Civil Engineering Reference
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2.1.3.3 Confounding
None of the musculoskeletal disorders examined in this report is uniquely caused by work exposures.
They are what the World Health Organization calls work-related conditions. “Work-related diseases
may be partially caused by adverse working conditions. They may be aggravated, accelerated, or exacer-
bated by workplace exposures, and they may impair working capacity. Personal characteristics and other
environmental and socio-cultural factors usually play a role as risk factors in work-related diseases, which
are often more common than occupational diseases” (World Health Organization, 1985).
In Chapter 3 we note that the epidemiologic study of causes related to health outcomes such as mus-
culoskeletal disorders requires careful attention to the several factors associated with the outcome.
The objective of a study will determine which factor or factors are the focus and which factors
might “confound” the association. In the case of musculoskeletal disorders, a study may have as its
objective the investigation of individual risk factors. Such a study, however, cannot evaluate individual
risk factors effectively if it does not also consider relevant work exposures; the work exposures are
potential confounders of the association with individual risk factors. Conversely, a study that evaluates
work exposures cannot effectively evaluate these factors if it does not also consider relevant individual
risk factors; the individual risk factors are potential confounders of the association with work
exposures.
Therefore, when studying the relationship of musculoskeletal disorders to work, it is necessary to con-
sider the other known factors that cause or modify the likelihood that the disorder will occur, such as
individual factors and nonwork exposures. For example, the frequency of many musculoskeletal
disorders is a function of age, so age has to be taken into account before attributing a musculoskeletal
disorder to a work exposure. Another common concern is whether a recreational exposure accounts
for an outcome that otherwise might be attributed to work.
In every epidemiologic study, confounders need to be measured and, when relevant, included in
the data analysis. The confounders selected for consideration in the analysis of data from a specific
study depend on the types of exposures studied, the types of outcomes measured, and the detail on
potential confounders that can be accurately collected on a sufficient number of the study subjects. As
a consequence, our approach to reviewing epidemiologic studies of work and musculoskeletal dis-
orders documented the attention given to a wide range of potential confounders (see the panel's
abstract form in Figure 2.1). No study can measure every possible confounder; however, the
papers included by the panel were judged to have given adequate attention to the primary individual
factors that might have confounded the work exposures under study. These include in particular age
and gender, as well as, when necessary and possible, such factors as obesity, cigarette smoking, and
comorbid states.
The role of potential confounders in epidemiologic studies and their proper management is often con-
fusing to the nonepidemiologist. The difficulty stems from the fact that the potential confounder is often
known to be associated with the disease, in this case musculoskeletal disorders. The association of a risk
factor such as age with the disease, however, does not make it a true confounder of the study's examin-
ation of a separate risk factor such as work exposures. True confounding occurs only when, for example,
both the risk factors being studied (age and work exposures) are associated with the outcome (muscu-
loskeletal disorders) and the two risk factors are also correlated (e.g., those with more work exposure are
also older). Fortunately, as noted in Chapter 3, there are statistical methods available to manage con-
founding that provide a way to “separate,” in this example, the effects of the work exposure from the
effects of age.
The panel recognizes that a number of nonwork factors are associated with or also cause the muscu-
loskeletal disorders under study. These were not separately studied, but they were considered, as necess-
ary, to evaluate the significance of the work factors that were studied. In our judgment, it is evident that
confounding alone is highly unlikely to explain the associations of musculoskeletal disorders with work
that are noted. More detailed consideration of confounding in future studies, however, should further
improve the precision and accuracy of risk estimates.
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