Introduction Dehydration is a major risk factor for Heat Related Illness (HRI) in farmworkers.

Methods 587 acclimatised Latina/o farmworkers were monitored once each, in the hot, dry, California Central Valley over the summers of 2014–2015. Weight was recorded before and after the shift in a minimum level of clothing to assess change in hydration. To assess activity, accelerometers were worn, and questionnaires were administered in Spanish to collect occupational characteristics.

Results 66.2% of the participants were male; both sexes had a mean age 38.7 years. Men drank more, either total or just water (adjusted for height) than women (mean volumes 112 v 77oz, or 97 v 67 oz, PVal <0.001 for both). However men were more likely to lose ≥1.5% of their body weight: 64 (16.5%) v 6 (3.0%) women PVal <0.0001. Shift lengths were similar, but both total and mean activity levels were higher in males 2 02 000 v 1 33 000 and 391 v 255 counts per minute, respectively PVal <0.0001 for both.

Being male, working any type of piece rate and higher mean activity were all independently associated with weight loss over the shift in a multivariable linear regression model. Parameter and (95% CI) respectively: - 0.31 (- 0.43 to - 0.19), 0.18 (0.05–0.309), and mean cpm/100 0.022 (0.001–0.045).

Conclusion Male Latino farmworkers are more at risk of dehydration especially those who work high activity tasks or any form of piece rate. Employers should focus special attention on the safety of these workers.