CURRENT TRENDS IN REDUCING GROUNDFALL ACCIDENTS IN U.S. COAL MINES 

C. Mark, NIOSH, Pittsburgh, PA 

D. M. Pappas, NIOSH, Pittsburgh, PA 

T. M. Barczak, NIOSH, Pittsburgh, PA 

ABSTRACT 

Ground falls (roof and rib) have historically been responsible for 

about 50% of all fatalities in bituminous underground coal mines. In 

some recent years, the number of ground fall fatalities has approached 

zero, indicating that significant progress has been made. On the other 

hand, last year’s Crandall Canyon twin mine disasters, in which 9 

miners perished in violent coal bumps, provided a stark reminder that 

complacency is premature. 

One important success has been a great reduction in the number 

of miners killed inby roof supports. Throughout the 1990’s these 

accounted for nearly half of all roof fall fatalities, but there have just 

been two inby incidents in the past 5 years. Progress has also been 

made in pillar recovery. Safer mining technologies have reduced the 

number of fatal incidents, but some recent failures have underlined the 

need for careful management of the retreat mining process. 

On the other hand, more than 400 miners continue to be injured 

each year by rock falling from between supports, and 100 more are 

injured by rib falls. Together, these two categories also account for a 

large percentage of recent ground fall fatalities. Available technologies 

such as roof screen, rib bolting, and inside control roof bolters could 

dramatically reduce injury and fatality rates if they were used more 

widely. Further advances in these areas will likely be the next big 

advance in ground control safety. 

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. 

INTRODUCTION 

On August 6, 2007, a violent coal bump occurred at the Crandall 

Canyon mine near Price, Utah. Six miners working at the time of the 

incident were presumed trapped. Once again, the nation was 

transfixed by the drama of a mine rescue. Ten days later, hope turned 

to horror when three rescuers were killed in a second bump. Rescue 

efforts were suspended, and the original six miners were entombed in 

the mine. 

The Crandall Canyon disaster was the greatest loss of life caused 

by a ground fall incident in a generation. Unfortunately, however, in 

other ways the Crandall Canyon miners were by no means unique. 

Ground falls have always been the single biggest killer of coal miners, 

accounting for approximately 50% of all underground fatalities in each 

decade of the last century. Nearly 50,000 coal miners have perished 

in ground falls in the U.S., mostly in small accidents claiming just one 

or two lives at a time. 

Recent decades have seen a substantial reduction in the number 

of ground fall fatalities, and in some years the toll has been measured 

in the low single digits. The improvement can be attributed to three 

factors. The first is a reduction in the number of miners. For every 

miner working underground today, there were approximately 15 at the 

industry’s peak in 1920. 

The second factor is the development of new technology. For 

ground control, the greatest single technological innovation was roof 

bolting. Other advances have included canopies on mobile equipment

and Automated Temporary Roof Supports (ATRS). 

The third factor is change in the “safety culture,” which determines 

the level of risk that miners take (or are exposed to) while 

underground. The safety culture includes both regulatory mandates 

and company-specific safety policies. Obviously, the safety culture 

has changed dramatically since the early 1900’s when, as the old 

saying had it, “it was cheaper to lose a man than a mule, because the 

company could always hire a new man, but had to buy a new mule.” 

A close look at the ground fall fatal incident rate during the last 

century underlines the importance of the safety culture. 1

 Between 1900 

and 1968, the incident rate was relatively constant, fluctuating between 

about 0.6 and 1.0 fatalities per million employee hours (Figure 1). 

Over the next eight years, following the passage of the 1969 Coal Mine 

Health and Safety Act, the rate fell to about 0.15, an unprecedented 

75% reduction. Careful analysis indicated that this reduction was not 

due primarily to new technology, but rather to widespread application 

of technologies that were already available. In particular, much of the 

improvement can be attributed to the requirement that essentially all 

roofs be supported according to a mandated roof control plan (Mark, 

2002). 

1

 Note that by analyzing incident rates, the effect of the 

number of miners is eliminated. 

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Year

Roof Fall Fatalities / Million Hours

Roof Bolts Introduced 1968 Farmington

 Disaster

1976

Figure 1. Roof fall fatality rates per million employee hours for 

underground bituminous coal mines, 1900-2005.

OVERVIEW OF GROUND FALL FATALITIES SINCE 1995 

In 1995, MSHA started creating electronic versions of their fatality 

reports, and these are available on their website, WWW.MSHA.gov. A 

total of 112 bituminous underground coal miners have been killed in 

groundfalls since 1995. Figure 2 shows that if Crandall Canyon is 

excluded, there has been a slight downward trend in the ground fall 

incident rate during this period. The r-squared for the trend is only 

0.21, however, indicating that it has little statistical significance. 

 
0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

1994 1996 1998 2000 2002 2004 2006 2008

Ground Fall Fatalities/Million Hours

Crandall Canyon 

Figure 2. Ground fall (roof and rib) fatality rates per million employee 

hours for underground coal mines, 1995-2008. The trendline was 

determined for the data set excluding the Crandall Canyon incident. 

For this paper, each of the fatality reports was analyzed in detail. 

In figure 3, the fatalities are categorized by the type of the ground fall 

hazard. The remainder of this paper will be devoted to evaluating each 

of the five largest categories, which together account for 78% of all the 

fatalities during the period: 

• Fatalities occurring inby support (21%); 

• Fatalities during retreat mining (21%); 

• Rib falls (13%); 

• Rock falls from between or around supports, (13%) and ; 

• Major roof falls (not associated with retreat mining) (10%) 

Inby

21%

Coal Bumps

11%

Other

7%

Longwall Face

4%

Roof Skin

13%

Rib

13% Large Roof Fall

10%

Pillar Recovery

21%

Figure 3. Classification of ground fall fatalities, 1995-2008. The total 

number of fatalities for the period is 112. 

For each category, this paper will discuss trends over time in 

fatalities (and injuries where appropriate). The state-of-the-art in 

ground control safety technology for each category will also be 

described. 

The remaining 20% of fatalities, or 23 total incidents, involved: 

• Bumps during retreat mining (11, discussed under “retreat 

mining”). 

• Longwall recovery (3). 

• Underground construction or roof fall rehabilitation (2). 

• Longwall face bumps (2). 

• Longwall face or roof falls (2). 

• Red zone violations (3, discussed under “inby roof support”). 

FATALITIES OCCURRING INBY ROOF SUPPORT 

One clear success story told by the ground fall statistics is the 

reduction in the number of fatalities occurring inby support. Although 

traveling inby for any reason has been illegal since the 1969 Act, 

throughout the 70’s and 80’s approximately half of all ground fall 

fatalities were attributed to this cause (Peters, 1992). Figure 4 shows 

that as late as 2002, there were still two or three fatalities occurring 

inby each year. During the past six years, however, there have been 

just two incidents in bituminous coal mines. 

The progress is associated with the industry-wide educational 

campaign under the slogan “Inby is Out!” Success has come none to 

soon, because it is essential that the new generation now entering the 

mining industry not pick up the bad habits of its predecessor. 

Another factor contributing to the reduction in inby fatalities is the 

development of the concept of the “Red Zone.” In many roof control 

plans, miners must now stay outby the second row of bolts, particularly 

when making extended cuts or turning a crosscut. There have been 

three fatalities (one each in 2002, 2004, and 2005) that occurred when 

miners were within the red zones defined for their mine. 

0

1

2

3

4

5

6

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Number of Inby Roof Fall Fatalities

Figure 4. Number of roof fall fatalities occurring inby roof support

during entry development, 1995-2008 (August). 

FATALITIES OCCURRING DURING PILLAR RECOVERY 

Pillar recovery accounts for no more than 10% of the coal mined 

underground, yet it has historically been associated with more than 

25% of all ground fall fatalities (Mark et al., 2003). However, the 

statistics do seem to indicate that some progress is being made. Prior 

to 2002, an average of two fatal incidents occurred each year during 

pillar recovery operations. During the past six years there have been 

just four incidents, though unfortunately two of those resulted in 

double-fatalities (figure 5). Moreover, the nature of the incidents has 

changed. Most incidents prior to 2002 occurred during the mining of 

final stump, while posts were being set, or because of insufficient roof 

bolt support. In many of these cases, the miners were following their 

roof control plan, but the plan itself proved to be inadequate (Mark et 

al., 2003). In contrast, in most of the recent incidents, the fatalities 

occurred when the victims were unnecessarily in unsafe locations and 

in violation of their roof control plans. 

A key cause of the change has been the widespread adoption of 

safer retreat mining techniques and technology. A concerted effort by 

MSHA and NIOSH (Mark and Zelanko, 2005; Mark et al.; 2003) 

promoted the following three steps to safer pillar recovery: 

• Global stability through proper pillar design; 

• Local stability with proper roof support, and; 

• Worker safety through proper section management. 

The Crandall Canyon incident, like the double bump fatality at the 

C-2 Mine in Kentucky 11 years earlier, was a clear example of global 

instability caused by improper pillar design. The MSHA Fatality 

Investigation Report concluded that the design at Crandall Canyon was 

“destined to fail” because the remaining production and barrier pillars 

were too small to carry the overburden load (Gates et al., 2008). The 

report documented how the two pillar design software packages used 

to develop the design, ARMPS and LaModel, were both employed 

improperly, resulting in the flawed design. In the wake of Crandall 

Canyon, MSHA published a Program Information Bulletin (MSHA, 

2008a) and a Procedure Instruction Letter (MSHA, 2008b) on ARMPS 

to help ensure that pillars are designed properly. 
0

1

2

3

4

5

6

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

Number of Pillar Recovery Fatalities

Double Fatality

Figure 5. Number of roof fall fatalities occurring during pillar recovery 

operations, 1995-2008 (August). 

ARMPS, for Analysis of Retreat Mining Pillar Stability, was 

developed by NIOSH in the mid 1990’s (Mark and Chase, 1997.) Its 

main strength is that it uses a large data base of actual mining case 

histories to suggest the proper stability factors (SF) to employ under 

different circumstances. The original data base was later updated with 

nearly 100 retreat mining case histories from mines operating at 

depths in excess of 750 ft. A key finding of the deep cover research 

was that substantial barrier pillars were essential to maintain stability 

when the mining depth exceeds 1,000 ft (Chase et al., 2002). 

ARMPS has been used extensively to design pillars and evaluate 

roof control plans in the central Appalachian coalfields for nearly a

decade. Since ARMPS came into widespread use, the number of pillar 

squeezes has been greatly reduced, and massive pillar collapses have 

been largely eliminated (Mark et al., 2003). Most recently, NIOSH 

developed the Analysis of Multiple Seam Stability (AMSS) program 

which extends ARMPS to multiple seam situations (Mark et al., 2007). 

Global stability is a necessary, but not sufficient, for creating a 

safe working area. Proper roof support is required to maintain local 

stability. The final pillar stump (sometimes called the “pushout”) 

provides critical roof support during pillar recovery. Traditionally, 

miners tried to extract all the coal during pillar recovery, and many 

fatalities occurred during the mining of the final stump. Research has 

now shown that the optimum pillar extraction plan leaves a final stump 

that is engineered to provide roof support without inhibiting caving 

(Mark and Zelanko, 2001). Most roof control plans that are now in use 

do not allow the mining of the pushout, and specify a cut-to-corner 

distance that ensures that the stump is properly sized. 

One striking feature of the pillar recovery fatalities is that in nearly 

every case, the victim was beneath bolted roof. In many cases, bolt 

failure was itself implicated in the fatality. Increasingly, mines are

using longer and/or stronger bolts to support areas that will be retreat 

mined. In addition, cable bolts or other special bolts are employed in 

intersections, which are the most hazardous locations for miners 

during pillar recovery. 

Traditionally, timber posts provided supplemental support for pillar 

recovery, but they have many disadvantages. Mobile Roof Supports 

(MRS) provide better ground control, and they can be set remotely, 

away from the dangers of the pillar line. Today, perhaps 50% of all 

retreat coal is mined with MRS, primarily in the thicker seams. 

Unfortunately, several of the recent victims in pillar recovery fatalities 

have been MRS operators that were standing unnecessarily in unsafe 

locations. These incidents have underlined the third factor, effective 

section management. 

Management begins before mining, with crew training that 

focuses on the roof control plan and hazard identification. A careful 

geologic survey of the section should be conducted before mining 

commences, and pre-shift evaluations and test holes should be used 

to identify hazards as the section is retreated. Good supervision 

means ensuring that the plan is followed precisely every time, that 

miners always are stationed in safe locations, and that safe operating 

procedures are followed for the MRS units and the other equipment on 

the section. 

RIB FALLS 

Falls of coal and/or rock from the sidewalls of coal mines or pillar 

ribs injure approximately 100 coal miners annually. As figure 6

indicates, the number has actually been trending upward in recent 

years. In addition, rib falls have killed 15 mineworkers since 1995. 

0

20

40

60

80

100

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140

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

Number of Rib Fall Injuries .

Figure 6. Number of rib fall injuries in underground coal mining 

(excluding bumps), 1995-2007. 

Increased risk of rib falls is associated with thicker coal seams 

and higher stress levels. Analysis of the 15 fatal incidents reveals that 

two-thirds occurred at depths exceeding 900 ft, and the mining height 

was at least 7 ft in every case. On the other hand, more than half of 

the rib fall injuries that occurred in the 2005-2007 period were in mines 

where the reported seam height is less than 7 ft. 

Technology is available to prevent rib fall injuries. Rib bolting can 

be highly effective, and it is routinely employed by many U.S. coal 

mines, particularly in the West where thicker seams are prevalent. A 

recent Australian research project concluded that under more difficult 

conditions, the rib support system “should incorporate mesh that is 

firmly secured to the ribline with steel bolts and plates” (Colwell and 

Mark, 2005). It is significant that apparently none of the U.S. fatality 

sites were even rib bolted. 

About a third of the rib fall fatalities were suffered by roof bolter 

operators. In every case, the machine involved was an “outside

control” roof bolter, where the operator works between the coal rib and 

the machine. A much safer alternative is the “inside control” (or “walk 

through”) roof bolter that removes the operator from direct contact with 

the rib. Such machines are available for mining heights as low as 50 

inches. 

ROCK FALLS FROM BETWEEN OR AROUND SUPPORTS 

Since the 1969 Act, any roof that coal miners work under is 

required to be supported. Yet, as figure 7 shows, more than 400 coal 

miners are injured each year by rock falls while they are beneath 

supported roof. Another 11 miners have been killed by these rock falls 

so far this decade. 

The problem is that roof supports, like bolts or ATRS, are 

designed primarily to prevent large rock falls and major roof collapses. 

They do not protect miners from smaller pieces of rock that fall from 

between or around the supports. Figure 8 shows the size of a rock 

that fell between roof bolts and killed one miner in 2008. 

Technology is available to prevent the vast majority of these 

injuries and fatalities. Surface controls like straps, headers, and large 

roof bolt plates can help, but by far the most effective prevention 

technique is roof screen. Screen works best because it can cover 
almost the entire roof (Robertson et al., 2003). Screen also offers a 

first line of defense for roof bolter operators by confining or deflecting 

small rocks that can come loose during drilling or bolt installation. 

Numerous studies have now shown that mines that use screen 

routinely have much lower rates of “struck by” rock fall injuries (Molinda 

and Klemetti, 2008). 

0

100

200

300

400

500

600

700

800

900

1995

1996

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1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

Number of Roof Fall Injuries .

Figure 7. Number of injuries caused by rock falls in underground coal 

mining, 1995-2007.

Figure 8. Photograph of a rock that fell from between roof bolts and 

fatally injured a mineworker (photo from MSHA). 

Unfortunately, many mines do not use roof screen because they 

think installing it would be awkward, time consuming, and expensive. 

However, studies reported by NIOSH have shown that simple machine 

modifications, well planned supply methods, and best practice 

installation techniques can minimize the effects of roof screen 

installation on a mine’s overall mining cycle. Moreover, at many 

mines, the savings in workers compensation costs alone could cover 

much of the cost of roof screen (Compton et al., 2007). 

Roof bolter machine manufacturers also offer a number of 

products that can directly reduce the risk of rock fall injuries. Inside 

control machines greatly facilitate screen handling and installation, 

particularly when equipped with semi-automated material handling 

systems (Robertson and Mark, 2004). Before purchasing a new roof 

bolting machine, or introducing an old machine into a different mine, 

mine operators should carefully evaluate all the potential ground fall 

hazards and ensure that the equipment will address them. 

Existing roof bolters can also be retrofitted with “flipper pads” and 

other safety devices that provide extra protection from rocks falling 

during roof bolting operations. Just this year, a 25-year-old roof bolting 

machine operator was killed in Illinois when a piece of rock fell from 

beyond permanent support and cantilevered outby, pinning him against 

the foot cylinder of the pressurized ATRS (MSHA, 2008c). 

MASSIVE ROOF FALLS 

Massive roof falls are those that extend higher into the roof than 

the anchorage horizon of the roof bolts. Such roof falls must be 

reported to MSHA if they occur in actively traveled parts of the mine, 

even if they do not result in an injury. Approximately 1,500 reportable 

“non-injury” roof falls occur each year, and the number has been 

trending upward (figure 9). These roof falls can threaten miners, 

damage equipment, disrupt ventilation, and block critical emergency 

escape routes. Roof collapses also helped trigger the 2001 mine 

disaster in Alabama which took 13 lives (MSHA, 2004). 

0

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1995

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2001

2002

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200

Number of Noninjury Reportable Roof Falls

7

 . Figure 9. Number of non-injury roof falls reported to MSHA, 1995-

2007. 

Massive roof falls usually provide enough warning that miners are 

seldom caught beneath them. However, during the past 4 years, six 

coal miners have been killed by roof falls that extended above the 

bolts. In one incident, three miners were traveling into the mine on a 

mantrip, when they realized the intersection they were passing through 

was about to collapse. While two miners barely escaped, the third, a 

26-year-old mechanic, was buried under 6 ft of rock (MSHA, 2006). 

Every massive roof fall represents a failure of the installed roof 

support system at that location. Wide mining spans can also increase 

roof fall rates (Mark and Barczak, 2000). This is why more than twothirds of all roof falls take place in intersections, even though 

intersections constitute much less than one-third of all drivage 

underground. Focusing additional support on intersections can be an 

effective control strategy. The extra support can consist of longer, 

stronger, primary bolts or supplemental supports like cable bolts. 

After studying roof falls at 40 coal mines, NIOSH developed the 

Analysis of Roof Bolt Systems (ARBS) software package to help mine 

planners select the most appropriate roof bolt system for their 

conditions (Mark et al., 2001). Unfortunately, even with tools like 

ARBS, roof support design is still more of an art than a science. 

However, if a mine is regularly experiencing roof falls, increased levels 

of support are clearly warranted. 

CONCLUSIONS 

Ground falls continue to be a major hazard in underground coal 

mining. The mining community can be proud of the progress that has 

been made in reducing fatalities occurring inby support and during 

pillar recovery. Unfortunately, the trends in other accident categories 

are not so encouraging. Hundreds of injuries and several fatalities 

occur each year as a result of rock falling from between supports, rib 

falls, and major roof collapses. 
Most ground fall incidents could be prevented by readily available 

technologies that are practical and proven, such as roof screen. 

However, too few mines currently make use of these technologies. It 

seems that the major barrier to further progress is not a lack of 

knowledge, but rather a safety culture that, in too many cases, accepts 

the risk that ground falls currently pose to miners. Today the most 

conscientious mining companies are taking steps to change the safety 

culture, by systematically implementing ground fall prevention 

technologies and lowering the risk to their miners. It is to be hoped 

that the rest of the industry will soon follow. 

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Chase, F.E., C. Mark and K.A. Heasley, (2002), "Deep Cover Pillar 

Extraction in the U.S," Proceedings of the 21st International

Conference on Ground Control in Mining, Morgantown, WV, pp. 

68-80. 

Colwell, M. and C. Mark, (2005), "Analysis and Design of Rib Support 

(ADRS): Rib Support Design Methodology for Australian 

Collieries," Proceedings of the 24th International Conference on 

Ground Control in Mining, Morgantown, WV, pp. 12-22. 

Compton, C.S., S. Gallagher, G.M. Molinda, C. Mark, and G. Wilson, 

(2008), "Roof Screening for Underground Coal Mines: Recent 

developments," Coal Age, June, pp. 34-36. 

Gates, R.A. et al., (2008), "Report of the Investigation, Fatal 

Underground Coal Burst Accidents," August 6 and 16, 2007, 

Crandall Canyon Mine, Available from 

http://www.msha.gov/Fatals/2007/CrandallCanyon/FTL07Crandall

Canyon.pdf. 

Mark, C. and F.E. Chase, (1997), "Analysis of Retreat Mining Pillar 

Stability, Paper in New Technology for Ground Control in Retreat 

Mining," Proceedings of the NIOSH Technology Transfer 

Seminar, NIOSH IC 9446, pp. 17-34. 

Mark, C. and T.M. Barczak, (2000), "Fundamentals of Coal Mine Roof 

Support," New Technology for Coal Mine Roof Support,

Proceedings of the NIOSH Open Industry Briefing, NIOSH IC 

9453, pp. 23-42. 

Mark, C., G.M. Molinda and D.R. Dolinar, (2001), "Analysis of Roof 

Bolt Systems," Proceedings of the 20th International Conference 

on Ground Control in Mining, Morgantown, WV, pp. 218-225. 

Mark, C. and J.C. Zelanko, (2001), "Sizing of Final Stumps for Safer 

Pillar Extraction," Proceedings of the 20th International 

Conference on Ground Control in Mining, Morgantown, WV, pp. 

59-66. 

Mark, C., (2002), "The Introduction of Roof Bolting to U.S. 

Underground Coal Mines (1948-1960): A Cautionary Tale,

"Proceedings of the 21st International Conference on Ground 

Control in Mining, Morgantown, WV, pp. 150-160. 

Mark, C., F.E. Chase and D.M. Pappas, (2007), "Analysis of Multiple 

Seam Stability," Proceedings of the 26th International Conference

on Ground Control in Mining, Morgantown, WV, pp. 1-18. 

Mark, C., F.E. Chase and D.M. Pappas, (2003), "Reducing the Risk of 

Ground Falls During Pillar Recovery," SME Transactions

314:153-160 (formerly SME Preprint 03-137). 

Mark, C. and J.C. Zelanko, (2005), "Reducing Roof Fall Accidents on 

Retreat Mining Sections," Coal Age, December, pp. 26-31. 

McKinney, R. et al., (2004), “Report of Investigation, Fatal 

Underground Mine Explosion, Jim Walter Resources No. 5 Mine, 

September 23, 2001,” available from 

http://www.msha.gov/fatals/2001/jwr5/ftl01c2032light.pdf.

MSHA, (2008a), "Precautions for the Use of the Analysis of Retreat 

Mining Pillar Stability (ARMPS) Computer Program," Program 

Information Bulletin No. P08-08, 

http://www.msha.gov/regs/complian/pib/2008/pib08%2D08.asp. 

MSHA, (2008b), "Technical Support Assistance in Review-ing Roof 

Control Plans," Procedure Instruction Letter No. 108-V-2, 

http://www.msha.gov/regs/complian/pils/2008/pil08%2Dv%2D2.as

p. 

MSHA, (2008c), "Coal Fatalgram 08-C12," http://www. 

msha.gov/FATALS/2008/FAB08c12.asp. 

MSHA, (2006), "Coal Fatalgram 06-C47," 

http://www.msha.gov/FATALS/2006/FTL06c47.asp. 

Molinda, G.M., and T. Klemetti, (2008), "Diagnosing and controlling 

moisture sensitive roof in coal mines," Electronic Journal of 

Geotechnical Engineering, 13(A):20. 

Peters, R.H., (1992), "Miners Views About How to Prevent People from 

Going Under Unsupported Roof," Paper in Preventing Coal Mine 

Groundfall Accidents: How to Identify and Respond to Geologic 

Hazards and Prevent Unsafe Worker Behavior, USBM IC 9332, 

pp. 25-31. 

Robertson, S.B., G.M. Molinda, D.R. Dolinar, D.M. Pappas and G.E. 

Hinshaw, (2003), "Best Practices and Bolting Machine 

Innovations for Roof Screening," SME Annual Meeting, Feb 24-

26, Preprint 03-158, 8 pp. 

Robertson, S.B. and C. Mark, (2004), "Make it Safer with Roof 

Screen," Downloadable video available from

http://www.cdc.gov/niosh/mining/products/product65.htm