Associations of Temperature Patterns with Occupational Heat-Related Illness among Washington State Workers

Abstract

Background: Outdoor workers face elevated risk of occupational heat-related illness (HRI), and climate change is projected to drive further increases in heat-related morbidity and mortality. Current workplace heat safety rules rely primarily on laboratory-derived evidence, with opportunities to integrate information from real-world epidemiological studies. Objectives: This study quantified associations between ambient temperature patterns and HRI risk among Washington State workers to inform evidence-based occupational heat policies. Methods: Using a time-stratified case-crossover design, 1347 Washington State Fund HRI workers’ compensation claims from 2006 to 2022 were spatiotemporally linked to daily high-resolution (4km2) PRISM meteorological data on the date and location of HRI. Conditional logistic regression was used to assess three specific aims: (1) associations between daily maximum temperature (Tmax) and HRI risk; (2) effect modification by temperature surge conditions (≥10°F above 5-day average) on Tmax-HRI relationships; and (3) associations of daily minimum temperatures (Tmin) with occupational HRI. Results: HRI claims mostly occurred during May through September (95.2%) and were most common among males (79.1%), workers under age 35 (53.6%), and in construction (23.0%, agriculture, forestry & fishing (17.1%), and public administration (13.3%). Tmax ≥90°F was associated with 5-fold higher odds of an HRI claim (OR=5.0, 95% CI: 4.2,5.9), with the strongest effect on the day of exposure (lag 0) and rapid attenuation thereafter. In categorical analyses, compared to days with Tmax < 75°F, HRI odds increased with higher temperature categories: OR = 3.5 (95% CI: 2.6, 4.6) at 75–<80°F, OR = 8.1 (95% CI: 6.3, 10.5) at 80–<90°F, and OR = 50.5 (95% CI: 34.4, 74.1) at ≥100°F. At Tmax thresholds of 90°F and 80°F, respectively, additive joint HRI ORs were 7.8 (95% CI: 6.2,9.7) and 10.6 (95%CI: 8.5,13.2), with a relative excess risk due to interaction (RERI) of 1.4 and 4.0, and 18% and 37% of the excess HRI risk attributable to the joint additive occurrence of the threshold exceedance and surge rather than to either main effect alone. Temperature surge did not show statistically significant effect modification on the multiplicative scale. After adjustment for Tmax, elevated Tmin retained an independent positive association with higher HRI (OR=1.7, 95%CI: 1.4,2.1) at Tmin ≥60°F, OR=2.2 (95%CI: 1.7,2.9) at Tmin ≥65°F, and OR=3.4 (95%CI: 2.0,5.6) at Tmin ≥70°F. Conclusions: In this study, an exposure-response relationship was observed for the association between increasing Tmax and HRI, with risk occurring primarily on the day of illness. A positive additive interaction was observed between Tmax and same-day temperature surge relative to the previous 5-day average, suggesting that abrupt temperature increases amplify the effect of absolute high temperature on HRI risk on the additive scale. After adjustment for Tmax, daily Tmin retained an independent association with HRI, consistent with the hypothesis that elevated nighttime temperatures impair physiological recovery from heat. This study complements laboratory-based physiological evidence with real-world observational support for occupational heat exposure–HRI relationships and contributes to the evidence base that can inform the development and refinement of occupational heat protection standards.