UC Santa Barbara

Abstract Farmworkers are particularly vulnerable to heat stress, which can escalate into heat‐related illnesses such as heat exhaustion or heat stroke, and can also impact productivity loss. Heat exposure varies considerably depending on season, work‐shift timing, acclimatization, and workload. We examine the frequency of heat stress exceedance in the Imperial and Coachella Valleys of southern California using wet bulb globe temperature (WBGT) calculated using the outputs from Weather Research and Forecasting (WRF) model for a reference year of 2020. The critical WBGT threshold of 80℉ is exceeded for more than 500 hr in August, with considerable exceedance in all key harvesting months of April, May, and June. The threshold is exceeded most frequently in August, with daytime (nighttime) exceedance of 96.3% (38.0%) in the Imperial and 72.5% (11.9%) in the Coachella Valleys. Occupational WBGT limits are also exceeded in most crop environments for unacclimatized workers, and even for acclimatized workers during date and sugarcane harvests. Adjusting the work schedule to an early morning or late evening shift can reduce workers' productivity loss by up to 40%. Based on our findings, we propose several strategies to mitigate heat risks among farmworkers: (a) adjusting work hours to include cooler morning or evening hours (b) using WBGT instead of air temperature to monitor heat exposure (c) adopting variable rest‐break schedules (d) reducing nighttime heat exposure during summer to allow recovery from daytime heat (e) taking extra precaution in high‐risk crop zones (f) avoiding nighttime irrigation during summer, and (g) revising thresholds and definitions used to identify heatwaves. Plain Language Summary Farmworkers are at high risk of heat stress because they work outdoors for prolonged hours under strenuous conditions. This can lead to serious illnesses such as heat stroke and can reduce productivity. We examined how often heat exposure exceeds safety limits in California's agricultural heartlands, the Imperial and Coachella Valleys, using the Wet‐Bulb Globe Temperature (WBGT), a scientific heat metric. We found that WBGT exceeded the safety limit for hundreds of hours during key harvest months such as April, May, and June. Based on our results, we recommend several heat‐exposure mitigation strategies to protect farmworkers: (a) adjusting work hours to include cooler morning or evening periods; (b) using WBGT rather than air temperature to monitor heat exposure; (c) adopting a variable rest‐break schedule; (d) reducing nighttime heat exposure during summer to support recovery from daytime heat; (e) taking extra precautions in high‐risk crop zones; (f) avoiding nighttime irrigation during summer; and (g) revising definitions used to identify heatwaves. Key Points Wet bulb globe temperature exceeds safe limits in key harvest months, putting farmworkers at high risk of heat‐related illness Heat risk varies by crop, season, workload, and shift timing; current work schedules often include the hottest hours of the day Proposed heat stress mitigation strategies include adjusting work hours and adopting variable rest‐breaks, especially in high‐risk crop zones