Quantifying humidity thresholds affecting work performance in protective clothing through nonlinear response analysis

Despite extensive research on thermal stress in protective clothing environments, little is known about how humidity-induced work performance degradation occurs across physiological and psychological domains. Here, we quantify the relationships between relative humidity (RH) exposure and human responses by examining twelve male participants wearing personal protective equipment (PPE) across six humidity levels (50%, 60%, 70%, 80%, 90%, and 100%) during controlled exercise protocols. We developed a three-stage analytical framework comprising one-way analysis of variance (ANOVA) to identify humidity-sensitive variables, polynomial regression to model relationships between RH and measured parameters, and marginal effect analysis to determine critical thresholds and optimal zones. The study revealed that skin temperature highly correlates with RH among physiological parameters. Distinct response thresholds emerged, with psychological discomfort manifesting at 60% RH, preceding physiological strain indicators that appeared at 70–90% RH. This temporal dissociation indicates that subjective perception responses occur before measurable physiological changes. Marginal effect analysis identified an optimal performance zone between 63% and 70% RH for physiological and 60% and 75% for psychological indicators at 25°C. Beyond these boundaries, performance deteriorated, with reaction times increasing by 21.5% at 100% RH. These thresholds inform current occupational standards that rely predominantly on physiological monitoring, potentially overlooking early stage thermal strain. Our findings provide a framework for refining workplace environmental guidelines under controlled conditions and demonstrate that integrating psychological assessments into heat stress protocols could improve early detection of adverse conditions, enhancing safety for workers in protective equipment.