Agreement between measured and self-reported physiological strain in young adults and older adults with and without common chronic diseases during simulated occupational heat stress

While monitoring physiological strain is recommended to safeguard workers during heat exposure, it is logistically challenging. The perceptual strain index (PeSI) is a subjective estimate thought to reflect the physiological strain index (PSI) that requires no direct monitoring. However, advanced age and chronic diseases (hypertension/type 2 diabetes [T2D]) influence the perception of heat stress, potentially limiting the utility of the PeSI. We therefore assessed whether the relation and agreement between the PeSI and PSI during simulated work in various environmental conditions is modified by age and T2D/hypertension. Thirteen young adults and 37 older adults without (<i>n</i> = 14) and with T2D (<i>n</i> = 10) or hypertension (<i>n</i> = 13) walked on a treadmill (∼200 W/m²) for 180 min or until termination (volitional fatigue, rectal temperature ≥39.5 °C) in 16, 24, 28, and 32 °C wet-bulb globe temperatures. Rectal temperature and heart rate were recorded to calculate PSI (0-10 scale). Rating of perceived exertion and thermal sensation were recorded to calculate PeSI (0-10 scale). The relation between hourly PSI and PeSI was assessed via linear mixed models. Mean bias (95% limits of agreement [LoA]) between PSI and PeSI was assessed via Bland-Altman analysis. PSI increased with PeSI (<i>p</i> < 0.001), but the slope of this relation was not different between young and older adults (<i>p</i> = 0.189) or as a function of chronic disease (within older adults; <i>p</i> = 0.183). The mean bias between PSI and PeSI was small (0.02), but the 95% LoA was wide (-3.3-3.4). Together, a linear relation between PeSI and PSI was observed but agreement between these measures varied considerably across individuals and thus PeSI should not be used as a surrogate marker of PSI. Caution should be taken when utilizing the PeSI to estimate physiological strain on workers.