<p>The Wet-Bulb Globe Temperature (WBGT) is the benchmark for occupational heat-stress assessment, yet its accuracy can be constrained by variability in humidity, air movement, and solar load. The Environmental Stress Index (ESI), derived from routinely available meteorological variables, has been proposed as a low-cost field surrogate. This study evaluated the agreement and physiological relevance of ESI versus WBGT among outdoor workers across arid and semi-arid climates in Iran. A cross-sectional study was conducted on 150 sand-and-gravel mine workers in arid and semi-arid regions of Iran. Environmental and physiological data were collected at different times. The WBGT (based on ISO 7243) and ESI (based on ambient parameters) were calculated. Between-climate differences were analyzed using t-test or Mann–Whitney U; agreement was assessed by ICC; and associations with meteorological and physiological variables were evaluated using GEE (α = 0.05). In the semi-arid site, the values of both WBGT and ESI were higher during the warmer hours (<i>p</i> &lt; 0.001). Regression analyses revealed statistically significant relationships between the thermal indices and most environmental parameters in both climates (<i>p</i> &lt; 0.05). The agreement between WBGT and ESI was high (ICC_arid:0.98, ICC_semi−arid:0.97) with statistically significant correlations in both climates (<i>p</i> &lt; 0.001). In the arid site, both WBGT and ESI showed significant positive correlations with physiological responses, whereas in the semi-arid site, only the ESI index exhibited significant correlations with physiological responses (<i>p</i> &lt; 0.05). Overall, ESI demonstrated performance equivalent to WBGT in the arid site and was suitable for field screening applications. ESI shows strong agreement with WBGT and meaningful physiological validity in arid sites, making it a viable low-cost screening tool when WBGT devices are unavailable. It is recommended that industries in arid regions adopt ESI for daily heat-stress monitoring.</p>