The most commonly used exposure assessment for heat stress is based on Wet Bulb Globe Temperature (WBGT), and the limits are based on empirical relationships that demonstrate a sustainable exposure. The ISO 7933 (2017) describes Predicted Heat Strain (PHS), which is a rational model for heat stress assessment that is used to assess time-limited exposures. Investigators have examined PHS validity under a variety of time-limited conditions. The purpose of this paper is to evaluate if PHS can predict sustainable exposures. The data used for this study were from two previous heat stress studies using a progressive heat stress protocol. The studies included 29 participants wearing four different ensembles (woven clothing, particle barrier, water barrier, and vapor barrier coveralls) at three levels of metabolic rate and relative humidity. Each trial provided data for a fully compensable (sustainable) exposure and an uncompensable (time-limited) exposure. The heat stress data from each condition were used to see if PHS demonstrated a steady-state response indicating a sustainable exposure. From the analysis, the sensitivity and specificity respectively for the ensemble types were: 1.00 and 0.18 for woven clothing, 0.95 and 0.60 for particle barrier, 0.91 and 0.83 for water barrier, 0.91 and 0.80 for vapor barrier, and 0.94 and 0.65 for all ensembles. The data show that while the sensitivity of PHS (correctly identifying unsustainable conditions) is good for the different ensembles, PHS specificity (correctly identifying sustainable exposures) was weak. From an occupational health and safety perspective, using PHS to identify sustainable exposures leads to protective decisions.