Physiological strain in outdoor workers: The hidden danger of high humidity.

Background: Outdoor workers in hot climates face significant heat strain, exacerbated by factors like air temperature and relative humidity (RH). While high temperatures' effects on health are well-documented, RH's critical role in influencing physiological strain is less explored. This study investigates RH's impact on the Physiological Strain Index (PSI) among outdoor workers, aiming to enhance safety in hot, humid conditions.

Methods: We conducted a cross-sectional study of 1452 outdoor workers across India (2014-2022), collecting data on air temperature, RH, wet bulb globe temperature (WBGT), and physiological heat strain indicators in the summer and winter. Generalised Linear Mixed Models (GLMM) and Generalised Additive Models (GAM) were utilised to assess RH's influence on PSI, potential non-linear relationships, and a heat-humidity threshold.

Results: Crude odds ratios (COR) indicated that workers exposed to high humidity were 2.5 times more likely to experience high PSI (COR = 2.5 [95 % CI: 1.82-3.44]). GLMM results confirmed RH's significant impact on PSI when adjusting for covariates (aOR = 1.6 [95 % CI: 1.24-2.29]). GAM analysis revealed non-linear relationships between air temperature, RH, and PSI. The predictive model derived from the GAM identified a heat-humidity threshold of 32 °C and 60 % RH.

Conclusions: Elevated PSI under high humidity conditions highlights the need for tailored protective measures, such as hydration strategies and adjusted work-rest cycles, to mitigate physiological strain in hot and humid environments. Conclusions: RH significantly exacerbates PSI among outdoor workers. These findings inform workplace safety guidelines, emphasizing the need for more aggressive heat stress management in high-humidity conditions.