acute traumatic injuries in underground bituminous coal miners

American Journal of Industrial Medicine 23:407415 (1993)

Acute Traumatic Injuries in Underground Bituminous Coal Miners

Terrence Lee, MPH, Craig Anderson, MPH, DrHS, and Jess F. Kraus, MPH, PhD

~~ ~

Using injury information from the 1986 Mine Safety and Health Administration database, and deniographic information from a 1986 sample survey of the miner population, acute traumatic injury rates for male workers in underground bituminous coal mines were computed by age, current job experience, and total mine work experience. Three groups of workers assessed in this study showed the highest injury rates: workers aged 15-30 years; workers with between 2 and 3 years of experience; and workers with 10 or more years of experience in the current job.

Injury rates decline with age in each time-interval of current job category and each total mine working experience category. Experience per se does not appear to be related to lower injury rates independent of age, which is paradoxical. The interaction of worker age with length of current or total mining experience is complex. The potential for catastrophic multiple fatalities and severe injuries suggests that additional work is needed to study, simultaneously, factors such as training and job task, and those that describe specific work exposures and mining hazards. o 1993 Wiiey-Liss, Inc

Key words: work experience, mining hazards, injury rates

INTRODUCTION

Mining continues to be one of the most dangerous industries in which workers are at risk of an acute traumatic injury. Mining had the highest traumatic injury death rate among all industries in the United States for 1980-1985, an average annual rate of 31.9 per 100,000 workers per year [Bell et al., 19901. In addition, there is some literature that documents the incidence of nonfatal injuries in the coal mining industry. Bliss et al. [I9771 reported an injury rate in the coal mining industry three times the average for all industries, and a rate of lost work days about eight times that of the overall industrial average. Zimmerman [ 198 11 concluded that disabling injury rates were two to three times greater than in other occupations. The US Department of Labor [1990] reported an injury incidence rate per 100 full-time workers in bituminous coal mining of 1 1.2 in 1988, which was only about one-third higher than the

Department of Epidemiology and Southern California Injury Prevention Research Center, School of Public Health, University of California, Los Angeles. Address reprint requests to Jess F. Kraus, MPH, PhD, Department of Epidemiology and Southern California Injury Prevention Research Center, School of Public Health, 10833 Le Conte Avenue, Uni- versity of California, Los Angeles, CA 90024-1772. Accepted for publication June 3, 1992.

0 1993 Wiley-Liss, Inc.

408 Lee et al.

injury rate for the entire private sector. The lost work days rate per 100 full-time workers was, however, 3.3 times higher in the coal mining industry compared with the private sector.

The underground bituminous coal mining workforce represents about 30% of the mining industry workforce [Butani and Bartholomew, 19881 and in 1986, 63% of the 136 deaths and 57% of the injuries in the mining industry were in coal mining operations [US Department of the Interior, 19861.

Many factors, possibly associated with the causes of the excess injury rates in coal mining, have been explored over the past several decades, including age, job experience, and training [National Research Council, 1982; Bennett and Passmore, 1984; Martin and Prather, 1973; Kobrick, 1973; Butani, 19881. However, even with information on these descriptive factors, it has not been possible to determine injury rates for specific groups of miners, because information on the number of miners at risk of injury was needed [National Research Council, 19821.

To compensate for lack of detailed information on the population of coal miners, the US Bureau of Mines conducted a two-stage sample survey in 1986 to determine the demographic features of the United States mining population [US Depart- ment of the Interior, 19861. The factors enumerated included: job title or occupation, principal equipment used, work location at the mine, experience at present job, experience with present company, total mining experience, job-related training during last 2 years, age, sex, race, and education. By linking injury reports for 1986 with the demographic survey for 1986, it is possible to derive injury rates for specific subgroups of the mining population. This current report describes injury rates for underground bituminous coal miners by age, years of experience at the current job, and total years of experience in the mining industry; only these three exposure factors are recorded in both the survey and the injury report form.

MATERIALS AND METHODS

The Mine Safety and Health Administration (MSHA) injury database for 1986 was obtained from the Division of Safety Research, NIOSH. An acute traumatic injury was defined as one that resulted from mechanical energy forces, and included falls, bodily contact with equipment, or being struck by falling or moving objects. Injuries occurring during 1986 in underground bituminous coal mines were included in the analyses if they resulted in death, in permanent or partial disability, or in work loss of at least 5 days. The reporting of minor injuries may be incomplete; hence, we chose to include only those injuries that were severe enough to ensure complete reporting. Because of the very small number of women employed in this industry, only injuries involving men were studied.

The types of incidents resulting in the injuries included: “struck against . . . ,” “struck by , . . ,” ‘‘fall’’ (higher to lower level or same level), “caught idunder/ between . . . ,” and “rub or abraded.” Injuries classified as strain, rupture, or internal injury were excluded, because of the uncertainty of time or place of occurrence or cause. Injuries from overexertion, electric shock, contact with heat or cold, caustics, toxins, flashburns, and water immersion and drowning were excluded, because of the wide variety of exposures involved, and differences in countermea- sures that are required for prevention.

Estimates of the coal mining workforce were obtained from the US Bureau of

Acute Traumatic Injuries in US Coal Miners 409

Total Reparts F i l e d ( N = 25,283) I

I 1.041

I 1 lnesses 2 .755 I

In jur ies I

I 2,437

Accidents u/o i n ju r ies

1, Minor

1 1 i 6, d 27 9 I Days Auay and

from Uork Restricted Ac t iv i t y In ju ry Perm. or Part. Days Auay

D isab i l i t y

4 Death

I I

6.295 I

6. 75 1 8 Male Female Not Specified

I 6. I 75 Male

I 1 8 I

Female 4

Not Specified

143 5,482 Diseases, hernias, Non-Diseases ( inc l . eye in jury) "other" in jur ies I

# days auay:

4,470 In jury Due to Inpact or FrictionIPressure

I

1.142 In jury Due to Overexertion

I I I I I I I 243 297 147 111 149 lib 113 3,b44

1 8 or mare + death

Feuer than 5 days away from uork More than 5 days

auay from work

Fig. 1 . classification and selection of homogeneous study group.

Total 1986 injury or illness reports with Mine Health and Safety Administration, and steps in

Mines report on characteristics of the 1986 coal mining workforce [US Department of the Interior, 19861. Rates were derived using this source for worker count estimates by each factor (age, years of experience at current job, and total years of experience in the mining industry) during 1986. The coefficient of variation for each subgroup of workers was used to compute 95% confidence intervals for the injury rate.

Logistic regression was employed to examine the main and interaction effects of age, total years of mine experience, and years of work in the current job on the risk of injury.

RESULTS

The process of database review and steps in selection of the injury/accident reports is schematically portrayed in Figure 1 . In 1986, 25,283 illness, injury, or

410 Lee et al.

TABLE I. Number, Percent, and Rate of Fatal and Nonfatal Acute Traumatic Injuries per 100 Male US Bituminous Underground Coal Miners, 1986

Ratc No. YO per 100

Injury severity injured In i ured workers

Death 37 Permanent or partial disability 8 2 5 days of work lost 3,342

1.1 2.8

97.1

0.05 1.02 4.91

Total 3,442 100 5.06

“accident” reports without an injury were filed with MSHA. Bituminous coal mining accounted for 12,132 of the 21,755 “injury” reports. Over 8,000 of these were associated with underground mining and 1,736 were a “minor injury.” Males accounted for 6,175 of the 6,295 non-minor injuries and 5,982 were injuries rather than diseases, hernias, or “other” injuries. Acute traumatic injuries represented 4,270 (71.4%) of the group of 5,982 injuries reported. The final study group of 3,442 sustained either a fatality, a permanent or partial disability, or an injury with 5 or more workdays away from work. The acute traumatic injury rate among 68,007 male underground bituminous coal miners during 1986 was 5.1 per 100 workers employed at any time during 1986, and included 1.1 % of the injuries resulting in death (Table I and Fig. 1).

Age-specific injury rates decrease with age: rates are highest in the younger age groups and are lowest in male underground miners aged 50 or older (Table 11).

Injury rates, according to years of total mining working experience, for male underground bituminous coal miners are summarized in Table 111. Rates per 100 workers are lowest in the work experience groups of less than 1 year and more than 20 years of total mining work experience.

Work experience is, of course, related to age of the miner: few mine workers aged 15-19 years would be expected to have more than a few years of mining experience. Injury rates per 100 workers by age and length of total mining experience (Fig. 2) show, with one exception, that injury occurrence declines by age within each of the years of total work experience group. Workers aged 50 years and older have a high injury rate within the 5-10 years work experience group. The highest injury rate observed is among mine workers aged 30-39 years who had 20 or more years of mining experience. This rate, however, was based on 8 injured persons among 82 miners with this combination of age and years of total work experience. Note that the black solid portion of the bars in Figures 2 and 3 shows the excess in injury rate over the overall injury rate of about 5 per 100 workers.

Injury rates by years of current job experience (Table IV) show two intervals with high injury rates: those with 2 or 3 years and those with 10 or more years of experience on the current job. The high rate for those with 20 or more years on the current job is based on 29 injured workers among a total of 314 worker-years. The association of age with current experience is shown in Figure 3. High injury rates among mine workers with 2 or 3 or 10 or more years in the current job appear to cluster in age groups of 20 to 29 and 30 to 39 years. In addition, a high injury rate is observed for those with 5-10 years of experience and aged 20-29 years. However, this rate is based on 16 injured workers among 7 1 in the mining workforce of this age


TABLE 11. Number of Workers, Number Injured and Injury Rates per 100 Male Workers by Age at Time of Injury. US Underground Bituminous Coal Miners, 1986

Age group

15-19 20-24 25-29 30-34 35-39 40-44 45 - 49 50 + All ages

No. of workers

No. injured

Injury Rate/ 100a

95 % CI

132 2,224

10,550

14,423 8,67 1 6,402 8,257

68,007

16,948

9 136 578 823 711 402 295 322

3,442

6.8 6.1 5.5 4.9 4.9 4.6 4.6 3.9 5.1

3.9 -26.7 4.8-8.3 5.0-6.0 4.6-5.2 4.6-5.3 4.4-4.9 4.1-5.3 3.7-4.1 -

'Rates exclude 166 injured and 400 uninjured miners with unknown or unrecorded ages during 1986.

TABLE 111. Number of Workers, Number Injured and Injury Rates per 100 Male Workers by Length of Total Mine Work Experience. US Underground Bituminous Coal Miners, 1986

Work experience No. of No. Injury 95 % (years) workers injured ratei100" CI

0-1.0 905 37 4.1 2.7-8.9 1.01-5.0 5,707 319 5.6 4.8-6.6 5.01-10.0 16,252 961 5.9 5.4-6.5

10.01-15.0 20,821 1,138 5.5 5.3 -5.7 15.01-20.0 7,656 47 1 6.2 5.7-6.7 20.01-30.0 2,778 133 4.8 4.4-5.3 30.01 + 1,681 83 4.9 4.5 -5.4

"Rates exclude 12,204 workers and 301 injured workers with unknown total mine work experience.

and current job experience. The highest injury rate observed (22.5/100 workers) is seen among those with 10 or more years on the current job and aged 20-29 years. This rate is based on 16 injured workers.

The main effects from the logistic regression model (Table V) show that age has a very small protective effect. Total mine experience also has a protective effect, that is, 5 years of mine experience reduces injury risk by 10%. Current job experience is associated with an increase in risk. Five years in the current job increased injury risk by 20%. Neither interactions among these factors nor years-squared terms contributed further to the logistic model.

DISCUSSION

Age and experience are closely related factors. While it is possible to have older age and short work experience, the opposite is not possible. In addition, while age is a fixed factor, and intervention may be achieved by restricting employment to workers of a specific age, experience (as a surrogate for expertise in job tasks) may be enhanced at any time through training or supervision.

Findings from the National Research Council [ 19821 show that younger miners have a disabling injury rate about twice as great as older miners. Experience was not

412 Lee et al.

Iar i e s per I 0 0 workers

........................... . . . . . . . _ .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

15-19 20-29 30-39 4 - 4 9 S O +

Fig. 2. work experience in years, 1986.

Acute injury rates per 100 male US underground bituminous coal miners by age and total mine

IMries per I00 workers

" 15-19 20-29 30-39 4 - 4 9 50 t *Rnt. based 011 < 20 W W L m Age Group

Fig. 3. experience in years, 1986.

Acute injury rates per 100 male US underground bituminous coal miners by age and current job

considered in this finding, but was offered as one explanation in the work of Hag- glund, cited by Bennett and Passmore [1984].

Total mining experience was inversely related to fatal or nonfatal injuries in coal miners in a 1973 report by Martin and Prather [ 19731. This same finding was reported when experience in the current job was used as the exposure measure. Kobrick [ 19731 came to the same conclusion when disabling injuries were studied in a large coal mine


TABLE IV. Number of Workers, Number Injured and Injury Rates per 100 Male Workers by Years of Experience in Current Job, US Underground Bituminous Coal Miners, 1986

Work experience No. of No. Injury 95% (vears) workers iniured rate/100" CI

51 .0 1.01-2.0 2.01-3.0 3.01-5 .O 5.01-10.0

10.0 1-20.0 20.01 +

14,229 553 3.9 3.5-4.3 10,793 410 3.8 3.5-4.1 6,451 537 8.3 7.4-9.5

10,364 355 3.4 3.0-4.0 14,987 837 5.6 5.0-6.3 5,893 467 7.9 7.2-8.8

314 29 9.3 7.2-1 3 .O

"Rates exclude 4,976 workers or 254 injured workers with missing data on length of current mine job experience.

TABLE V. Association of Age, Total Years of Mining Experience, and Years in Current Job on Risk of Acute Injury, US Underground Bituminous Coal Miners, 1986

Model Standard Rate ratio and

Beta error P 95% CI

Intercept -0.048 0.105 0.645 Age" -0.004 0.003 0.196 0.96 (0.89, 1.02) Total years experience -0.021 0.005 <0.001 0.90 (0.85, 0.95) Years in current iobb 0.036 0.006 >0.001 1.20 (1.13, 1.27)

"Rate ratio is for a 10-year difference in age. bRate ratio is for a 5-year difference in years of experience in current job

company. Age was not considered in the analysis in either of these earlier reports, but was controlled for in the analyses of 1986 data on bituminous coal mine injuries reported by Butani [ 19881.

Our findings show that, generally, inexperience level is more related to high injury rates than age. Workers 15 to 30 years of age have the highest injury rates with an inverse gradation in rate from youngest to oldest in this age interval (Table 11). The interrelationship of age, total mining experience, and experience on the current job at time of injury during 1986 is complex, as evidenced by the mixed findings in the earlier published research and our findings reported here. Injury rates decrease generally with age in all total years of work experience categories except those with 5 to 10 years of experience. When length of current job experience is used as the quali- fying factor, there is a decline in injury rates for all categories. If level of experience were related to increased injury rates, high rates would be expected in every age category. Our findings show that the injury rates are low in all age groups with less than 2 years of work experience. It must be noted that a few of the higher injury rates were based on small numbers; hence, the rates are subject to wide variation.

Less clear is an explanation for the high injury rates among workers with 2 or 3, or 10 or more years of current job experience. Work rules, practices, and supervision may protect workers who are new to a job from the most hazardous tasks of that job. It will be difficult to reconcile this observation without data on work task coupled with specific information on training, current job hazards, and injury control coun- termeasures. In addition, factors such as size of workforce and of the mine may also be related to these work-task factors [National Research Council, 19821.

414 Lee et al.

The results of the logistic regression were paradoxical. Total mine work experience was protective, but experience in the current job had a greater but opposite effect. It is conceivable that the level of exposure to underground mining hazards might decline with increasing overall mining experience, but total years in the current job should logically reflect this same change in the level of hazardous exposure,

In assessing the results of this study, the reader should bear in mind the homo- geneity of the workforce, definitions of injury exposure (and type), and categories of age and exposure experience. The findings were aggregated in somewhat standard groupings, but it is conceivable that they may mask high (or low) risk subgroups within such groups. Certain of the findings suggest additional investigation, in par- ticular the relationship of age with increased or decreased injury rates in some experience categories. Even these findings must be cautiously interpreted, because of the potential for underreporting inherent in the current injury reporting system, and the possibility of undernumeration in the 1986 census used to estimate features of the bituminous coal mining worker population.

The size of the worker population, the high injury rate, and the potential for catastrophic events with multiple fatalities and severe injuries require a careful scru- tiny of all injury data available. This report is an initial step, in which we attempted to control for the interaction of age in the job experience injury rates.

ACKNOWLEDGMENTS

Many persons helped with this project, including Carol Conroy, Tom Bobick (NIOSH), and Anne Coulson (UCLA). We are grateful to Me1 Myers (NIOSH) who sponsored Terrence Lee as an intern during the time he worked on this project, and David McArthur, PhD, who developed the graphics presented in this paper. This research was supported in part by the Southern California Injury Prevention Research Center Grant (Centers for Disease Control; R49/CCR 903622-Ol), and the UCLA Center for Occupational and Environmental Health.

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Bell CA, Stout NA, Bender TR, Conroy CS, Crouse WE, Myers JR (1990): Fatal occupational injuries in the United States, 1980 through 1985. JAMA 263:3047-3050.

Bennett JD, Passmore DL (1984): Correlates of coal mine accidents and injuries: A literature review. Accid Anal Prev 16:37-45.

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acute traumatic injuries in underground bituminous coal miners

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