Overview of coal mine ground control 

issues in the Illinois Basin

G.M. Molinda, C. Mark, D.M. Pappas and T.M. Klemetti

Gerologist, mining engineer, civil engineer and mining engineer, respectively, 

National Institute for Occupational Safety and Health (NIOSH), Pittsburgh, Pennsylvania

Abstract

Some of the most difficult coal mine roof in the United States can be found in the Illinois Basin. Factors 

contributing to the high roof fall rate include weak, moisture-sensitive roof rock; high horizontal stress; 

and limited longwall mining. The depth of cover ranges from 27 to 300 m (90 to 1,000 ft), and roof damage from horizontal stress can be severe. Moisture-sensitive roof rock, which contributes to roof skin 

deterioration and roof fall, is common above the Springfield-Harrisburg Herrin #5 and #6 seams in the 

Illinois Basin. The roof fall rate increases significantly in the humid summer months. Using laboratory 

and field studies, the National Institute for Occupational Safety and Health (NIOSH) has shown that 

highly moisture-sensitive roof rock can be directly correlated to poor roof conditions. Controlling the 

skin is the key to reducing rock fall injuries, and roof screening is, by far, the best remedy. Illinois Basin 

coal operators have been successful in reducing the number of rock fall injuries in recent years. NIOSH 

has documented best practices for screen installation, which has resulted in safe, efficient operations. 

Other solutions to skin failure include the use of denser five bolts/row patterns to reduce spans between 

bolts; systematic supplemental support in intersections; straps and large pans to protect operators; and 

air conditioning to remove moisture from the intake air.

Introduction

Rock fall injuries continue to present a significant hazard to U.S. 

coal miners. In addressing the problem, NIOSH has reviewed 

ground control issues contributing to this hazard. A number of 

these geotechnical issues are present in the Illinois Basin. The 

following overview documents the current state of coal mine 

ground control in the Illinois Basin and the efforts operators 

have made to prevent rock fall injuries.

The Illinois Basin is a major coal-producing basin in the 

United States, with more than 86 Mt (95 million st) of coal 

mined in 2006 (MSHA, 2006). Of this total, 9 Mt (10 million 

st) of coal were mined by longwall methods, 47 Mt (52 million st) were mined by room and pillar methods and 30 Mt 

(33 million st) were surface mined. The basin includes Illinois, 

southwestern Indiana and western Kentucky (Fig. 1). 

More than 75 individual coal seams have been identified in 

the basin, of which 20 have been mined (Archer, 1975). The 

primary producing coal seams in the basin, the Herrin #6 and 

the Springfield-Harrisburg #5, are middle Pennsylvanian in 

age. There are now 30 underground coal mines operating in the 

basin (Fig. 1). Twenty-seven of the mines are room and pillar 

operations and three are longwalls. (Two additional longwall 

mines are permitted but are not yet operating.) Annual production of the active mines ranges from 58 kt (64,000 st) to more 

than 6.5 Mt (7.2 million st).

In 2005-2006, the Illinois Basin had a roof fall rate that 

was significantly higher than other coal-producing regions in 

the United States (Fig. 2). One of the reasons for this is that 

the Illinois Basin has few longwall mines. Longwall mining 

has fewer roof falls than room and pillar mining, because 

there is far less entry development per ton of coal mined. The 

Illinois Basin has only two producing longwall mines (a third 

mine has not yet begun its first panel) and had only 15.6% of 

its production from longwall mining in 2005-2006 (MSHA, 

2006). However, the lack of longwall mining cannot explain 

all of the increase in roof fall rate. The southern Appalachian 

Basin has a similar proportion of longwall mining (15.0%), 

and yet its roof fall rate is 35% lower than that of the Illinois 

Basin. There may be two other reasons for the high roof fall 

rate in the Illinois Basin: A strong biaxial horizontal stress field 

and weak, highly moisture-sensitive roof rocks.

High regional horizontal stresses in Illinois, and the damage 

resulting from them, have been both measured and documented 

by underground observation and mapping (Nelson and Bauer, 

1987; Ingram and Molinda, 1988; Mark and Mucho, 1994; 

Mark et al., 2004). The Wabash mine in southeastern Illinois 
(now closed) had hundreds of long, running roof falls. Entries 

oriented north-south had severe damage because the regional 

stress field is approximately N80°E. To minimize the damage 

caused to entries oriented perpendicular to the regional stress 

direction, the mine turned its development 45° to the nearly 

E-W stress field. Roof conditions improved as a result of the 

reorientation.

Figure 1 — Underground mines in the Illinois Basin 

(2007).

Figure 2 — Roof-fall rates in selected coal basins in the 

United States (2005 through 2006).

Weak roof rocks are easily damaged by high stresses. Rusnak 

and Mark (2000) documented the relative weakness of Illinois 

Basin mudrocks (clay-rich rocks) compared to similar rock 

types from the Appalachian Basin. There is also abundant 

evidence that roof sequences respond to changes in seasonal 

humidity and that some mudrocks deteriorate when exposed to 

moisture. Data from the NIOSH roof rock moisture sensitivity 

database shows a higher average moisture sensitivity of roof 

rocks in the Illinois Basin than in roof rocks from the northern 

and southern Appalachian Basins (Fig. 3). NIOSH tested more 

than 840 rock samples for moisture sensitivity. A wet/dry 

cycling test was used to determine moisture sensitivity. The 

index value representing moisture sensitivity ranges from 0% to 

100%, with 100% indicating total disintegration of the sample 

(Unrug, 1997; Molinda et al., 2006). Rocks that deteriorate on 

contact with water can generate high swelling pressures that 

can bulk the roof and result in roof falls (Molinda et al., 2006). 

The roof fall rate (roof falls per 100 employees) increases in 

the humid summer quarter in most coal regions in the United 

States, but it is most pronounced in the Illinois Basin (Fig. 4). 

During the summer quarter (July, August and September) the 

roof fall rate in the Illinois Basin was more than double the 

rate of most other U.S. regions through the 2004-2006 period. 

In response to these difficult mining conditions, Illinois Basin 

operators adopted roof control methods aimed at improving 

safety during mining. 

Figure 3 — Average moisture sensitivity index of roof rocks 

in NIOSH database by basin.

Figure 4 — Seasonal roof fall rates for U.S. coal basins 

(2004 through 2006). The highest roof fall rates occur in 

the Illinois Basin during the summer months.

Overview of ground control issues and practices 

in the Illinois Basin

Roof geology. NIOSH has been actively gathering groundcontrol information in the Illinois Basin in an effort to understand and control difficult mining conditions. The following 

information was gathered from numerous mine visits and from 
discussions with MSHA District 8 and District 10 roof control 

specialists and mine operators.

Thirty active underground mines are operating in four coal 

seams in the basin. Eighteen mines are currently working in the 

Springfield-Harrisburg #5 seam, and 10 mines work the Herrin 

#6 seam (Fig. 5). These seams will hereafter be referred to as 

the #5 and #6 seams, respectively. In Kentucky, the #5 seam is 

equivalent to the #9 seam, and the #6 seam is equivalent to the 

#11 seam. Two other seams are being mined, with one mine in 

the Danville #7 and one mine working the deeper Kentucky 

# 6 seam (Davis). 

Black shale commonly occurs as the immediate roof rock in 

20 of 30 operating mines. In the #5 seam, this rock is known 

as the Turner Mine shale, and in the #6 seam it is called the 

Anna shale. The black shale ranges from 0 to 1.8 m (6 ft) thick 

and averages about 0.6 m (2 ft) thick. In both seams, the black 

shale can transition into a gray shale facies. The black shale is 

resistant to moisture deterioration and can protect the overlying 

gray shale, which is typically moisture sensitive.

Limestone can be present in the roof of both the #5 and #6 

coal seams and can dictate the roof conditions and support 

practices. Sixteen of the 30 operating mines have limestone 

within the bolted horizon, and many select roof bolt lengths 

in order to obtain anchorage in the limestone. In mines operating in the #6 seam, eight of 10 have limestone that can occur 

in the bolted horizon. In the #5 seam, eight of 18 mines have 

limestone that can occur in the bolted horizon. In the #5 seam, 

the limestone is the St. David limestone, and in the #6 seam 

it is called the Brereton limestone.

Thick gray shale is also an important component in Illinois 

Basin roof rock. Called the Dykersburg shale when it is above 

the #5 seam, and the Energy shale when it overlies the #6 seam, 

it is typically weak and moisture-sensitive. Gray shale forms 

the immediate roof rock in nine of 30 operating mines. Various other rock types occur in the immediate roof, including 

stackrock and fireclay.

Eleven of 26 reporting mines have faults on the property 

that are large enough to cause mining issues, either adverse 

roof or change in mine plans. The major faulting is concentrated in the southern part of the basin, with most of the mines 

in extreme southern Illinois and western Kentucky reporting 

some sizeable faults on their property.

Figure 5 — Underground coal mining by seam in the Illinois Basin (2007).

Table 1 — Moisture sensitivity of roof rocks for selected 

mines in the Illinois Basin.

 Moisture sensitivity 

 index, %

 Sample Gray Black 

 Mine No. Rock Type1 shales shales

 A AQW-1 Gray shale (124) 63.8 

 W-702 Gray shale (124) 1.2 

 W-709 Gray shale (328) 19.3 

 W-715 Gray shale (333) 28.3 

 B W-497 Gray shale (124) 73.3 

 W-510 Gray shale (124) 65.3 

 W-517 Black shale (114) 8.3

 C W-465 Gray shale (124) 73.6 

 W-469 Black shale (112) 27.7

 W-483 Gray shale (124) 78.3 

 D W-305 Gray shale (127) 32.7 

 W-312 Gray shale (127) 73.9 

 E GW-1 Gray shale (124) 48.8 

 F VG-1 Gray shale (122) 96.3 

 G W-321 Gray shale (137) 100.0 

 W-327 Black shale (114) 84.5

 W-336 Black shale (112) 5.6

 H W-345 Gray shale (124) 18.6 

 W-354 Gray shale (324) 20.6 

 W-364 Gray shale (324) 33.5 

 W-374 Gray shale (324) 49.0 

 

 Average: 51.6 31.5

1 Numbers in parentheses refer to the “Ferm” rock 

 classification number (Ferm, 1981).

Moisture sensitivity of roof rocks. Roof rocks that absorb 

humidity and swell can deteriorate over time causing skin 

control problems and roof falls (Molinda et al., 2006). Fourteen 

of 30 operating mines report problems with slaking roof. These 

problems range from thin skin flaking to chandeliered bolts 

to severe guttering requiring supplemental support (Fig. 6). 

Roof falls in the Illinois Basin spike in August and September, 

indicating that high humidity in intake air plays a role in roof 

instability (Fig. 4). NIOSH has tested roof rock for moisture 

sensitivity and found extremely moisture sensitive roof rock 

in a number of Illinois Basin mines (Table 1). 

Table 1 shows a number of moisture sensitivity values 

greater than 40%. Mines that have roof rock moisture sensitivity values greater than 40% have had roof damage from 

slaking (Molinda, 2006). Typically, gray shales are much more 

sensitive to moisture than the black shale immediate roof. 

Where black shale is present, it serves to seal the overlying 
moisture sensitive gray shale from moisture, preserving it. 

NIOSH has documented poor roof conditions directly related 

to the lack of a protective black shale layer. At one western 

Kentucky mine, the immediate black shale roof was removed 

to increase the roof height. The exposed gray shale weathered 

quickly in contrast to the flat roof in the adjacent crosscut 

(Fig. 7). Black shale provides a natural barrier to humidity 

exposure, but spray-on roof sealants have also been effective 

in stopping moisture infiltration (Molinda, 2007). In extreme 

cases, weathering around roof bolts can compromise roof 

bolts. “Ker-Thobs” or other tensioning devices are used to 

reestablish rock contact and restore plate loads (Fig. 8). The 

Ker-Thob1 is a pipe extension inserted between the loose 

roof bolt plate and the roof that allows reestablishment of 

roof/plate contact.

1 Reference to company name or product does not imply endorsement by the National Institute for Occupational Safety 

and Health.

An Illinois mine in the #6 seam used air conditioning as a 

novel remedy for extreme weathering. In addition to causing 

injuries, extensive seasonal deterioration of weak clay shale 

around bolts was blocking airways and required extensive 

cleanup. The mine installed surface air conditioners to cool 

the humid summer air to within 4 degrees of the ambient mine 

air. This effectively reduced roof slaking. Additionally, a cost 

analysis showed that the reduced cost of cleanup and resupport 

would be enough to pay for the cost of the air conditioning 

(Laswell, 1999).

Figure 6 — Roof fall in moisture sensitive roof.

Figure 7 — Gray shale weathers badly after protective black 

shale in removed.

Figure 8 — A “Ker-Thob” is used to reestablish plate contact 

with the roof and maintain roof bolt integrity.

Horizontal stress. A strongly biaxial regional horizontal stress 

field is currently acting on coal mine roof in the Illinois Basin. 

Maximum horizontal stresses ranging from 8.32 to 22.00 MPa 

(1,207 to 3,191 psi) and oriented from N73°E to N86°E have 

been measured using a variety of methods (Ingram and Molinda, 

1988). As a result, significant roof damage in the form of N-S 

oriented falls and cutter roof has occurred. Eight of 29 mines 

reported moderate to severe roof damage, including guttering, 

kink zones and running falls (Fig. 9). The depth of cover for 

mines currently operating in the Illinois Basin ranges from 

27 to 300 m (90 to 1,000 ft) (Fig. 10). Four mines have less 

than 60 m (200 ft) of cover. Seventeen of 30 mines operate 

under shallow-moderate cover between 60 and 120 m (200 

and 400 ft), and four mines have 240 to 300 m (800 to 1,000 

ft) of cover. Three of four mines working in 240 to 300 m (800 

to 1000 ft) of cover have moderate-severe roof damage from 

horizontal stress. Stress damage is not just related to cover. 

Two mines with cover of 27 to 60 m (90 to 200 ft) also have 

cutter roof and horizontal stress damage. At the deepest part 

of the basin in Wayne Co., Illinois, the #5 seam will be less 

than 365 m (1,200 ft) of cover.

Many times reorienting the mines to minimize drivage in 

the N-S direction has provided some relief. In other cases, 

roof rock is so weak that even minimum stress magnitudes are 

enough to cause roof damage (Mark et al., 2004).

Roof support. Primary roof support in the Illinois Basin varies 

with the mine and roof condition. Of the 30 mines, 14 use a 

fully grouted bolt system. Completely encapsulating the bolt 

with resin locks in the strata from horizontal movement keeps 

excessive loads off the plate and prevents humidity from entering the bolt hole. In very weak strata with high horizontal 

stress, fully grouting a bolt can be very important.
Figure 9 — Severe guttering due to horizontal stress.

Figure 10 — Distribution of Illinois Basin mines by depth 

of cover (bars include mines at all depths between that bar 

and the previous bar).

Tensioned roof bolt systems were used in 16 of the 30 mines. 

Three of these mines were using conventional roof bolts, and 

the rest were using fully grouted resin. When the tensioned bolts 

function by suspending the shale from the nearby limestone, 

their length is determined by the bolter who may carry as many 

as four different bolt lengths depending on the limestone location. Often the goal in limestone roof is to achieve at least 0.3 

m (1 ft) of anchorage in the strong limestone. If the limestone 

was thin or absent, a longer bolt was used for beam building. 

In many cases, shorter or lighter support was used in panels 

— 1.2 m (4 ft) fully grouted bolts was typical — with longer 

bolts or tensioned systems used in mains. Only four mines 

mixed bolt lengths in a row of bolts. 

The predominant roof bolt row pattern employed four 

bolts per row. Five mines out of 30 used five bolts across in a 

row. In looking at 2006 data, mines using a five-bolt pattern 

had a slightly lower roof fall rate than mines using four bolts 

across (Fig. 11). In addition to building a stronger beam, the 

five-bolt pattern has value in reducing the span between bolts, 

particularly in very weak rock. One mine in Illinois also uses 

a 1.2-m (4-ft) “cutter” bolt in the corners on the same row or 

staggered between rows. This bolt is designed to support highly 

stressed corners and prevent the propagation of cutter roof, in 

addition to supporting roof screen close to the rib. 

Figure 11 — The effect of roof bolt pattern on the roof fall 

rate for Illinois Basin mines (2005 through 2006).

In the Illinois Basin, an estimated 75% to 80% of roof 

falls occur in intersections. In response, 13 of 30 mines install 

systematic supplemental support to reinforce intersections. 

Systematic support is usually installed in mains and at the 

panel mouths. Intersection support includes cable bolts, longer 

double lock bolts, mega bolts, trusses and timbers. There is some 

variation in cable bolt patterns and number of bolts used in the 

intersections. An “X” pattern consisting of five bolts with one 

center bolt is used. A variation on this pattern is the “diamond 

pattern,” which is a rotated “X” pattern. A “box-in-box” pattern 

consisting of four corner bolts with an inside pattern of four 

more corner bolts is used by several mines. While not installing systematic intersection supplemental support, a number of 

mines would install intersection support “triggered” by absent 

limestone. In these cases, when the limestone thickness is 

insufficient for roof bolt anchorage, the roof control plan calls 

for the installation of supplemental support.

Surface control. Illinois Basin operators have responded to 

moisture sensitive roof rock with increased skin control. A 

number of mines use straps and large pans to increase surface 

coverage. 

Ten Illinois Basin mines currently use welded steel screen in 

a systematic application to control sloughing roof and prevent 

rock fall injuries. With limestone present above, the contact 

between the limestone and underlying shale unit is sharp. The 

underlying black shale can separate and fall away with time. 

An 8-gauge screen is capable of holding 0.3 to 0.6 m (1 to 2 

ft) of scale, which would otherwise be on the floor (Fig. 12). 

The alternative would be to take down the draw rock, which 

could result in increased waste product. 
Figure 12 — Roof screen loaded with rock.

Typically, mines that use screen will install it in the belt, 

travelway, one of the intakes and one of the return entries. By 

installing screen in a total of four entries and in the crosscuts 

between the belt and travelway, these mines can cover approximately 50% of the total exposed roof in a typical seven 

to nine entry development mining system. This systematic 

coverage is typically used only in mains or other long-term 

entries. Only one longwall mine was screening the roof 

everywhere. 

One of the major barriers to increased use of screen is seam 

height. It is difficult to handle screen in low seams. Twentyeight percent of mines in the #5 seam, with an average seam 

height of 1.65 m (65 in.), use screen systematically, while 50% 

of mines in the thicker #6 seam, with an average seam height 

of 1.9 m (75 in.), install screen systematically. Several mines 

have documented a significant reduction in injuries coinciding 

with the onset of systematic roof screening (Fig. 13). 

Figure 13 — Reduction in rock fall injuries at an Indiana mine after this introduction of on-cycle 

screening.

Several simple procedures can dramatically increase the 

safety and efficiency of screen installation (Compton et al., 

2007):

• To insure that screen handling does not take place inby 

supported roof, after each row of bolts is installed, the 

bolting machine should be backed up by one row of bolts 

and located under supported roof. Then screen can safely 

be loaded on to the ATRS.

• Rails can be installed on top of either side of the roof 

bolter to facilitate screen handling. The screen is loaded 

onto the rails from the rear of the machine and then 

slid up the machine over the ATRS and into place. This 

practice reduces snags on the machine and the potential 

for back injuries. 

• Screen storage racks can be installed on the bolter to 

provide easier handling and less damage to the screen.

• Once the screen is in place on the ATRS, it can be secured 

in place with wire ties. This insures correct location when 

the ATRS is raised to the roof and eliminates shifting of 

the screen. 

Operators should be alert to several hazards that may occur 

during the screen installation. As the roof bolter is installing 

bolts and moving towards the face, it may become impossible 

to install the last screen without having extra screen hanging 

down from the roof. The continuous miner would tear up this 

screen as it advanced the next cut. As a result, some operators 

may finish bolting the place without installing the last screen. 

This leaves a gap in the roof coverage when screen installation is resumed after the next cut. Injuries have occurred from 

rock falling through this gap (Fig. 14). It is better to double 

bolt the last row when starting to bolt and screen the next cut 

in order to anchor the next screen. This practice will ensure 

no screen gaps are left.

The roof may break into very small pieces, depending on 

the composition and weathering characteristics of the roof 

rock. One mine had injuries from small pieces of rock falling 

between the 100 mm (4 in.) openings on a standard 8-gauge 

screen. They went to a screen with 75 mm (3 in.) openings 

to solve the problem. Corners are always an area of concern 

because screens typically do not extend to the rib. One mine 

in Illinois typically extends screen all the way to the rib. They 

use an extra 100 mm (4 ft) angled, conventional bolt to fasten 

the screen at the rib line. In places where the room is cut a 

little wide and the screen does not reach the rib, the corner 

cuts and gutters several feet above the corner. This gutter has 
lead to time-dependant roof falls. At this mine roof screen is 

now installed around the roof corner and about half way down 

the rib. Where this rib screening has been installed, the corner 

stays intact. The condition of the returns and travelways has 

dramatically improved due to rib and corner screening. In this 

case, not only does the screen function as a surface control, but 

it also prevents roof falls that occur with time. Short channel 

extensions can also be used on the last bolt in the row to support wider screens reaching closer to the rib.

One operator experimented with installing a lighter gauge 

wire screen (10 gauge) to save money. The wire itself was strong 

enough, but the screen failed at the welds. NIOSH is currently 

conducting tests on screen products to determine the limits of 

rock load that can be carried. One mine uses a screen-handling 

system on a roof bolter with inside controls and a central walkway 

(Fletcher Walk Thru bolter). The walkway keeps bolters away 

from dangerous ribs. The handling system includes a winch to 

pull a screen bundle onto a lift that raises the screen in place. 

The system reduces material handling injuries. The Walk Thru 

bolter also protects operators from rib, brow or cutter falls where 

screen does not reach the rib. Two other Illinois Basin mines currently use Walk Thru roof bolters without the material-handling 

system. Unfortunately, many of the operators cited low mining 

height as a barrier to using Walk Thru bolters.

While additional steel products installed on the roof will 

add to support costs, data compiled by NIOSH show that the 

additional cost of installing screen could easily be overtaken by 

the cost of a single rock fall injury (Compton, 2007). Further 

savings can be realized from the use of steel screen. Because 

workman’s compensation premiums are directly tied to accident 

rates, a reduction in premiums can be realized by a reduction 

in rock fall accidents (Bhatt, 2007). Another perceived barrier 

to screen installation is the additional time requirement. Many 

super sections today have plenty of roof bolter capacity to make 

up for the additional installation time. 

Data from U.S. longwalls shows that mines that use roof 

screen can also be very productive, as well as being safer 

(Fig. 15). At numerous safe and productive longwall mines, 

screen installation has not impeded development or negatively 

effected production.

Figure 14 — Gaps left in roof screen coverage have resulted 

in injuries.

Multiple seam interactions. Currently, eight of 30 mines 

report multiple seam mining situations somewhere on the 

property. Seven of the eight cases involve the #5 and #6 

seams. There is only one case where ground interactions have 

been reported.

Figure 15 — Safety and productivity in U.S. longwall mines that use roof screen vs. longwall mines that 

do not use roof screen (2003 through 2005).

Retreat mining. Although full retreat mining has been done 

in the past in the Illinois Basin, no mines report any current 

full retreat mining activity. Several mines reported a practice 

called perimeter mining. This type of mining is primarily 

for increased recovery, but has also been used for stress 

control. Perimeter mining involves taking 12 m (40 ft) cuts 

in the solid boundary on one side of a panel. No roof bolts 

are installed, but the opening seldom caves. Similar cuts are 

taken on the opposite solid boundary of the panel on the way 

out of the panel.
Summary

Coal mining in the Illinois Basin is making a comeback due to 

the rise of clean coal technologies and reduced reserves in the 

Appalachian Basin. However, difficult roof conditions resulting 

from horizontal stress and weak, moisture sensitive roof rock 

has made safety a top priority for mine operators. 

NIOSH is currently conducting research into developing 

diagnostic tests that would accurately predict the onset of 

weak roof rock. A large number of roof rock samples have 

been tested for moisture sensitivity, and a database has been 

compiled. 

Some roof rocks from the Illinois Basin show extreme 

deterioration when exposed to moisture. Field observation of 

slaking roof confirms the value of rock testing and provides a 

guideline for predicting future roof deterioration. 

The most difficult roof conditions are being managed by 

reorienting mine openings to minimize horizontal stress damage and by installing supplemental support in intersections. 

Surface control is the single most effective intervention in 

preventing rock fall injuries, especially when installed at 

the face. 

Mine operators in the Illinois Basin have been proactive in 

introducing wire screen and developing innovations that make 

the installation process efficient and productive. Injury reductions show the effectiveness of wire screen. Continued vigilance 

and a willingness to adopt new technologies in controlling the 

roof will make Illinois Basin coal mines safer.

Disclaimer

The findings and conclusions in this report have not been formally disseminated by the National Institute for Occupational 

Safety and Health and should not be construed to represent 

any agency determination or policy.

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