Author ORCID Identifier:
Date of Graduation
7-2026
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Health, Sport and Exercise Science (PhD)
Degree Level
Graduate
Department
Health, Human Performance and Recreation
Advisor/Mentor
McDermott, Brendon
Committee Member
Butts, Cory
Second Committee Member
Lo, Wen-Juo
Third Committee Member
McKenna, Zachary
Keywords
National Institute for Occupational Safety (NIOSH), health recommendations, workers, heat, Perceptual Strain Index (PSI)
Abstract
Occupational heat stress is an increasing global health concern due to rising environmental temperatures and the growing frequency of extreme heat events. Workers exposed to hot environments experience elevated physiological and perceptual strain, increased sweat loss, and greater risk of dehydration and heat-related illness. Despite existing occupational heat stress recommendations, limited research has examined how environmental characteristics influence thermoregulatory, hydration, and perceptual responses during simulated work. Therefore, the purpose of this dissertation was to investigate the influence of hot-dry (HD) and warm-humid (WH) environments on physiological strain, fluid balance, hydration responses, and perceptual strain during simulated occupational work performed in accordance with National Institute for Occupational Safety and Health (NIOSH) heat stress recommendations. This dissertation consisted of three experimental studies utilizing randomized repeated-measures crossover designs in young, healthy adults. Participants completed moderate-intensity intermittent walking under HD and WH environmental conditions while adhering to standardized occupational work-rest and hydration recommendations. Study 1 examined thermal and cardiovascular responses under HD and WH conditions with equivalent adjusted ambient temperatures. Study 2 evaluated fluid balance and hydration responses while participants adhered to NIOSH hydration recommendations. Study 3 investigated the relationship and agreement between the Perceptual Strain Index (PeSI) and the Physiological Strain Index (PSI) across environmental conditions. Collectively, the findings demonstrated that environmental characteristics influenced physiological, perceptual, and fluid balance responses despite similar adjusted environmental heat stress indices. HD conditions generally produced greater physiological strain and whole-body sweat rates, whereas WH conditions resulted in greater net fluid balance and hydration efficiency despite similar hydration biomarkers. Adherence to current NIOSH hydration recommendations prevented excessive body mass loss in both environments; however, the prescribed fixed-volume hydration strategy also promoted positive fluid balance and body mass gain, suggesting that it may provide more fluid than many workers require during moderate-intensity work. Additionally, although PeSI demonstrated a strong relationship with PSI, agreement between indices was limited, indicating that perceptual measures should not be used as standalone indicators of physiological heat strain. Overall, these findings demonstrate that environmental characteristics influence physiological strain, fluid balance, and perceptual responses during occupational heat exposure even when workers adhere to current NIOSH recommendations. Collectively, the results support the need for more individualized and flexible hydration strategies that account for environmental conditions, workload, and worker characteristics, while also highlighting the limitations of perceptual monitoring tools for estimating physiological heat strain. This dissertation contributes to the growing body of occupational heat stress literature and provides practical implications for improving worker hydration practices, heat stress management, and physiological monitoring in hot occupational environments.
Citation
Zhao, X. (2026). The efficacy of National Institute for Occupational Safety and Health recommendations to protect workers in the heat. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/6386
