Introduction Farmworkers are at risk of heat related illness (HRI). The main contributors are thought to be environmental temperatures and activity levels, but the association has not been objectively assessed.

Methods California farmworkers were monitored for one work-shift each in the summers of 2014 and 2015. Interviewers recorded personal and demographic information. Wet bulb globe temperature (WBGT) was collected in minute intervals over the shift as were accelerometer activity data from each participant. This data was grouped into physiologically meaningful 15 min increments, with the average used for analysis. Data from the previous 15 min were used to predict current activity. Generalized Additive Modeling was used to determine the form of the association between WBGT and activity level. This information was used to build an appropriate repeated measures model including demographics and work-related attributes.

Results Final analyses included 318 Latina/o farmworkers. Only 70 subjects (22%) were a healthy BMI, and 115 subjects (36%) were considered obese (BMI ≥30). None were underweight. The majority of participants were paid hourly (n=233, 73%) as opposed to piece-rate (n=85, 27%). Activity counts averaged 452, or a low intensity activity level. In the final repeat measures model, work activity was inversely associated with WBGT, for every oC increase in WBGT activity decreased by 4.5 counts (95% CI 1.2–7.6) P 0.01. Other independent negative associations with activity were found with age and male gender. An interaction was found between piece-rate workers and gender. Male piece rate workers did not decrease activity levels with WBGT, but females and hourly paid workers did.

Conclusion In general, farmworkers decrease their work activity with increasing environmental heat. Men, especially piece-rate workers are less likely to decrease their activity levels putting them at increased risk of HRI.