Comparison of 2001 and 2018 ACGIH<sup>®</sup> Threshold Limit Values<sup>®</sup> for hand activity and risk of carpal tunnel syndrome in international prospective cohort studies

Introduction. Large multinational prospective studies have shown an association between carpal tunnel syndrome (CTS) risk and the 2001 ACGIH® Threshold Limit Value (TLV®) for hand activity score. However, because prior studies showed no difference in CTS risk among workers with scores between the action limit and the threshold limit value and workers with scores greater than the threshold limit value, the ACGIH Threshold Limit Value for hand activity score was revised in 2018 to better separate workers requiring additional surveillance of work-related musculoskeletal disorders from those at unacceptably increased risk. Objectives. The purpose of this study was to validate the 2018 ACGIH TLV for hand activity using a large multinational data set. Methods. Prospective data collected on 3,276 individuals for up to 7 years (USA) were merged with data from 2,858 workers followed for up to 10 years (Italy). All study participants (N=6,134) were full-time employees aged 18 or older who performed hand-intensive, repetitive, and forceful activities and were employed at 61 companies across various industries, including manufacturing, construction, food production, and healthcare. Incident CTS were those with symptoms and abnormal electrodiagnostic findings using thresholds that increased specificity. were compared using the chi-square test. Hazard ratios were estimated using Cox proportional hazard models to evaluate the rate of CTS by three 2001 TLV categories and three 2018 TLV categories. Results and Conclusions. After excluding prevalent CTS cases at baseline, subjects with no follow-up measurements, and subjects with missing biomechanical exposure data, the cohort was reduced to 5,147, with an incidence rate of 40.23 cases per 100 person-years of follow-up. There were substantial differences in exposure classification between the 2018 and 2001 TLVs. Overall, 3,012 (60%) individuals changed from a lower exposure category (2001 TLV) to a higher exposure category (2018 TLV). Specifically, compared to the 2001 TLV, when classified using the 2018 TLV, 2,149 non-cases and 69 cases were reclassified from the low exposure group to the medium exposure group, and 732 non-cases and 62 cases were reclassified from the medium exposure group to the high exposure group. Using the 2001 TLV approach, those above the action limit but below the TLV (HR=1.86; 95% CI: 1.34-2.58) had the same CTS rate as those above the TLV (HR=1.86; 95% CI: 1.38-2.51). Using the 2018 TLV approach, those above the action limit but below the TLV (HR=1.68; 95% CI: 1.05-2.68) had the same rate of CTS as those above the TLV (HR=2.60; 95% CI: 1.70-3.99). The 2018 TLV approach showed a stronger exposure-response relationship with CTS. The 2018 TLV approach, when used properly by experienced practitioners, serves as an effective tool to evaluate risk for CTS.