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Researchers Identify Dose-Response Relationship Between ACGIH Hand Activity Level TLV and Risk of Carpal Tunnel Syndrome

Researchers Roberta Bonfiglioli, MD, Stefano Mattioli, MD, Thomas Armstrong, PhD, Francesca Graziosi, MSc, Francesco Marinelli, MStat, Andrea Farioli, MD, and Francesco Violante, MD, all from the University of Bologna, Italy, with the exception of Armstrong, from the University of Michigan, USA have published a study that they say validates the ACGIH TLV© for HAL (American Congress of Governmental Industrial Hygienists Threshold Limit Value for Hand Activity Level). If so, this could be a ground-breaking study in understanding and preventing work-related carpal tunnel syndrome (CTS), one type of musculoskeletal disorder (MSD).

In their introduction, the authors provide the following evidence related to carpal tunnel syndrome (CTS):

  • the prevalence of CTS in the general population is estimated to be between 1% – 5%;
  • incident rates range from 0.5% – 3.8% per 1000 people;
  • personal risk factors for CTS include gender, body mass index (BMI), height, marital status, “familiar history”, and pathologies like rheumatoid arthritis and diabetes;
  • a number of epidemiological studies show a relationship between manual work (“especially repetitive and forceful exertions) and CTS, although those studies are cross-sectional and therefore cannot conclude cause-and-effect relationships;
  • biomechanical models and laboratory studies support a dose-response relationship between CTS and mechanical loading;
  • certain postures of the wrist increase pressure on the median nerve inside the carpal tunnel;
  • high contact stress tendon loads increase fibrous connective tissue and thickening of the flexor synovia and the walls of arterioles inside the carpal tunnel;
  • tendon load evidence and epidemiological evidence suggest hand-wrist tendinitis and CTS are related to repetitive work;
  • wrist angle deviations from neutral are associated with increased pressure inside the carpal tunnel; and
  • “These studies provide support for the contribution of repetitive exertions, high hand forces and certain postures commonly involved in manual work to chronic tendon and nerve injuries in the wrist.”

The goal of this study was to “investigate biomechanical load as a causal factor for CTS, using the ACGIH TLV© based on hand activity level (HAL) and normalized peak force (PF).”


The experimental methods for this study are somewhat complex and involved, beyond the scope of this review, and interested readers are encouraged to read the complete research article, cited below and available at no charge at the time of this writing.

The researchers provide this abbreviated description:

"Industrial and service workers were followed from 2000–2003. We classified subjects with respect to action limit (AL) and TLV. Case definitions were: (i) self-reported symptoms; and (ii) combination of symptoms and positive nerve conduction studies. Poisson regression models including age, gender, body mass index, and presence of predisposing pathologies were conducted to estimate incidence rate ratios (IRR) of CTS."

“AL” refers to the Action Limit, one of the components of the ACGIH HAL assessment tool. ACGIH proposed the tool as a means to assess “mono-task” jobs such as “assembly line” and “keyboard” work. This studies authors state that “There is no clear cut-off between mono- and multi-task jobs,” and therefore argue that the tool “…applies to jobs in which there is a regular or predictable pattern of exertions over the course of each work shift.” Readers are again referred to the full article for more information on the ACGIH HAL assessment tool.

The researchers used data from the Occupational Carpal Tunnel Syndrome Observational Prospective Unified Study (OCTOPUS), “a longitudinal study of different industrial and service worker groups started in 2000,” and reported previously by other researchers.


Study results include:

  • 4332 workers were invited to participate;
  • 174 were eliminated because the ACGIH HAL methods didn’t apply to their work; 61 were eliminated because they previously had CTS surgery; 236 never responded to the invitation; 939 were lost after follow-up after the baseline measurements; 1 was eliminated due to missing BMI information;
  • of the 2921 remaining participants, 2599 (67.3%) were women (mean age 39.2 (SD 9.5) years, mean BMI of 23.2 (SD 4.1) kg/m2) and 1261 (32.7%) were men (mean age 35.6 (SD 9.2) years, mean BMI of 25.1 (SD 3.3) kg/m2)
  • 1076 were observed at twice at 1 year; 429 at were observed twice at 2 year; and 1416 were observed thrice at 1 year and 2 years into the study;
  • Subjects exhibiting symptoms of CTS were invited to undergo nerve conduction studies; 239 (32.3%) refused to undergo examination; 310 presented incident symptoms; 84 of them had a positive electroneurographic testing.
  • preliminary analysis identified the following variables to be included in two separate statistical models: gender, age, BMI, and presence of pathologies predisposing to CTS onset;
  • the researchers tested two different multivariate models, and both present a dose–response relationship between the ACGIH classification and the risk of CTS.
  • the risk of CTS for females was more than double that of males;
  • the effect of BMI and age appeared stronger when studying CTS diagnosed with nerve conduction studies;
  • ultimately, the researchers conclude that the “ACGIH TLV© for HAL predicted a dose-response relationship for the incidence of CTS;”
  • there was “an appreciable increase in risk of CTS for subjects below the TLV but above the AL,” which they postulate might suggest current TLV cut-off may not be sufficiently protective, leaving some workers at risk even for exposures below the TLV

The researchers present much more detail and additional results for interested readers.

What This Might Mean to Ergonomists

This could be one of the more influential studies in the area of MSDs in many years, though time will tell as other researchers review and further replicate and validate, or not, the findings in this study. One of the long-standing criticisms of the ergonomics scientific knowledge base has been the absence of “dose-response” relationships between physical risk factors and MSDs. In the minds of researchers and ergonomics professionals, the lack of risk-factor-to-injury dose-response relationships is due to the difficulties in conducting statistically valid studies. In the eyes of critics, the lack of scientific evidence is indicative that the relationships are weak or non-existent.

It’s a fair question: Why is it so difficult to identify dose-response relationships for Workplace MSDs (WMSDs)? The answer lies in the types of scientific methods and evidence that are necessary to demonstrate such cause-and-effect relationships. Assume for the moment that researchers could control all of the forces, motions and repetitions an experimental subject conducted over the many days, weeks or years it might take to develop an MSD under different conditions. If this were possible, researchers could carefully control their experiments and could expose human subjects to various levels of risk factors like force, repetition and posture to see what exposure levels lead to specific MSDs over time. In essence, they would have to injure a lot of people in order to prove statistically valid cause-and-effect relationships. However, intentionally injuring people for the sake of knowledge is heavily frowned upon in civilized societies, so researchers must instead turn to what are generally known as epidemiology studies.

In epidemiological studies, researchers lack controls over specific risk factor exposures. Instead, they must increase the number of subjects they include in their studies, and then they must attempt to measure and quantify risk factor exposures and MSD outcomes over some extended period of time and look for statistically valid patterns in their data. What they lack in specific controls over some smaller set of subjects in a direct cause-and-effect controlled experiment, they attempt to make up for with a larger subject population and less specific knowledge of the full-spectrum of risk factor exposures their subjects experience. Isolating specific exposures is very difficult, especially in a real-world work environment where people may perform many tasks with different risk exposures.

A primary “take-home” message from the study, however, is that the ACGIH HAL TLV© assessment method, when applied by properly trained assessors, appears to be a good predictor of CTS risk among workers.


Roberta Bonfiglioli, MD, Stefano Mattioli, MD, Thomas Armstrong, PhD, Francesca Graziosi, MSc, Francesco Marinelli, MStat, Andrea Farioli, MD, and Francesco Violante, MD (2012), Validation of the ACGIH TLV for hand activity level in the OCTOPUS cohort: a two-year longitudinal study of carpal tunnel syndrome, Scand J Work Environ Health – online first. doi:10.5271/sjweh.3312.


This article originally appeared in The Ergonomics Report™ on 2012-07-18.