Introduction Heat-related illness (HRI) poses a serious occupational health risk for farmworkers, particularly considering harvest season corresponds to peak summertime heat. The California Heat Illness Prevention Study (CHIPS) collected objective data on the physiological responses of farmworkers to environmental heat, covering a wide range of crops and working conditions.

Objectives The primary objective of this analysis is to identify risk factors for elevated core body temperature (CBT).

Methods A convenience sample of farmworkers was recruited through farms and farm labor contractors. Bilingual researchers administered questionnaires pre- and post-shift. CBT and work rate were measured using an ingestible wireless thermistor and an accelerometer, respectively. Ambient weather conditions, including wet bulb globe temperature (WBGT), were recorded using two weather stations at the work site. Multiple logistic modeling was used to identify risk factors for elevated CBT (≥38.5°C).

Results In total, 587 farmworkers on 30 farms throughout California participated over the summers of 2014 and 2015. After data cleaning, 507 participants were included in analyses using elevated CBT as the primary outcome. In multiple logistic modeling (AOR [95% CI]) male sex (3.74 [1.22 – 11.54]), WBGT (1.22 [1.08 – 1.38]), work rate (1.004 [1.002 – 1.006]), and increased BMI (1.11 [1.10 – 1.29]) were all independently associated with elevated CBT.

Conclusion Despite high summer temperatures, most farmworkers were able to keep their CBT below 38.5°C. Risk of HRI was exacerbated by work rate and environmental temperature despite farms following Cal/OSHA regulations.