Article Spotlight: Heat-related rest-break recommendations for farmworkers in California based on wet-bulb globe temperature

Royani Saha | SCORCH Graduate Intern & MEZCOPH Doctoral Student

We are excited to highlight current peer-reviewed publications that further our understanding of climate and health issues in this new section of the SCORCH Building Climate Resilience newsletter. This issue’s vignette recaps the use of wet-bulb globe temperature to inform protections for outdoor workers during extreme heat events and explains, in plain language, the research article Heat-related rest-break recommendations for farmworkers in California based on wet-bulb globe temperature authored by Parajuli, Biggs, Galvez, et al. (full citation below) and published by Communications Earth & Environment on May 9, 2025. 

What is the concern/issue? Outdoor workers are among the most at risk group for heat related illness and mortality globally. In the United States the Occupational Safety and Health Administration (OSHA) regards agricultural, construction, landscaping, mail and packaging, and oil and gas well operations as industries with heightened outdoor heat risk exposure. Outdoor laborers experience heat exposure from environmental heat (temperature, humidity, solar radiation, and wind), which is exacerbated by metabolic heat generated by physical labor. When these stressors collide workers become vulnerable to dehydration, heat cramps, fainting, kidney dysfunction, and even heat stroke if insufficient rest, shade, and hydration are provided. In extreme cases, intense physical exertion under high heat can lead to muscle rupture and fatal cardiovascular collapse.

This featured research explores the heat risk and safety needs of agricultural workers in California with implications for outdoor worker safety policies both globally and here at home in Arizona.

Arizona’s stakes are just as high: More than 4,320 Arizonans died from excessive‐heat exposure between 2012 and 2023, and in 2024 Maricopa County alone confirmed 602 heat deaths. While these figures represent the general population, they nonetheless underscore the urgent need for stronger, evidence-based worker protections. 

Study at a glance:  In the study published in the May 2025 issue of Nature Communications Earth and Environment, Parajuli et al. compared rest minutes per hour (RMPH) using three different heat stress indicators: wet-bulb globe temperature (WBGT), dry-bulb temperature (DBT), and the heat index (HI) – the criterion on which the newly proposed OSHA rule, Heat Injury and Illness Prevention in Outdoor and Indoor Work Settings, is based. The authors of this study calculated rest minutes for farmworkers in California’s Imperial and Coachella Valleys (ICV) region accounting for geography, crop environment, acclimatization status, clothing, work shift hours, and workload.

 The authors created a hyper-local, hour-by-hour “digital twin” of California’s ICV, which is among the nation’s hottest agricultural regions, by simulating irrigation, land cover, wind, and solar radiation outputs from a validated Weather Research and Forecasting Model (WRF) at a 1-km spatial resolution. They calculated recommended RMPH for farmworkers considering “factors that affect heat exposure including location, work shift, season, clothing, acclimatization, and workload conditions” (Table 2). The resulting RMPH recommendations account for the time of year and provide policy makers and employers WBGT-based RMPH thresholds, eliminating the need to conduct continuous WBGT monitoring and measurements. 

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Key Findings:

• The results “demonstrate that the rest minutes based on WBGT yields much higher rest break requirements than other measures” (e.g., DBT and HI used in OSHA’s new rule) and are “most protective”.
• “Existing air temperature-based heat illness prevention regulations are likely inadequate and not protective of farmworkers in hot regions and seasons.”
• The RMPH recommendations are based on unacclimatized workers. 

Limitations: The authors acknowledge several limitations in their approach to calculating the RMPH including the limits of the WBGT which does not “fully consider the individual’s physical condition, age, or personal tolerance to heat.” Therefore, the recommended rest minutes per hour estimates need to be modified for more specific, tailored rest breaks to ensure protection for outdoor workers who may be older or have pre-existing conditions. While the RMPH recommendations are based on healthy, unacclimatized workers, the authors note that the recommendations can be protective for both unacclimatized and acclimatized workers across seasons and work conditions. 

Applications: The study recommends adapting their WBGT‐based work–rest framework to local climates by deriving region‐specific rest‐minute tables from high‐resolution WBGT climatology, rather than relying on one‐size‐fits‐all temperature rules. Policy makers and employers are encouraged to embed these tables directly into policies to protect both acclimatized and unacclimatized workers under real-world conditions. The research study can inform worker safety policies. Some recommendations stemming from this study include implementing:

• More frequent work breaks
• Work breaks of varying duration to account for extreme heat conditions and seasonal variation
• Improved access to shade
• Increased access to hydration
• Implementing earlier work shifts to avoid heat exposure during peak times of the day while still being paid for all hours in the work shift
• Enhanced worker and employer education

This study provides evidence-based outdoor worker protection recommendations such as rest minutes per hour based on wet-bulb globe temperatures. By grounding safety practices in high-resolution heat science and emphasizing practical implementation, policymakers and employers can take targeted steps to keep outdoor workers safe as global temperatures climb.