches, and by another estimated 66 percent (to 135 ug/m[3]) by LEV applied to conveyors, transfer belts, drop points, and hoppers. An additional 63-percent reduction (to 50 ug/m[3]) will occur when site misting is applied. 
OSHA preliminarily concludes that employers can reduce exposures below 50 μg/m[3] for the 50 percent of ancillary support workers who require additional controls using the same combination of engineering controls as described for fracturing sand workers. Such controls would include ventilated equipment for conveying and transferring proppant in sand movers, conveyors, transfer belts, and blender hoppers. Wet site-misting methods will also be required.
Feasibility Finding for Remote/Intermittent Support Workers   
OSHA preliminarily concludes that employers can reduce exposures below 50 μg/m[3] for the 58 percent of ancillary support workers who require additional controls by using the same combination of engineering controls as described for fracturing sand workers. Such controls would include ventilated equipment for conveying and transferring proppant in sand movers, conveyors, transfer belts, and blender hoppers. Wet site-misting methods will also be required.
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