Underground coal mining injury: A look at how age and experience relate to

days lost from work following an injury

Katherine A. Margolis *

* Tel.: +1 412 386 4627.

E-mail address: KMargolis@cdc.gov.

Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, PO Box 18070, Pittsburgh, PA 15236, USA

abstract

Coal has been mined in the United States since colonial times and coal mining has always been a dangerous occupation. Despite the dangers involved in coal mining, coal is essential to the functioning of our

society. Coal provides energy for products, businesses, and homes. Not only is coal mining a dangerous

occupation, but, like many other industries, coal mining has also been referred to as a ‘‘graying occupation” as many coal miners are reaching retirement age. Younger workers possess certain advantages as

older workers may have age-associated decrements in cognitive function, health, and recuperative ability. Although there are documented decreases in health and safety associated with age, there are also

benefits at the workplace associated with increasing age. Increasing age brings about more experience

and familiarity with the work environment. This study used the Mine Safety and Health Administration’s

(MSHA) database on accidents, injury, and illness from the years 2003 through 2007 to examine how age,

experience at the current mine, total years experience as a coal miner, and experience in the current job

affects injury severity. The results of the data indicated that there was a relationship between age and

days lost as well as total mining experience and days lost following an injury. Furthermore, the data indicated an increased risk of overexertion injuries as age increases. These are important findings for the coal

mining industry as many miners are more experienced and older.

1. Background

Coal mining has been dangerous occupation since mining of

coal began during colonial times (Reardon, 1993). The deadliest

year in US coal mining history was 1907 when 3242 people died

(Mine Safety Health Administration, 2007). That year was highlighted by an explosion in Monongah, WV which killed 358 people

(Mine Safety Health Administration, 2007). Due to high fatality

rates during the early 1900s at coal mines, in 1910 the US Bureau

of Mines was founded. The US Bureau of Mines was created to conduct research and develop technologies to reduce safety and health

risks to miners. Since its inception, the US Bureau of Mines has

achieved a significant reduction in mining injuries and fatalities

and improvements in health. This reduction in the number of

deaths is partly due to ‘‘pioneering research on hazard identification and control,” (National Research Council, 2002, p. 4) which

has been conducted by the US Bureau of Mines and later became

part of the National Institute for Occupational Safety and Health

(NIOSH). More stringent legislation later followed in 1969 and

1977 which led to a number of safety improvements. Between

2003 and 2007, the number of deaths per year of coal miners ranged from 23 to 47 deaths; which is a substantial reduction from

3242 deaths in 1907.

Despite increased safety measures and decreased death toll,

coal mining has received national attention as a dangerous industry, partly from recent highly publicized disasters. Even though research and new practices continue to reduce the dangers of the

industry, its reputation is somewhat warranted when coal mining

is compared to other industries. The 1992 census found that workers in mining have a higher risk of workplace fatalities relative to

their share of employment (Toscano and Windau, 1993). A look

at fatal occupational injuries from 1980 to 1997 found that mining

had the highest death rates (Centers for Disease Control and Prevention (CDC), 2001). In more recent years, coal miners continue

to experience a high injury rate. In 2006, there were 3021 non-fatal

lost-time injuries among coal miners occurring at a rate of 3.3 injuries per 100 full time equivalent employees (US Department of

Health and Human Services (DHHS), 2008). The Bureau of Labor

Statistics (BLS) has also found that employees in coal mining are

more likely to be killed or sustain an injury or illness than workers

in private industry. These injuries are also likely to be more severe

(BLS, 2007a). In fact, the rate of non-fatal injuries and illnesses in

coal mining in 2005 was 11% higher than for total private industry.

The injury rate is even higher in underground bituminous coal

mining; the rate of non-fatal injuries and illnesses was 63% higher
than that of all private industry (BLS, 2007a). Fig. 1 illustrates the

higher rate of fatality for coal mining compared to metal/nonmetal

mining and private industry in 2007.

Fig. 1. Fatality rate for coal and metal/nonmetal mining and private industry, 2007. (Data comes from the Bureau of Labor Statistics (BLS), the Census of Fatal Occupational

Injuries (CFOI) and the Current Population Survey (CPS). Metal/nonmetal mining includes metal, nonmetal, stone, and sand and gravel mining. Data reported for the metal/

nonmetal industry are not available separately from the CFOI program).

Despite the dangers involved in coal mining, coal is essential to

the functioning of our society. Coal provides energy for products,

businesses, and homes. More than 51% of electricity in the United

States is generated through burning coal (United Mine Workers of

America (UMWA), n.d.). For this reason, coal production continually rises and will likely continue to do so as the US is estimated

to have over 250 years of coal reserves available for mining

(UMWA, n.d.). Because coal is essential to society and there appears to be no immediate limit to its mining, it is important to

understand injuries in coal mining. Furthermore since coal mining

injuries tend to be more severe (BLS, 2007a) it is important to

understand who is suffering the most severe injuries in coal mining

in hopes of reducing their occurrence and severity.

Not only is coal mining a dangerous occupation, but, like many

other industries (Pransky et al., 2005) coal mining has also been referred to as a ‘‘graying occupation” as many coal miners are reaching retirement age. According to the National Mining Association

(NMA), in 2006 the mean age of a coal miner was 50 and the median was 47 (NMA, 2007). The age of a coal miner can be compared

with the entire civilian labor force where the median age has been

steadily increasing and was 40.8 in 2006 (BLS, 2007b).

This growth in older workers across all industries, not just coal,

can be attributed to the aging Baby Boomer Generation (Schwerha

and Mallett, 2005). Among those aged 55–64, 60% are in the workforce and 14% of those over the age of 65 are in the workforce (Rogers and Wiatrowski, 2005). Furthermore, certain policies and

budget cuts indicate that older workers will comprise a larger portion of the workforce in the future and will need to continue working past retirement age (Besl and Kale, 1996). It is estimated that

by the year 2025, 30% of the North American workforce will be

over the age of 55 (Ilmarinen, 2001). Therefore, there is a trend

in the US of an increasingly older workforce; however, this trend

is even more pronounced for coal mining.

An aging workforce has certain implications for health and

safety as workers aged 65 and above have the highest age-specific

occupational death rate (Centers for Disease Control and Prevention (CDC), 2001). An older workforce also has important implications for coal mining. First, this means that there will be an influx

of young and inexperienced coal miners. Secondly, there are

numerous miners who are older and possibly more prone to severe

injury. These two factors could have important effects on the injuries in coal mining.

Due to the current demographics of the mining industry, the

first hypothesis is advanced. It is predicted the older a worker is,

the more severe an injury that he/she will suffer. Specifically, it

is posited that age will be positively related to days lost from work.

The evidence for how experience impacts miners’ safety is less lucid. Therefore, a research question is posed which asks how experience at the current mine, total years experience as a coal miner,

and experience in the current job is related to days lost from work.

2. Literature review

2.1. Age

Younger workers in many cases possess certain capabilities over

older workers including increased strength, speed, and precision

(Ilmarinen, 2001). They do, however, have less experience and

training which can hinder both their performance and safety. Often, injuries and fatalities are not only a matter of youth but are

also due to inexperience. Workers often experience higher injury

rates during their 1 year on a new job or when they start working

for a new employer (Siskind, 1982). Looking at the impact that

younger workers have on injury rates, Mitchell (1988) found that

employees under the age of 25 are more likely to become injured

on the job compared to older workers. Mitchell (1988) attributes

this to a higher prevalence of temporary injuries due to inexperience. Although Mitchell (1988) does not define temporary injuries

these are usually injuries which are not permanent, such as paralysis, and will heal such as a sprained ankle.

Breslin and Smith (2005) examined age-related differences in

work injuries with a representative sample of Canadians aged 15

and older. It was found that the younger groups of workers, were

more likely to sustain a work injury than were adults over 35

(Breslin and Smith, 2005). Although this provides evidence that

younger workers are more likely to become injured on the job, it

clusters all older workers above the age of 35 in one group. This

clustering might miss some of the injury trends for older workers

as there are large differences in the capabilities of a 35 year old

and a 55 year old. Furthermore, this study only looked at the frequency of injury and did not include severity statistics.

Increased age also becomes a concern as workers may have ageassociated decrements in cognitive function, health, and recuperative ability (Pransky et al., 2005). Older workers also have decreased physical abilities such as changes in their maximal

oxygen consumption and musculoskeletal capacity after the age
of 45–50 (Ilmarinen, 2001). Kowalski-Trakolfer et al. (2005) point

out that the aerobic capacity of a 65 year old is about 70% of a

25 year old. For these reasons, there currently exist numerous

studies on the effects of age and injuries in the workplace. Meyer

(2005) found that a worker aged 55 or older in farming was more

than 10 times more likely to be fatally injured than workers as a

whole.

Another study that looked at how age and injury are related

only examined older workers and clustered younger workers with

middle-aged workers. In this study, Pransky et al. (2005) found

that among workers who had lost time due to injury in New Hampshire, older workers reported more comorbidities than did those

who were under the age of 55. Duration of lost time, however,

was not different for those above 55 and those below 55 (Pransky

et al., 2005).

Another way to look at how age relates to injury is to look at age

as a continuous variable and not break it into clusters. Feuerstain

et al. (1999) did examine age as a continuous variable and found that

among US Army soldiers every increase in age by 1 year brought

along an increased risk for a low back disability. This study did

not, however, examine lost work time or severity of injury by age.

2.2. Experience

Although there are documented decrements in health and

safety associated with age, there are also benefits at the workplace

associated with increasing age. Increasing age brings about more

experience and familiarity with the work environment. Some studies have found that older workers are not more prone to injury because they are able to use experience and resources more

efficiently to avoid injury (Laflamme and Menckel, 1995). As workers age they do gain improved cognitive function in some areas

including the ability to process complex problems in insecure situations and control of the use of language (Ilmarinen, 2001). These

are important skills to have during a mine emergency. Furthermore, older workers may be able to overcome shortcomings in

speed, strength, and precision with increased motivation, experience, and wisdom which they have acquired during their work tenure (Ilmarinen, 2001). Laflamme and Menckel (1995) reviewed 22

studies and found evidence that older workers do possess a compensatory ability to reduce difficulties in meeting job demands.

Breslin and Smith (2006) studied Ontario workers and also found

that injury claim rates declined as time on the job increased.

2.3. Severity of injury

Although frequency of injury may not increase as workers age,

severity may. Severity of injury is often considered the number of

days lost from work after an injury. In a later study, Laflamme et al.

(1996) specifically examined male iron-ore miners in Sweden. It

was found that as age increased there was no escalation in frequency of accident. There was, however, a rise in the severity of

accidents as age increased. In this study, severity of injury was

operationalized as the number of days lost from work and it was

found that workers aged 55–65 tended to lose a greater number

of days from work due to an injury than younger workers (Laflamme et al., 1996). This study is one of the few studies to examine

how age affects injury in a mining setting. However, it examines

iron-ore workers in Sweden. Underground coal mining is known

to be the most dangerous form of mining in the US. The US also

has different safety regulations from Sweden. Therefore, this study

cannot be generalized to US coal mining but it does provide a good

foundation for future research in the US.

Because there is mixed evidence about exactly how age relates

to injury, some studies have examined both rate of injury and

severity of injury. Mitchell (1988) found that younger workers

had a higher rate of injury but employees aged 65 and above suffer

more serious job related injuries. Wiatrowski (2005) examined the

BLS data for the year of 2003 and also found that those 65 and older

had a lower incidence of injury but had the highest median days

away from work due to an occupational injury or illness. Furthermore, approximately 40% of injuries that involved days lost among

those 65 and older involved 31 days or more off from the job (Wiatrowski, 2005).

There are, however, very few studies that examine the effect of

age on mining safety, specifically coal mining. Coal mining must be

studied separately from other occupations and types of mining as it

has a higher average age of worker than the overall work force and

it is more dangerous. The work environment of coal mining also

provides unique challenges to safety and health. It is, therefore,

important to examine how aging specifically affects safety in coal

mining.

3. Methods

Data came from the Mine Safety and Health Administration’s

(MSHA) database on accidents, injury, and illness, which includes

injury data reported from all underground coal mines. Data from

the years 2003 through 2007 were combined to form one data

set with a sample size of 10,345. Data were only included from coal

operator employees and not contractors. The data set only reports

data on characteristics of injured miners. It does not include demographic data about all workers or frequency of injury to various

categories of workers. It is important to note that mining differs

vastly by method of mining, type and mineral. Therefore, this study

chose to examine only underground coal mining so that the results

are more interpretable. Furthermore, coal mining was chosen because it has the highest rate of non-fatal injuries among all types

of mining (McDermott, 1997). Within coal mining, underground

coal mining had substantially more injuries than surface mining

(Bureau of Labor Statistics, 2007a). For these reasons, the present

study will only examine underground bituminous coal mining as

it is the most dangerous form of mining. Therefore, it is important

to understand these injuries in order to reduce them.

In order to test the hypothesis and research question, the data

were restricted to only include injuries where one or more days of

work were lost due to injury. Age was generated from the birth date

where the median was 43.00 (SD = 11.22) and the range was

17–79 years. Age was normally distributed; yet, slightly platykurtic.

Experience was measured with three different variables. Total

mining experience was measured in years with a median of

13.90 years (SD = 14.85) and a range of 0.02–46.00 years. Experience at this mine had a median of 2.00 years (SD = 7.65) with values ranging from 0.02 to 38.06 years. Experience in this job was

also measured in years and had a median of 3.00 years

(SD = 7.76). The range was 0.02–44.00 years. All three experience

variables exhibited slight positive skews.

The severity of injury was operationalized as the number of

days lost from work after an injury and was measured using

MSHA’s actual days lost variable. This variable does not include

statutory days lost but instead measures the actual number of days

lost due to injury. Only values greater than or equal to 1 day lost

were included in the analysis. The median for this variable was

32.00 days (SD = 77.23) with a range from 1 to 376. Days lost

was slightly positively skewed.

In order to assess the hypothesis, a multiple regression analysis

was conducted with age and the three experience variables as

independent variables and days lost as the dependent variable.

The analysis appeared to meet all of the assumptions of multiple

regression. The plot of residuals versus predicted values implied

that there were independent errors and that the assumption of
homoscedasticity was not violated. There also did not appear to be

any outliers that should be removed from the analysis. There also

was no evidence of multicollinearity as there were no tolerance

values less than 0.10. Although the distribution of the data was

not perfectly normal, other studies have shown that in large data

sets normally distributed data are not necessary for regression

and t-tests (Lumley et al., 2002). This data set included 9627 cases

list-wise so it is large enough to withstand slight violations to the

assumption of normality.

The results of the multiple regression analysis revealed that

there was a significant relationship between the independent variables and the actual days lost due to injury, F(4, 9623) = 30.24,

p < .001. R for the model was 0.11 and adjusted R2 was 0.01. The

relationship between age and days lost was statistically significant,

b = 0.78, t(9623) = 4.69, p < .001. The relationship between total

mining experience and days lost was also statistically significant

b = 0.37, t(9623) = 1.97, p < .001.

Because age and total mining experience were the only significant relationships, the other two experience variables were removed from the model and the regression analysis was

conducted again. When days lost from work was regressed onto

age and mining experience total, the relationship was significant,

F(2, 9761) = 59.08, p < .001. R for the model was 0.11 and adjusted

R2 was 0.01. The relationship between age and days lost remained

statistically significant, b = .07, t(9761) = 4.52, p < .001. The relationship between total mining experience and age also remained

statistically significant, b = 0.04, t(9761) = 2.46, p = .014.

Therefore, these data show that age and total mining experience

have an influence on severity of injury such that as age and total

mining experience increases the number of days away from work

following an injury also increases. This relationship, although statistically significant, is slight and greatly influenced by the large

sample size. Furthermore, experience at this mine and experience

in this job do not have an effect on severity of injury.

Because the data allowed for further assessment of the role that

age plays in injury, an analysis to gain a more nuanced understanding of this relationship was sought. One type of injury that may be

more likely to occur as age increases is the overexertion injury. Kumar (1994) points out that overexertion injury can occur due to direct trauma, a single overexertion, or frequent or sustained load to

the body. As miners age, they have likely not only sustained load to

the body but their physical strength also decreases (Kumar, 1994).

Therefore, the body has weakened capacity and is more likely to

suffer from overexertion. Due to this, the relationship between

age and overexertion injury was assessed.

Increasing age was hypothesized to result in an increased risk of

overexertion injuries compared to all other types of injury. In order

to assess this relationship, a logistic regression analysis was conducted. The accident type variable was recoded so that all types

of overexertion injuries were given a value of 1 and all other types

of injuries were recoded to a value of 0. Age was initially used as a

continuous-scaled variable; however, it did not meet the assumption of linearity in the logit. Three design variables were then created for age, less than 30 years, 31–50 years, and more than

50 years. The reference group was the less than 30 years. The results of the logistic model showed an increased risk of overexertion

injuries as age increases. For the 31–50 age group, the estimated

odds of an overexertion injury, compared to the reference group

were 1.554, p < .0001, while for the over 50 group, the odds were

equal to 1.937, p < .000.

4. Discussion

McDermott (1997) reports that injuries in mining are more severe than other industries as miners often miss more days of work

after an injury. It is, therefore, important to study the severity of

mining injuries and what is associated with these injuries in an attempt to reduce them. Once more is known about variables associated with mining injury severity, steps can be taken to reduce the

severity of injury. For example, specific safety programs could be

developed for older workers to avoid injury.

This study takes a step in attempting to reduce the severity of

mining injuries by trying to better understand variables that may

be affecting injury. One of the biggest shifts in the mining industry

as of late is an aging workforce and the addition of younger, inexperienced workers to the industry. Inexperience in the workforce

may affect mining injury severity as mining requires a great deal

of safety training. And the present study found that total mining

experience was related to an increase in the number of days lost

from work.

As more inexperienced miners enter the workforce an emphasis

on training will be necessary to reduce injury among the new

workers. Improved training for coal miners has been linked to better health and safety outcomes (Braithwaite, 1985). Instead of simply training workers on how to operate equipment or perform a

task, training should also include how to avoid an injury. Castillo

and Rodriguez (1997) conducted a national survey of people who

were injured while working and found that 66% of workers had

not received training on how to avoid injury while doing their line

of work. Shannon et al. (1997) found that a greater amount of

training was related to lower injury rates. In a review of 80 studies,

Cohen and Colligan (1998) found that training not only increases

worker knowledge of job hazards but also affects safer work practices. Training could also focus on how to avoid certain types of injury such as overexertion injuries.

The literature often suggests a link between job experience and

injury. However, the present study only found a slight relationship

between total mining experience and severity of injury. As total

mining experience increases, miners miss more days of work after

suffering an injury. There was also, a relationship between age and

severity of injury. As age increases, miners miss more days of work

after an accident. Although it is often assumed that an older miner

is likely to miss more days of work than a younger miner after an

injury, there have not been any studies to show if this relationship

does exist. Furthermore, before the present study there was no evidence as to how strong this relationship might be. Therefore, the

present study confirms that there is a statistically significant and

positive relationship between age and days lost from work after

an injury. There was also a relationship between age and overexertion injuries such that as age increases the likelihood of an overexertion injury increases. It should be noted, however, that the

relationship between age and severity of injury is not as strong

as some might have expected. Furthermore, this relationship was

also more likely significant because of the large sample size.

Also there are other factors that could account for differences in

severity of injury. As coal miners become more experienced, the

type of jobs that they perform may change. For example, a more

experienced worker is likely to be in a supervisory role. This study

is not experimental so it cannot be shown that age or experience

causes increased severity of injury but can only highlight that there

is a relationship. Future studies can, therefore, look to see what

other variables may have an impact on severity of injury.

This study also points to future directions for research in this

area. The present study examined the severity of injury using days

lost. However, future studies can expand on these results by collecting data and examining how the frequency of injury relates

to age and experience. When looking at frequency of injury, as opposed to severity of injury, there may be a stronger relationship between experience and injury.

This study also points to the importance of collecting more

safety and demographic data on coal mining in the future. There
are certain inherent limitations in the data set used in this paper as

there is no measure of frequency of injury for coal miners. Currently MSHA only collects data on age and experience when an injury occurs so it is not possible to measure frequency of injury as

there is no data on the age of all miners in the workforce. Although

BLS also collects data on coal mining, it does not report how age relates to frequency of injury for underground coal mining. The National Institute for Occupational Safety and Health (NIOSH) is

currently working to solve this problem by conducting a national

survey of mining that may allow researchers to examine how age

relates to frequency of injury. This is important as little research

has been conducted on mining demographics and injury since

1986. It is important that these data be collected more often as

changes in technology, technique, and demographics could affect

rate of injury. These data could play a crucial role in reducing fatalities and injuries by pinpointing specific characteristics that lead to

an increased risk of injury.

Despite the limitations with these data, this paper is a first step

for the mining industry. Very little has been done to analyze data

related to age and experience and other variables that may be related to safety and health in mining. This study found a relationship between increasing age and increasing time away from

work after an injury. This is an important finding when considering

that the average age of a coal miner is 50 (National Mining Association, 2007). Safety trainers should consider the safety of older

workers and consider training them about behaviors that would

be unique for older workers to help avoid injuries and, therefore,

avoid time away from work. Possible directions for training include

focusing on preventing musculoskeletal injuries or preventing falls.

New interventions are being developed to help improve balance

training and reduce falls among older adults (Maki et al., 2008).

Interventions such as these can be adopted for miners in the workplace. An even more comprehensive training program could be

used to train the entire workforce about the effects of aging on

safety and health (i.e. Porter et al., 2008). Such training may help

miners understand early warning signs of cumulative trauma disorders and what they can do to prevent lost-time injuries.

It is also important to note that the finding that there is not a

statistically significant relationship between experience in this

job and experience at this mine and severity of injury is as valuable

as finding a statistically significant result. This study shows that

experience may not play as crucial a role in severity of injury as

has been previously thought. Since this study did not find a statistically significant relationship between these types of experience

and severity of injury. Future research could look at the relationship between injury and variables such as length of shift, amount

of sleep, number of breaks, and drug use.

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